Java is the high-level, object-oriented, robust, secure programming language, platform-independent, high performance, Multithreaded, and portable programming language. It was developed by James Gosling in June 1991. It can also be known as the platform as it provides its own JRE and API.
There are the following features in Java Programming Language. Simple: Java is easy to learn. The syntax of Java is based on C++ which makes easier to write the program in it. Object-Oriented: Java follows the object-oriented paradigm which allows us to maintain our code as the combination of different type of objects that incorporates both data and behavior. Portable: Java supports read-once-write-anywhere approach. We can execute the Java program on every machine. Java program (.java) is converted to bytecode (.class) which can be easily run on every machine. Platform Independent: Java is a platform independent programming language. It is different from other programming languages like C and C++ which needs a platform to be executed. Java comes with its platform on which its code is executed. Java doesn't depend upon the operating system to be executed. Secured: Java is secured because it doesn't use explicit pointers. Java also provides the concept of ByteCode and Exception handling which makes it more secured. Robust: Java is a strong programming language as it uses strong memory management. The concepts like Automatic garbage collection, Exception handling, etc. make it more robust. Architecture Neutral: Java is architectural neutral as it is not dependent on the architecture. In C, the size of data types may vary according to the architecture (32 bit or 64 bit) which doesn't exist in Java. Interpreted: Java uses the Just-in-time (JIT) interpreter along with the compiler for the program execution. High Performance: Java is faster than other traditional interpreted programming languages because Java bytecode is "close" to native code. It is still a little bit slower than a compiled language (e.g., C++). Multithreaded: We can write Java programs that deal with many tasks at once by defining multiple threads. The main advantage of multi-threading is that it doesn't occupy memory for each thread. It shares a common memory area. Threads are important for multi-media, Web applications, etc. Distributed: Java is distributed because it facilitates users to create distributed applications in Java. RMI and EJB are used for creating distributed applications. This feature of Java makes us able to access files by calling the methods from any machine on the internet. Dynamic: Java is a dynamic language. It supports dynamic loading of classes. It means classes are loaded on demand. It also supports functions from its native languages, i.e., C and C++.
Java Virtual Machine is a virtual machine that enables the computer to run the Java program. JVM acts like a run-time engine which calls the main method present in the Java code. JVM is the specification which must be implemented in the computer system. The Java code is compiled by JVM to be a Bytecode which is machine independent and close to the native code.
JVM is an acronym for Java Virtual Machine; it is an abstract machine which provides the runtime environment in which Java bytecode can be executed. It is a specification which specifies the working of Java Virtual Machine. Its implementation has been provided by Oracle and other companies. Its implementation is known as JRE. JVMs are available for many hardware and software platforms (so JVM is platform dependent). It is a runtime instance which is created when we run the Java class. There are three notions of the JVM: specification, implementation, and instance.
JRE stands for Java Runtime Environment. It is the implementation of JVM. The Java Runtime Environment is a set of software tools which are used for developing Java applications. It is used to provide the runtime environment. It is the implementation of JVM. It physically exists. It contains a set of libraries + other files that JVM uses at runtime.
JDK is an acronym for Java Development Kit. It is a software development environment which is used to develop Java applications and applets. It physically exists. It contains JRE + development tools. JDK is an implementation of any one of the below given Java Platforms released by Oracle Corporation: Standard Edition Java Platform Enterprise Edition Java Platform Micro Edition Java Platform
Many types: Class(Method) Area: Class Area stores per-class structures such as the runtime constant pool, field, method data, and the code for methods. Heap: It is the runtime data area in which the memory is allocated to the objects Stack: Java Stack stores frames. It holds local variables and partial results, and plays a part in method invocation and return. Each thread has a private JVM stack, created at the same time as the thread. A new frame is created each time a method is invoked. A frame is destroyed when its method invocation completes. Program Counter Register: PC (program counter) register contains the address of the Java virtual machine instruction currently being executed. Native Method Stack: It contains all the native methods used in the application.
Just-In-Time(JIT) compiler: It is used to improve the performance. JIT compiles parts of the bytecode that have similar functionality at the same time, and hence reduces the amount of time needed for compilation. Here the term “compiler” refers to a translator from the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.
A platform is the hardware or software environment in which a piece of software is executed. There are two types of platforms, software-based and hardware-based. Java provides the software-based platform.
There are the following differences between the Java platform and other platforms. Java is the software-based platform whereas other platforms may be the hardware platforms or software-based platforms. Java is executed on the top of other hardware platforms whereas other platforms can only have the hardware components.
The bytecode. Java compiler converts the Java programs into the class file (Byte Code) which is the intermediate language between source code and machine code. This bytecode is not platform specific and can be executed on any computer.
Classloader is a subsystem of JVM which is used to load class files. Whenever we run the java program, it is loaded first by the classloader. There are three built-in classloaders in Java. Bootstrap ClassLoader: This is the first classloader which is the superclass of Extension classloader. It loads the rt.jar file which contains all class files of Java Standard Edition like java.lang package classes, package classes, java.util package classes, package classes, java.sql package classes, etc. Extension ClassLoader: This is the child classloader of Bootstrap and parent classloader of System classloader. It loads the jar files located inside $JAVA_HOME/jre/lib/ext directory. System/Application ClassLoader: This is the child classloader of Extension classloader. It loads the class files from the classpath. By default, the classpath is set to the current directory. You can change the classpath using "-cp" or "-classpath" switch. It is also known as Application classloader.
Yes, Java allows to save our java file by .java only, we need to compile it by javac .java and run by java classname Let's take a simple example:
//save by .java only class A{ public static void main(String args[]){ System.out.println("Hello java"); } } //compile by javac .java //run by java A compile it by javac .java run it by java A
The program compiles and runs correctly because the order of specifiers doesn't matter in Java.
The local variables are not initialized to any default value, neither primitives nor object references.
In Java, access specifiers are the keywords which are used to define the access scope of the method, class, or a variable. In Java, there are four access specifiers given below. Public The classes, methods, or variables which are defined as public, can be accessed by any class or method. Protected Protected can be accessed by the class of the same package, or by the sub-class of this class, or within the same class. Default Default are accessible within the package only. By default, all the classes, methods, and variables are of default scope. Private The private class, methods, or variables defined as private can be accessed within the class only.
The methods or variables defined as static are shared among all the objects of the class. The static is the part of the class and not of the object. The static variables are stored in the class area, and we do not need to create the object to access such variables. Therefore, static is used in the case, where we need to define variables or methods which are common to all the objects of the class. For example, In the class simulating the collection of the students in a college, the name of the college is the common attribute to all the students. Therefore, the college name will be defined as static.
There are various advantages of defining packages in Java. Packages avoid the name clashes. The Package provides easier access control. We can also have the hidden classes that are not visible outside and used by the package. It is easier to locate the related classes.
Explanation In the first case, 10 and 20 are treated as numbers and added to be 30. Now, their sum 30 is treated as the string and concatenated with the string Javatpoint. Therefore, the output will be 30Javatpoint. In the second case, the string Javatpoint is concatenated with 10 to be the string Javatpoint10 which will then be concatenated with 20 to be Javatpoint1020.
class Test { public static void main (String args[]) { System.out.println(10 + 20 + "Javatpoint"); System.out.println("Javatpoint" + 10 + 20); } }
The output of the above code will be 30Javatpoint Javatpoint1020
Explanation In the first case, The numbers 10 and 20 will be multiplied first and then the result 200 is treated as the string and concatenated with the string Javatpoint to produce the output 200Javatpoint. In the second case, The numbers 10 and 20 will be multiplied first to be 200 because the precedence of the multiplication is higher than addition. The result 200 will be treated as the string and concatenated with the string Javatpointto produce the output as Javatpoint200.
class Test { public static void main (String args[]) { System.out.println(10 * 20 + "Javatpoint"); System.out.println("Javatpoint" + 10 * 20); } }
The output of the above code will be 200Javatpoint Javatpoint200
class Test { public static void main (String args[]) { for(int i=0; 0; i++) { System.out.println("Hello Javatpoint"); } } } The above code will give the compile-time error because the for loop demands a boolean value in the second part and we are providing an integer value, i.e., 0.There is given more than 50 OOPs (Object-Oriented Programming and System) interview questions. However, they have been categorized in many sections such as constructor interview questions, static interview questions, Inheritance Interview questions, Abstraction interview question, Polymorphism interview questions, etc. for better understanding.
It is a programming paradigm based on objects having data and methods defined in the class to which it belongs. Object-oriented paradigm aims to incorporate the advantages of modularity and reusability. Objects are the instances of classes which interacts with one another to design applications and programs. There are the following features of the object-oriented paradigm. Follows the bottom-up approach in program design. Focus on data with methods to operate upon the object's data Includes the concept like Encapsulation and abstraction which hides the complexities from the user and show only functionality. Implements the real-time approach like inheritance, abstraction, etc. The examples of the object-oriented paradigm are C++, Simula, Smalltalk, Python, C#, etc.
The Object is the real-time entity having some state and behavior. In Java, Object is an instance of the class having the instance variables as the state of the object and the methods as the behavior of the object. The object of a class can be created by using the new keyword.
There are the following basic differences between the object-oriented language and object-based language. Object-oriented languages follow all the concepts of OOPs whereas, the object-based language doesn't follow all the concepts of OOPs like inheritance and polymorphism. Object-oriented languages do not have the inbuilt objects whereas Object-based languages have the inbuilt objects, for example, JavaScript has window object. Examples of object-oriented programming are Java, C#, Smalltalk, etc. whereas the examples of object-based languages are JavaScript, VBScript, etc.
The constructor can be defined as the special type of method that is used to initialize the state of an object. It is invoked when the class is instantiated, and the memory is allocated for the object. Every time, an object is created using the new keyword, the default constructor of the class is called. The name of the constructor must be similar to the class name. The constructor must not have an explicit return type.
Based on the parameters passed in the constructors, there are two types of constructors in Java. Default Constructor: default constructor is the one which does not accept any value. The default constructor is mainly used to initialize the instance variable with the default values. It can also be used for performing some useful task on object creation. A default constructor is invoked implicitly by the compiler if there is no constructor defined in the class. Parameterized Constructor: The parameterized constructor is the one which can initialize the instance variables with the given values. In other words, we can say that the constructors which can accept the arguments are called parameterized constructors.
The purpose of the default constructor is to assign the default value to the objects. The java compiler creates a default constructor implicitly if there is no constructor in the class.
class Student3{ int id; String name; void display(){System.out.println(id+" "+name);} public static void main(String args[]){ Student3 s1=new Student3(); Student3 s2=new Student3(); s1.display();
yes, The constructor implicitly returns the current instance of the class (You can't use an explicit return type with the constructor).
No, The constructor is not inherited.
No, the constructor can't be final.
Yes, the constructors can be overloaded by changing the number of arguments accepted by the constructor or by changing the data type of the parameters. Consider the following example. In the above program, The constructor Test is overloaded with another constructor. In the first call to the constructor, The constructor with one argument is called, and i will be initialized with the value 10. However, In the second call to the constructor, The constructor with the 2 arguments is called, and i will be initialized with the value 15.
class Test { int i; public Test(int k) { i=k; } public Test(int k, int m) { System.out.println("Hi I am assigning the value max(k, m) to i");
There is no copy constructor in java. However, we can copy the values from one object to another like copy constructor in C++. There are many ways to copy the values of one object into another in java. They are: By constructor By assigning the values of one object into another By clone() method of Object class In this example, we are going to copy the values of one object into another using java constructor. //Java program to initialize the values from one object to another class Student6{ int id; String name; //constructor to initialize integer and string Student6(int i,String n){ id = i; name = n; } //constructor to initialize another object Student6(Student6 s){ id =; name; } void display(){System.out.println(id+" "+name);} public static void main(String args[]){ Student6 s1 = new Student6(111,"Karan"); Student6 s2 = new Student6(s1); s1.display(); s2.display(); } }
The static variable is used to refer to the common property of all objects (that is not unique for each object), e.g., The company name of employees, college name of students, etc. Static variable gets memory only once in the class area at the time of class loading. Using a static variable makes your program more memory efficient (it saves memory). Static variable belongs to the class rather than the object.
//Program of static variable class Student8{ int rollno; String name; static String college ="ITS"; Student8(int r,String n){ rollno = r; name = n; } void di
A static method belongs to the class rather than the object. There is no need to create the object to call the static methods. A static method can access and change the value of the static variable.
Two main restrictions are applied to the static methods. The static method can not use non-static data member or call the non-static method directly. this and super cannot be used in static context as they are non-static.
Because the object is not required to call the static method. If we make the main method non-static, JVM will have to create its object first and then call main() method which will lead to the extra memory allocation. More Details.
No, we can't override static methods.
Static block is used to initialize the static data member. It is executed before the main method, at the time of classloading.
class A2{ static{System.out.println("static block is invoked");} public static void main(String args[]){ System.out.println("Hello main"); } }
Ans) Yes, one of the ways to execute the program without the main method is using static block. More Details.
Program compiles. However, at runtime, It throws an error "NoSuchMethodError."
As we know that the static context (method, block, or variable) belongs to the class, not the object. Since Constructors are invoked only when the object is created, there is no sense to make the constructors static. However, if you try to do so, the compiler will show the compiler error.
In Java, if we make the abstract methods static, It will become the part of the class, and we can directly call it which is unnecessary. Calling an undefined method is completely useless therefore it is not allowed.
Yes, we can declare static variables and methods in an abstract method. As we know that there is no requirement to make the object to access the static context, therefore, we can access the static context declared inside the abstract class by using the name of the abstract class. Consider the following example.
abstract class Test { static int i = 102; static void TestMethod() { System.out.println("hi !! I am good !!"); } } public class TestClass extends Test { public s
The this keyword is a reference variable that refers to the current object. There are the various uses of this keyword in Java. It can be used to refer to current class properties such as instance methods, variable, constructors, etc. It can also be passed as an argument into the methods or constructors. It can also be returned from the method as the current class instance.
There are the following uses of this keyword. this can be used to refer to the current class instance variable. this can be used to invoke current class method (implicitly) this() can be used to invoke the current class constructor. this can be passed as an argument in the method call. this can be passed as an argument in the constructor call. this can be used to return the current class instance from the method.
No, this cannot be assigned to any value because it always points to the current class object and this is the final reference in Java. However, if we try to do so, the compiler error will be shown. Consider the following example.
public class Test { public Test() { this = null; System.out.println("Test class constructor called"); } public static void main (String args[]) { Test
Yes, It is possible to use this keyword to refer static members because this is just a reference variable which refers to the current class object. However, as we know that, it is unnecessary to access static variables through objects, therefore, it is not the best practice to use this to refer static members. Consider the following example.
Constructor chaining enables us to call one constructor from another constructor of the class with respect to the current class object. We can use this keyword to perform constructor chaining within the same class. Consider the following example which illustrates how can we use this keyword to achieve constructor chaining.
As we know, that this refers to the current class object, therefore, it must be similar to the current class object. However, there can be two main advantages of passing this into a method instead of the current class object. this is a final variable. Therefore, this cannot be assigned to any new value whereas the current class object might not be final and can be changed. this can be used in the synchronized block.
Inheritance is a mechanism by which one object acquires all the properties and behavior of another object of another class. It is used for Code Reusability and Method Overriding. The idea behind inheritance in Java is that you can create new classes that are built upon existing classes. When you inherit from an existing class, you can reuse methods and fields of the parent class. Moreover, you can add new methods and fields in your current class also. Inheritance represents the IS-A relationship which is also known as a parent-child relationship. There are five types of inheritance in Java. Single-level inheritance Multi-level inheritance Multiple Inheritance Hierarchical Inheritance Hybrid Inheritance Multiple inheritance is not supported in Java through class.
There are various advantages of using inheritance in Java that is given below. Inheritance provides code reusability. The derived class does not need to redefine the method of base class unless it needs to provide the specific implementation of the method. Runtime polymorphism cannot be achieved without using inheritance. We can simulate the inheritance of classes with the real-time objects which makes OOPs more realistic. Inheritance provides data hiding. The base class can hide some data from the derived class by making it private. Method overriding cannot be achieved without inheritance. By method overriding, we can give a specific implementation of some basic method contained by the base class.
The object class is the superclass of all other classes in Java.
To reduce the complexity and simplify the language, multiple inheritance is not supported in java. Consider a scenario where A, B, and C are three classes. The C class inherits A and B classes. If A and B classes have the same method and you call it from child class object, there will be ambiguity to call the method of A or B class. Since the compile-time errors are better than runtime errors, Java renders compile-time error if you inherit 2 classes. So whether you have the same method or different, there will be a compile time error.
class A{ void msg(){System.out.println("Hello");} } class B{ void msg(){System.out.println("Welcome");} } class C extends A,B{//suppose if it were Public Static void main(String args[]){ C o
Aggregation can be defined as the relationship between two classes where the aggregate class contains a reference to the class it owns. Aggregation is best described as a has-a relationship. For example, The aggregate class Employee having various fields such as age, name, and salary also contains an object of Address class having various fields such as Address-Line 1, City, State, and pin-code. In other words, we can say that Employee (class) has an object of Address class. Consider the following example.
Holding the reference of a class within some other class is known as composition. When an object contains the other object, if the contained object cannot exist without the existence of container object, then it is called composition. In other words, we can say that composition is the particular case of aggregation which represents a stronger relationship between two objects. Example: A class contains students. A student cannot exist without a class. There exists composition between class and students.
Aggregation represents the weak relationship whereas composition represents the strong relationship. For example, the bike has an indicator (aggregation), but the bike has an engine (composition).
The pointer is a variable that refers to the memory address. They are not used in Java because they are unsafe(unsecured) and complex to understand.
The super keyword in Java is a reference variable that is used to refer to the immediate parent class object. Whenever you create the instance of the subclass, an instance of the parent class is created implicitly which is referred by super reference variable. The super() is called in the class constructor implicitly by the compiler if there is no super or this.
class Animal{ Animal(){System.out.println("animal is created");} } class Dog extends Animal{ Dog(){ System.out.println("dog is created"); } } class TestSuper4{ public static void main(String args[]){
There are the following uses of super keyword. super can be used to refer to the immediate parent class instance variable. super can be used to invoke the immediate parent class method. super() can be used to invoke immediate parent class constructor.
There are the following differences between this and super keyword. The super keyword always points to the parent class contexts whereas this keyword always points to the current class context. The super keyword is primarily used for initializing the base class variables within the derived class constructor whereas this keyword primarily used to differentiate between local and instance variables when passed in the class constructor. The super and this must be the first statement inside constructor otherwise the compiler will throw an error.
The object cloning is used to create the exact copy of an object. The clone() method of the Object class is used to clone an object. The java.lang.Cloneable interface must be implemented by the class whose object clone we want to create. If we don't implement Cloneable interface, clone() method generates CloneNotSupportedException.
protected Object clone() throws CloneNotSupportedException
Method overloading is the polymorphism technique which allows us to create multiple methods with the same name but different signature. We can achieve method overloading in two ways. Changing the number of arguments Changing the return type Method overloading increases the readability of the program. Method overloading is performed to figure out the program quickly.
In Java, method overloading is not possible by changing the return type of the program due to avoid the ambiguity.
class Adder{ static int add(int a,int b){return a+b;} static double add(int a,int b){return a+b;} } class TestOverloading3{ public static void main(String[] args){ System.out.println(Adder.add(11,11));//ambiguity
No, We cannot overload the methods by just applying the static keyword to them(number of parameters and types are the same). Consider the following example.
public class Animal { void consume(int a) { System.out.println(a+" consumed!!"); } static void consume(int a) { System.out.println("consumed static "+a); }
Yes, we can have any number of main methods in a Java program by using method overloading.
If a subclass provides a specific implementation of a method that is already provided by its parent class, it is known as Method Overriding. It is used for runtime polymorphism and to implement the interface methods. Rules for Method overriding The method must have the same name as in the parent class. The method must have the same signature as in the parent class. Two classes must have an IS-A relationship between them.
No, you can't override the static method because they are the part of the class, not the object.
It is because the static method is the part of the class, and it is bound with class whereas instance method is bound with the object, and static gets memory in class area, and instance gets memory in a heap.
No, we cannot override the private methods because the scope of private methods is limited to the class and we cannot access them outside of the class.
Yes, we can change the scope of the overridden method in the subclass. However, we must notice that we cannot decrease the accessibility of the method. The following point must be taken care of while changing the accessibility of the method. The private can be changed to protected, public, or default. The protected can be changed to public or default. The default can be changed to public. The public will always remain public.
Yes, we can modify the throws clause of the superclass method while overriding it in the subclass. However, there are some rules which are to be followed while overriding in case of exception handling. If the superclass method does not declare an exception, subclass overridden method cannot declare the checked exception, but it can declare the unchecked exception. If the superclass method declares an exception, subclass overridden method can declare same, subclass exception or no exception but cannot declare parent exception.
Yes, all functions in Java are virtual by default.
Now, since java5, it is possible to override any method by changing the return type if the return type of the subclass overriding method is subclass type. It is known as covariant return type. The covariant return type specifies that the return type may vary in the same direction as the subclass.
In Java, the final variable is used to restrict the user from updating it. If we initialize the final variable, we can't change its value. In other words, we can say that the final variable once assigned to a value, can never be changed after that. The final variable which is not assigned to any value can only be assigned through the class constructor.
If we change any method to a final method, we can't override it.
If we make any class final, we can't inherit it into any of the subclasses.
final class Bike{} class Honda1 extends Bike{ void run(){System.out.println("running safely with 100kmph");} public static void main(String args[]){ Honda1 honda= new Honda1();; }
A final variable, not initialized at the time of declaration, is known as the final blank variable. We can't initialize the final blank variable directly. Instead, we have to initialize it by using the class constructor. It is useful in the case when the user has some data which must not be changed by others, for example, PAN Number. Consider the following example:
Yes, if it is not static, we can initialize it in the constructor. If it is static blank final variable, it can be initialized only in the static block
Yes, We can declare the main method as public static final void main(String[] args){}.
The constructor can never be declared as final because it is never inherited. Constructors are not ordinary methods; therefore, there is no sense to declare constructors as final. However, if you try to do so, The compiler will throw an error.
No, we cannot declare an interface as final because the interface must be implemented by some class to provide its definition. Therefore, there is no sense to make an interface final. However, if you try to do so, the compiler will show an error.
The main difference between the final method and abstract method is that the abstract method cannot be final as we need to override them in the subclass to give its definition.
Runtime polymorphism or dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime rather than at compile-time. In this process, an overridden method is called through the reference variable of a superclass. The determination of the method to be called is based on the object being referred to by the reference variable.
No, because method overriding is used to achieve runtime polymorphism and data members cannot be overridden. We can override the member functions but not the data members. Consider the example given below.
In case of the static binding, the type of the object is determined at compile-time whereas, in the dynamic binding, the type of the object is determined at runtime.
The instanceof in Java is also known as type comparison operator because it compares the instance with type. It returns either true or false. If we apply the instanceof operator with any variable that has a null value, it returns false.
Abstraction is a process of hiding the implementation details and showing only functionality to the user. It displays just the essential things to the user and hides the internal information, for example, sending SMS where you type the text and send the message. You don't know the internal processing about the message delivery. Abstraction enables you to focus on what the object does instead of how it does it. Abstraction lets you focus on what the object does instead of how it does it. In Java, there are two ways to achieve the abstraction. Abstract Class Interface
Abstraction hides the implementation details whereas encapsulation wraps code and data into a single unit.
A class that is declared as abstract is known as an abstract class. It needs to be extended and its method implemented. It cannot be instantiated. It can have abstract methods, non-abstract methods, constructors, and static methods. It can also have the final methods which will force the subclass not to change the body of the method.
No, if there is an abstract method in a class, that class must be abstract.
No, because we need to override the abstract method to provide its implementation, whereas we can't override the final method.
No, the abstract class can never be instantiated even if it contains a constructor and all of its methods are implemented.
The interface is a blueprint for a class that has static constants and abstract methods. It can be used to achieve full abstraction and multiple inheritance. It is a mechanism to achieve abstraction. There can be only abstract methods in the Java interface, not method body. It is used to achieve abstraction and multiple inheritance in Java. In other words, you can say that interfaces can have abstract methods and variables. Java Interface also represents the IS-A relationship. It cannot be instantiated just like the abstract class. However, we need to implement it to define its methods. Since Java 8, we can have the default, static, and private methods in an interface.
No, because methods of an interface are abstract by default, and we can not use static and abstract together.
No, because an interface needs to be implemented by the other class and if it is final, it can't be implemented by any class.
A Marker interface can be defined as the interface which has no data member and member functions. For example, Serializable, Cloneable are marker interfaces. The marker interface can be declared as follows.
public interface Serializable{ }
An object reference can be cast to an interface reference when the object implements the referenced interface.
A class can be made read-only by making all of the fields private. The read-only class will have only getter methods which return the private property of the class to the main method. We cannot modify this property because there is no setter method available in the class.
A class can be made write-only by making all of the fields private. The write-only class will have only setter methods which set the value passed from the main method to the private fields. We cannot read the properties of the class because there is no getter method in this class.
There are the following advantages of Encapsulation in Java? By providing only the setter or getter method, you can make the class read-only or write-only. In other words, you can skip the getter or setter methods. It provides you the control over the data. Suppose you want to set the value of id which should be greater than 100 only, you can write the logic inside the setter method. You can write the logic not to store the negative numbers in the setter methods. It is a way to achieve data hiding in Java because other class will not be able to access the data through the private data members. The encapsulate class is easy to test. So, it is better for unit testing. The standard IDE's are providing the facility to generate the getters and setters. So, it is easy and fast to create an encapsulated class in Java.
A package is a group of similar type of classes, interfaces, and sub-packages. It provides access protection and removes naming collision. The packages in Java can be categorized into two forms, inbuilt package, and user-defined package. There are many built-in packages such as Java, lang, awt, javax, swing, net, io, util, sql, etc. Consider the following example to create a package in Java.
By defining packages, we can avoid the name conflicts between the same class names defined in different packages. Packages also enable the developer to organize the similar classes more effectively. For example, one can clearly understand that the classes present in package are used to perform io related operations.
If you are using the programming IDEs like Eclipse, NetBeans, MyEclipse, etc. click on file->new->project and eclipse will ask you to enter the name of the package. It will create the project package containing various directories such as src, etc. If you are using an editor like notepad for java programming, use the following steps to create the package. Define a package package_name. Create the class with the name class_name and save this file with Now compile the file by running the following command on the terminal. javac -d . The above command creates the package with the name package_name in the present working directory. Now, run the class file by using the absolute class file name, like following. java package_name.class_name
There are two ways to access a class in another class. By using the fully qualified name: To access a class in a different package, either we must use the fully qualified name of that class, or we must import the package containing that class. By using the relative path, We can use the path of the class that is related to the package that contains our class. It can be the same or subpackage.
No. It is by default loaded internally by the JVM.
One can import the same package or the same class multiple times. Neither compiler nor JVM complains about it. However, the JVM will internally load the class only once no matter how many times you import the same class.
By static import, we can access the static members of a class directly, and there is no to qualify it with the class name.
There are mainly two types of exceptions: checked and unchecked. Here, an error is considered as the unchecked exception. According to Oracle, there are three types of exceptions: Checked Exception: Checked exceptions are the one which are checked at compile-time. For example, SQLException, ClassNotFoundException, etc. Unchecked Exception: Unchecked exceptions are the one which are handled at runtime because they can not be checked at compile-time. For example, ArithmaticException, NullPointerException, ArrayIndexOutOfBoundsException, etc. Error: Error cause the program to exit since they are not recoverable. For Example, OutOfMemoryError, AssertionError, etc.
Exception Handling is a mechanism that is used to handle runtime errors. It is used primarily to handle checked exceptions. Exception handling maintains the normal flow of the program. There are mainly two types of exceptions: checked and unchecked. Here, the error is considered as the unchecked exception.
The java.lang.Throwable class is the root class of Java Exception hierarchy which is inherited by two subclasses: Exception and Error.
1) Checked Exception The classes that extend Throwable class except RuntimeException and Error are known as checked exceptions, e.g., IOException, SQLException, etc. Checked exceptions are checked at compile-time. 2) Unchecked Exception The classes that extend RuntimeException are known as unchecked exceptions, e.g., ArithmeticException, NullPointerException, etc. Unchecked exceptions are not checked at compile-time.
The Throwable class is the base class for Error and Exception.
It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. So whatever exceptions are likely to be thrown should be declared in the throws clause of the method.
The "finally" block is used to execute the important code of the program. It is executed whether an exception is handled or not. In other words, we can say that finally block is the block which is always executed. Finally block follows try or catch block. If you don't handle the exception, before terminating the program, JVM runs finally block, (if any). The finally block is mainly used to place the cleanup code such as closing a file or closing a connection. Here, we must know that for each try block there can be zero or more catch blocks, but only one finally block. The finally block will not be executed if program exits(either by calling System.exit() or by causing a fatal error that causes the process to abort).
Yes, According to the definition of finally block, it must be followed by a try or catch block, therefore, we can use try block instead of catch.
Finally block will not be executed if program exits(either by calling System.exit() or by causing a fatal error that causes the process to abort)
An exception is first thrown from the top of the stack and if it is not caught, it drops down the call stack to the previous method, If not caught there, the exception again drops down to the previous method, and so on until they are caught or until they reach the very bottom of the call stack. This procedure is called exception propagation. By default, checked exceptions are not propagated.
String pool is the space reserved in the heap memory that can be used to store the strings. The main advantage of using the String pool is whenever we create a string literal; the JVM checks the "string constant pool" first. If the string already exists in the pool, a reference to the pooled instance is returned. If the string doesn't exist in the pool, a new string instance is created and placed in the pool. Therefore, it saves the memory by avoiding the duplicacy.
The simple meaning of immutable is unmodifiable or unchangeable. In Java, String is immutable, i.e., once string object has been created, its value can't be changed.
Because Java uses the concept of the string literal. Suppose there are five reference variables, all refer to one object "sachin". If one reference variable changes the value of the object, it will be affected by all the reference variables. That is why string objects are immutable in java.
1) String Literal Java String literal is created by using double quotes. For Example: String s="welcome"; Each time you create a string literal, the JVM checks the "string constant pool" first. If the string already exists in the pool, a reference to the pooled instance is returned. If the string doesn't exist in the pool, a new string instance is created and placed in the pool. String objects are stored in a special memory area known as the string constant pool For example: 2) By new keyword String s=new String("Welcome");//creates two objects and one reference variable In such case, JVM will create a new string object in normal (non-pool) heap memory, and the literal "Welcome" will be placed in the constant string pool. The variable s will refer to the object in a heap (non-pool).
String s1="Welcome"; String s2="Welcome";//It doesn't create a new instance
String s1="Welcome"; String s2="Welcome"; String s3="Welcome"; Only one object will be created using the above code because strings in Java are immutable.
To make Java more memory efficient (because no new objects are created if it exists already in the string constant pool).
Two objects, one in string constant pool and other in non-pool(heap).
String s = new String("Welcome");
The toString() method returns the string representation of an object. If you print any object, java compiler internally invokes the toString() method on the object. So overriding the toString() method, returns the desired output, it can be the state of an object, etc. depending upon your implementation. By overriding the toString() method of the Object class, we can return the values of the object, so we don't need to write much code.
String stays in the string pool until the garbage is collected. If we store the password into a string, it stays in the memory for a longer period, and anyone having the memory-dump can extract the password as clear text. On the other hand, Using CharArray allows us to set it to blank whenever we are done with the password. It avoids the security threat with the string by enabling us to control the memory.
There are the following classes and interfaces present in java.util.regex package. MatchResult Interface Matcher class Pattern class PatternSyntaxException class
Metacharacters have the special meaning to the regular expression engine. The metacharacters are ^, $, ., *, +, etc. The regular expression engine does not consider them as the regular characters. To enable the regular expression engine treating the metacharacters as ordinary characters, we need to escape the metacharacters with the backslash.
The regular expression for the above criteria will be: ^[a-zA-Z][a-zA-Z0-9]{8,19} where ^ represents the start of the regex, [a-zA-Z] represents that the first character must be an alphabet, [a-zA-Z0-9] represents the alphanumeric character, {8,19} represents that the length of the password must be in between 8 and 20.
There are two types of advantages of Java inner classes. Nested classes represent a special type of relationship that is it can access all the members (data members and methods) of the outer class including private. Nested classes are used to develop a more readable and maintainable code because it logically groups classes and interfaces in one place only. Code Optimization: It requires less code to write.
The nested class can be defined as the class which is defined inside another class or interface. We use the nested class to logically group classes and interfaces in one place so that it can be more readable and maintainable. A nested class can access all the data members of the outer class including private data members and methods. The syntax of the nested class is defined below.
There are the following main disadvantages of using inner classes. Inner classes increase the total number of classes used by the developer and therefore increases the workload of JVM since it has to perform some routine operations for those extra classes which result in slower performance. IDEs provide less support to the inner classes as compare to the top level classes and therefore it annoys the developers while working with inner classes.
Yes, inner classes are non-static nested classes. In other words, we can say that inner classes are the part of nested classes.
No, the local variable must be constant if you want to access it in the local inner class.
Anonymous inner classes are the classes that are automatically declared and instantiated within an expression. We cannot apply different access modifiers to them. Anonymous class cannot be static, and cannot define any static fields, method, or class. In other words, we can say that it a class without the name and can have only one object that is created by its definition.
An Interface that is declared inside the interface or class is known as the nested interface. It is static by default. The nested interfaces are used to group related interfaces so that they can be easy to maintain. The external interface or class must refer to the nested interface. It can't be accessed directly. The nested interface must be public if it is declared inside the interface but it can have any access modifier if declared within the class.
Yes, an interface can be defined within the class. It is called a nested interface.
Yes, they are static implicitly.
Garbage collection is a process of reclaiming the unused runtime objects. It is performed for memory management. In other words, we can say that It is the process of removing unused objects from the memory to free up space and make this space available for Java Virtual Machine. Due to garbage collection java gives 0 as output to a variable whose value is not set, i.e., the variable has been defined but not initialized. For this purpose, we were using free() function in the C language and delete() in C++. In Java, it is performed automatically. So, java provides better memory management.
The gc() method is used to invoke the garbage collector for cleanup processing. This method is found in System and Runtime classes. This function explicitly makes the Java Virtual Machine free up the space occupied by the unused objects so that it can be utilized or reused. Consider the following example for the better understanding of how the gc() method invoke the garbage collector.
Garbage collection is managed by JVM. It is performed when there is not enough space in the memory and memory is running low. We can externally call the System.gc() for the garbage collection. However, it depends upon the JVM whether to perform it or not.
There are many ways: By nulling the reference By assigning a reference to another By anonymous object etc.
The finalize() method is invoked just before the object is garbage collected. It is used to perform cleanup processing. The Garbage collector of JVM collects only those objects that are created by new keyword. So if you have created an object without new, you can use the finalize method to perform cleanup processing (destroying remaining objects). The cleanup processing is the process to free up all the resources, network which was previously used and no longer needed. It is essential to remember that it is not a reserved keyword, finalize method is present in the object class hence it is available in every class as object class is the superclass of every class in java. Here, we must note that neither finalization nor garbage collection is guaranteed.

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