Java Interview Questions

1. What is meant by Object Oriented Programming?

OOP is a method of programming in which programs are organised as cooperative collections of objects. Each object is an instance of a class and each class belong to a hierarchy.

2.What is a Class?

Class is a template for a set of objects that share a common structure and a common behaviour.

3.What is an Object?

Object is an instance of a class. It has state,behaviour and identity. It is also called as an instance of a class.

4.What is an Instance?

An instance has state, behaviour and identity. The structure and behaviour of similar classes are defined in their common class. An instance is also called as an object.

5. What are the core OOP's concepts?

Abstraction, Encapsulation,Inheritance and Polymorphism are the core OOP's concepts.

6.What is meant by abstraction?

Abstraction defines the essential characteristics of an object that distinguish it from all other kinds of objects. Abstraction provides crisply-defined conceptual boundaries relative to the perspective of the viewer. Its the process of focussing on the essential characteristics of an object. Abstraction is one of the fundamental elements of the object model.

7.What is meant by Encapsulation?

Encapsulation is the process of compartmentalising the elements of an abtraction that defines the structure and behaviour. Encapsulation helps to separate the contractual interface of an abstraction and implementation.

8.What is meant by Inheritance?

Inheritance is a relationship among classes, wherein one class shares the structure or behaviour defined in another class. This is called Single Inheritance. If a class shares the structure or behaviour from multiple classes, then it is called Multiple Inheritance. Inheritance defines "is-a" hierarchy among classes in which one subclass inherits from one or more generalised superclasses.

9.What is meant by Polymorphism?

Polymorphism literally means taking more than one form. Polymorphism is a characteristic of being able to assign a different behavior or value in a subclass, to something that was declared in a parent class. 

10.What is an Abstract Class?

Abstract class is a class that has no instances. An abstract class is written with the expectation that its concrete subclasses will add to its structure and behaviour, typically by implementing its abstract operations.

11.What is an Interface?

Interface is an outside view of a class or object which emphaizes its abstraction while hiding its structure and secrets of its behaviour.

12.What is a base class?

Base class is the most generalised class in a class structure. Most applications have such root classes. In Java, Object is the base class for all classes.

13.What is a subclass?

Subclass is a class that inherits from one or more classes.

14.What is a superclass?

Superclass is a class from which another class inherits.

15.What is a constructor?

Constructor is an operation that creates an object and/or initialises its state.

16.What is a destructor?

Destructor is an operation that frees the state of an object and/or destroys the object itself. In Java, there is no concept of destructors. Its taken care by the JVM.

17.What is meant by Binding?

Binding denotes association of a name with a class

18.What is meant by static binding?

Static binding is a binding in which the class association is made during compile time. This is also called as Early binding.

19.What is meant by Dynamic binding?

Dynamic binding is a binding in which the class association is not made until the object is created at execution time. It is also called as Late binding.

20.Define Modularity?

Modularity is the property of a system that has been decomposed into a set of cohesive and loosely coupled modules.

21.What is meant by Persistence?

Persistence is the property of an object by which its existence transcends space and time.

22.What is colloboration?

Colloboration is a process whereby several objects cooperate to provide some higher level behaviour.

23.In Java, How to make an object completely encapsulated?

All the instance variables should be declared as private and public getter and setter methods should be provided for accessing the instance variables.

24.How is polymorphism acheived in java?

Inheritance, Overloading and Overriding are used to acheive Polymorphism in java.

 25.What are the most important features of Java?

Java is object oriented, platform independent,secure,robust,simple,etc

26.Which one of them do you consider the best feature of Java?

Platform independence.

27.What do you mean by platform independence?

Platform independence means that we can write and compile the java code in one platform (eg Windows) and can execute the class in any other supported platform eg (Linux,Solaris,etc).

28.What is byte code?

Byte code is a set of instructions generated by the compiler. JVM executes the byte code. 

29.How does Java acheive platform independence?

A Java source file on compilation produces an intermediary .class rather than a executable file. This .class file is interpreted by the JVM. Since JVM acts as an intermediary layer.

30.What is a JVM?

JVM is Java Virtual Machine which is a run time environment for the compiled java class files.

31.Are JVM's platform independent?

JVM's are not platform independent. JVM's are platform specific run time implementation provided by the vendor. A Windows JVM cannot be installed in Linux.

32.Who provides the JVM?

Any software vendor can provide a JVM but it should comply to the Java langauge specification.

33.What is the difference between a JDK and a JVM?

JDK is Java Development Kit which is for development purpose and it includes execution environment also. But JVM is purely a run time environment and hence you will not be able to compile your source files using a JVM.

34.What is a pointer and does Java support pointers?

Pointer is a reference handle to a memory location. Improper handling of pointers leads to memory leaks and reliability issues hence Java doesn't support the usage of pointers. 

35.How is an object reference different from a pointer?

Both object reference and pointer point to the memory location of the object. You can manipulate pointers but not object references.

36.Does Java support multiple inheritance?

Java doesn't support multiple inheritance.

37.Why Java doesn't support multiple inheritance?

When a class inherits from more than class, it will lead to the diamond problem - say A is the super class of B and C & D is a subclass of both B and C. D inherits properties of A from two different inheritance paths ie via both B & C. This leads to ambiguity and related problems, so multiple inheritance is not allowed in Java.

38.Is Java a pure object oriented language?

Java is a pure object oriented language. Except for the primitives everything else are objects in Java.

39.What is the difference between Path and Classpath?

Path and Classpath are operating system level environment variales. Path is used define where the system can find the executables(.exe) files and classpath is used to specify the location .class files.

40.Why does Java not support operator overloading?

Operator overloading makes the code very difficult to read and maintain. To maintain code simplicity, Java doesn't support operator overloading.

41.What are keywords?

Keywords cannot be used as identifiers.

42.What are reserved keywords? And can you name few reserved keywords?

Few of the words are reserved as keywords for future usage. Compiler forces the developer not to use a reserved keyword.

43.What is the overhead of introducing a new keyword?

When a new keyword is used in a new version of the JDK, there is high chances that has been used by developers as identifiers. This make tougher for the code base to migrate to new version since it requires code change, recompilation,testing and release cycle.

44.Have you come across difficulties due to introduction of a new keyword?

Yes. enum keyword was used extensively as identifier in one of our project - we had to change the code in a lot of places to migrate it to newer v ersion.

45.What are identifiers?

Identifiers are names given to a variable, method, class and interface. Identifiers must conform to the following rules:a. The identifiers can contain a to z, A to Z,0 to 9,_ and $.b. Special characters other than _ and $ cannot be used in identifiers.c. Identifiers cannot start with numbers. d. keywords cannot be used as identifiers.

46.What is meant by naming conventions? And what are the naming conventions followed in Java?

Naming conventions are part of coding standards which are prescribed for better readability and maintenance. The following are simple conventions followed in Java:1. Instance and local Variables should start with a lowercase and subsequent word should start with a capital letter. Examples: int quantity; double unitPrice; 2. Class level variables ie constants should be in capital letters and _ is used word seprator final static double PI = 3.14; final static int MAX_PRIORITY = 10;3. Method names should start with small case and subsequent word should be capital letter. public double getAvailableBalance(String accountNo) throws InvalidAccountException{}4. Classes and Interfaces should start with a capital letter and subsequent words should also be capital letters. public class HelloWorld{}

47.If you dont follow coding standards, will it result in compilation error?

No. These are standards. Each company or fot that matter each software unit might have its own coding standards. These are not enforced by the compiler.

48.How to make sure that all programmers are following the coding standards?

The best and simple way is to do peer reviews and code walkthroughs. You can use external plugins to your IDE (integrated development environment) to enforce during coding itself.

49.What are literals?

Literals are source code representation of primitive data types.

50.Name the eight data types which are available in Java?

boolean, byte, short, int, long, float, double and char.

51.Are primitive data types objects in Java?

Primitive data types are not objects.

52.What are all the number data types? And are they signed or unsigned?

Excpet boolean and char, others are number data types and they are all signed which means that they can hold both positive and negative values.

53.What are the possible values that a boolean data type can hold?

The boolean is the simplest data type which can hold true or false.

54.What are default values?

Values which are defaulted during object initialisation are called default values. Each data type has a default value.

55.What are the default values of primitive data types?

For boolean data type, it is false. For byte,short,int and long, it is 0. For float and double, it is 0.0. For char, the default value is 'u000'.

56.Are object references defaulted?

Yes. Object references are defaulted to null.

57.Are arrays defaulted?

If arrays is just declared but not initialised then the array reference will be defaulted to null. This is because arrays are objects in Java.int arr[]; // Here the arr reference is defaulted to null.If array values are not assigned, then will be defaulted to their respective default values.double priceRange[] = new double[3]; // Here all the elements in the array will be defaulted to 0.0 - the default value of double.String str[] = new String[3]; // Here all the elements will be defaulted to null - the default value for object references.

58.Can you explain keyword , identifier and literal with an example?

Consider the below statement:int i = 10;Here int is the keyword which has special meaning attached Java programming langauge ie whatever is declared is an integer value.i is the identifier or the variable name.10 is the literal or the actual value.

59.What are the 3 ways to represent an integer value?

The following are the 3 different ways to represent an integer value:int i = 123 // this is the usual decimal representation.int i = 0123 // this is octal representation. Octal values start with a zero.int i = 0XCAAD // this is hexadecimal representation. Hexadecimal values start with 0X.

60.In how many ways a char value be represented?

Char value can be represented in 3 ways. They are as follows:char ch = 'A'; // represented using single quotes.char ch = 'u0041'; // represented using unicode representation.char ch = 41; // represented using integer value.

61.How is it possible to represent char using a integer value?

char is internally represented as a unsigned 16 bit integer value ie it will accept integer values from 0 to 65536.

62.Can char be used when an integer value is expected?

Yes. A fine example is switch statement will accept char value for multiway condition checking.

63.Can char be manipulated like integers?

Yes possible. The below is an example.char ch = 'A';System.out.println(ch++); The above statement will print B. ++ is a numeral operand and since char is internally represented as integer, ++ operand can be applied on char value.

64.What is Unicode?

A universal, 16-bit, standard coded character set for the representation of all human scripts.

65.What should i have to do if i have to print my name in Spanish?

Provide code...

66.How will the below literal value be internally represented?
float f = 21.22;

It will be represented as a double value. Floating point literals are always double by default. If you want a float, you must append an F or f to the literal.

67.Give your observation on the below statement.
int i = 10/0;

The statement will result in RuntimeException (DivideByZeroException). Integer values cannot be divided by zero.

68.Give your observation on the below statement.
double d = 10.12/0;

This will compile and execute fine. The result will be Infinity

69.What is a Variable?

a variable is a facility for storing data. The current value of the variable is the data actually stored in the variable.

70.What are the 3 types of variables?

There are 3 types of variables in Java. They are :1. Local variables.2. Instance variables.3. Class variables.

71.What are Local variables?

Local varaiables are those which are declared within a block of code like methods, loops, exception blocks, etc. Local variables should be initialised before accessing them. Local variables are stored on the stack, hence they are sometimes called stack variables. They are also called as method variables or block variables.

72.When are local variables eligible for garbage collection?

As soon as the block is completed the variables are eligible for GC. Block could be a condition, a loop, a exception block or a methodConsider the below class:public class Test { public static void main (String str[]){ String name = str[0]; for (int i=0; i<=10; i++){ System.out.println(name + i); } }}In the above the class, the variable i is eligible for garbage collection immediately after the completion of for loop.name string variable and str[] argument are eligible for GC after the completion of main method.

73.What are Instance variables?

Instance variables are those which are defined at the class level. As we know, object have identity and behaviour - the identity is provided by the instance variables. These are also called as object variables. Instance variables need not be initialized before using them. Instance variables will be initialized to their default values automatically when not initialized.

74.Are arrays primitive data types?

No. Arrays are not primitives - they are objects.

75.What are different ways to declare an array?

Object obj[]; // most frequently used form of array declaration.Object []obj; // nothing wrong with this. Object[] obj; // nothing wrong with this. int i[][]; // most frequently form multi-dimensional arrayint[] i[]; // nothing wrong with this. int[] i[],j; // nothing wrong with this. Important : i is double dimensional whereas j is single dimensional.

76.What are the 3 steps in defining arrays?

declaration,initialization and assignmentint i[]; // array declaration.i[] = new int[2]; // array initialization.i[0] = 10; i[1] = 7; i[2] = 9; // element value assignment. What is the simplest way to defining an primitive array? The below statement merges declaration,initialization and assignment into a single step:int i [] = {10, 20, 30 40}; What is wrong with the below code segment?int i[] = new int[5]System.out.println(i.length()) length is an instance variable of array object - here it is given a method.

77.What is wrong with the below code segment?
int i[5] = new int[]System.out.println(i.length)

length of an array should be given when it is initialized. Here it is given during declaration. It will result in compilation error.

78.What will be the output of the below code segment?
int i[] = new int[5]System.out.println(i.length)

It will print 6. Remember array indexes start with 0.

79.What are the frequent RuntimeException's encountered because of improper coding with respect to arrays?

ArrayIndexOutOfBoundsException and NullPointerException.

80.Should a main method be compulsorily declared in all java classes?

No not required. main method should be defined only if the source class is a java application.

81.What is the return type of the main method?

Main method doesn't return anything hence declared void.

82.Why is the main method declared static?

main method is the entry point for a Java application and main method is called by the JVM even before the instantiation of the class hence it is declared as static. static methods can be called even before the creation of objects.

83.What is the argument of main method?

main method accepts an array of String objects as argument.

84.How to pass an argument to main method?

You should pass the argument as a command line argument. Command line arguments are seprated by a space. The following is an example:java Hello Tom JerryIn the above command line Hello is the class, Tom is the first argument and Jerry is the second argument.

85.What will happen if no argument is passed to the main method?

If you dont access the argument, the main method will execute without any problem. If try to access the argument, NullPointerException will be thrown.

86.Can a main method be overloaded?

Yes. You can have any number of main methods with different method signature and implementation in the class.

87.Can a main method be declared final?

Yes. Any inheriting class will not be able to have it's own default main method.

88.Does the order of public and static declaration matter in main method?

No it doesn't matter but void should always come before main().

89.Can a source file contain more than one Class declaration?

Yes. A single source file can contain any number of Class declarations but only one of the class can be declared as public.

90.If a source file has 2 class declaration in it, on compilation how many class files will be created?

2 class files will be generated by the compiler.

91.Can the first line of the source code be a comment?

Yes. Comments can appear anywhere in the code. It is just a "skip this line" instruction to the conpiler.

92.Can the source file name and the class name in the file be different?

If the class in the source is not of public access, then the name can be anything but should confirm to identifier rules.

93.Explain Inheritance?

Inheritance is a concept where the properties and behaviour of a class is reused in another class. The class which is being reused or inherited is called the super class or the parent class. The class which is inheriting is called the subclass or the child class of the inherited class.

94.Does Java support multiple inheritance?

No. Java Supports only single inheritance.

95.Why Java doesn’t support muliple inheritance?

When a class inherits from more than class, it will lead to inheritance path amibiguity (this is normally calledthe diamond problem). Say A is the super class of B and C & D is a subclass of both B and C. D inherits properties of A from two different inheritance paths ie via both B & C. This leads to ambiguity and related problems, so multiple inheritance is not allowed in Java.

96.Which keyword is used for inheriting from another class?

extends keyword is used for inheritance.

97.Can a subclass be referenced for a super class object?

No. If Vehicle is super class and Car is the subclass then the following reference would be wrong – Car c = new Vehicle();

98.Can a parent class be referenced for a subclass object?

Yes. The below is an example :Vehicle v = new Car();

99.Can an interface be referenced for any object?

Yes. We can create a object reference for an interface. The object should have provided the implementation for the object.Runnable r = new Test(); Test class should have implemented the runnable interface and overridded the run method.

100.What are constructors?

Constructors are used to initialise an object. Constructors are invoked when the new operator on the class are called.

101.What are the decalaration of a constructor?

Constructor name should be the same as class name.Constructors doesn’t return anything – not even void.

102.Does constructors throw exceptions?

Yes. Like methods constructors can also throw exceptions.

103.Can constructors be overloaded?

Yes. Constructors can be overloaded.

104.Is it compulsory to define a constructor for a class?

No. If you don’t define a constructor, the compiler will provide a default constructor.

105.What is a default constructor?

Default constructor is a no argument constructor which initialises the instance variables to their default values. This will be provided by the compiler at the time of compilation. Default constructor will be provided only when you don’t have any constructor defined.

106.Explain about “this” operator?

“this” is used to refer the currently executing object and it’s state. “this” is also used for chaining constructors and methods.

107.Explain about “super” operator?

“super” is used to refer to the parent object of the currently running object. “super” is also to invoke super class methods and classes.

108.What is the sequence of constructor invocation?

When you instantiate a subclass, the object will begin with an invocation of the constructor in the base class and initialize downwards through constructors in each subclass till it reaches the final subclass constructor. Note: This top down imagery for class inheritance, rather than a upward tree-like approach, is standard in OOP but is sometimes confusing to newcomers.

109.Can a constructor be defined for an interface?

No

110.Explain Polymorphism?

The dictionary definition of polymorphism refers to a principle in biology in which an organism or species can have many different forms or stages. This principle can also be applied to object-oriented programming and languages like the Java language. Subclasses of a class can define their own unique behaviors and yet share some of the same functionality of the parent class. Polymorphism is the capability of an action or method to do different things based on the object that it is acting upon. Overloading and overriding are two types of polymorphism.

111.What is Overloading?

In a class, if two methods have the same name and a different signature,it is known as overloading in Object oriented concepts.

112.When to use overloading?

Overloading is a powerful feature, but you should use it only as needed. Use it when you actually do need multiple methods with different parameters, but the methods do the same thing. That is, don't use overloading if the multiple methods perform different tasks. This approach will only confuse other developers who get a peek at your code.

113.Explain Overriding?

Overriding means, to give a specific definition by the derived class for a method implemented in the base class.

114.What is a package?

Package is a collection of related classes and interfaces. package declaration should be first statement in a java class.

115.Which package is imported by default?

java.lang package is imported by default even without a package declaration.

116.Is package statement mandatory in a Java source file?

It's not mandatory.

117.What will happen if there is no package for Java source file?

The classes will packaged into a no name default package. In practice, we always put classes into a meaningful package.

118.What is the impact using a * during importing(for example import java.io.*;?

When a * is used in a import statement, it indicates that the classes used in the current source can be available in the imported package. Using slightly increases the compilation time but has no impact on the execution time.

119.Can a class declared as private be accessed outside it's package?

A class can't be declared as private. It will result in compile time error.

120.Can a class be declared as protected?

A class can't be declared as protected. only methods can be declared as protected.

121.What is the access scope of a protected method?

A protected method can be accessed by the classes within the same package or by the subclasses of the class in any package.

122.What is the impact of marking a constructor as private?

Nobody can instantiate the class from outside that class.

123.What is meant by default access?

default access means the class,method,construtor or the variable can be accessed only within the package.

124.Is default a keyword?

Yes. default is a keyword but is associated with switch statement not with access specifiers.

125.Then how to give default access to a class?

If you dont specify any access, then it means the class is of default access.

126.Can i give two access specifiers to a method at the same time?

No its not possible. It will result in compile time error.

127.What is the purpose of declaring a variable as final?

A final variable's value can't be changed. final variables should be initialized before using them.

128.Can a final variable be declared inside a method?

No. Local variables cannot be declared as final.

129.What is the impact of declaring a method as final?

A method declared as final can't be overridden. A sub-class doesn't have the independence to provide different implementation to the method.

130.I don't want my class to be inherited by any other class. What should i do?

You should declared your class as final. A class declared as final can't be inherited by any other class.

131.When will you declare a class as final?

When a class is independent and completely concrete in nature, then the class has to be marked as final.

132.Can you give few examples of final classes defined in Java API?

java.lang.String,java.lang.Math are final classes.

133.How to define a constant variable in Java?

The variable should be declared as static and final. So only one copy of the variable exists for all instances of the class and the value can't be changed also.static final int PI = 3.14; is an example for constant.

134.Can a class be declared as static?

No a class cannot be defined as static. Only a method,a variable or a block of code can be declared as static.

135.When will you define a method as static?

When a method needs to be accessed even before the creation of the object of the class then we should declare the method as static.

136.I want to print "Hello" even before main is executed. How will you acheive that?

Print the statement inside a static block of code. Static blocks get executed when the class gets loaded into the memory and even before the creation of an object. Hence it will be executed before the main method.

137.What is the use of a static code block?

static code blocks could be used for one time initialisation activities.

138.Cant you use the constructor for initialisation rather than static block?

Constructors are used for object level initialisation whereas the static block are used for class level initialisation ie to initialise constants.

139.Will the static block be executed for each object?

No. It will be executed only once for each class ie at the time of loading a class.

140.What are the restriction imposed on a static method or a static block of code?

A static method should not refer to instance variables without creating an instance.It cannot use "this" or "super". A static method can acces only static variables or static methods.

141.When overriding a static method, can it be converted to a non-static method?

No. It should be static only.

142.What is the importance of static variable?

static variables are class level variables where all objects of the class refer to the same variable. If one object changes the value then the change gets reflected in all the objects.

143.Can we declare a static variable inside a method?

Static varaibles are class level variables and they can't be declared inside a method. If declared, the class will not compile.

144.What is an Abstract Class and what is it's purpose?

A Class which doesn't provide complete implementation is defined as an abstract class. Abstract classes enforce abstraction.

145.What is the use of a abstract variable?

Variables can't be declared as abstract. only classes and methods can be declared as abstract.

146.Can a abstract class be declared final?

Not possible. An abstract class without being inherited is of no use and a final class cannot be inherited. So if a abstract class is declared as final, it will result in compile time error.

147.What is an abstract method?

An abstract method is a method whose implementation is deferred to a subclass.

148.Can a abstract class be defined without any abstract methods?

Yes it's possible. This is basically to avoid instance creation of the class.

149.What happens if a subclass has inherited a abstract class but has not provided implementation for all the abstract methods of the super class?

Then the subclass also becomes abstract. This will be enforced by the compiler.

150.What happens if a class has implemented an interface but has not provided implementation for a method in a interface?

Its the same as the earlier answer. The class has to be marked as abstract. This will be enforced by the compiler.

151.Can you create an object of an abstract class?

Not possible. Abstract classes are not concrete and hence can't be instantiated. If you try instantiating, you will get compilation error.

152.Can I create a reference for a an abstract class?

Yes. You can create a reference for an abstract class only when the object being has provided the implementation for the abstract class - it means that the object should be of a concrete subclass of the abstract class. This applies to interfaces also. Below is an example for interface referencing an object:java.sql.Connection con = java.sql.DriverManager.getConnection("");

153.Can a class be marked as native?

No. Only methods can be marked as native.

154.What is the use of native methods?

When a java method accesses native library written in someother programming language then the method has to be marked as native.

155.What is the disadvantage of native methods?

By using native methods, the java program loses platform independence - the accessed platform might be tightly coupled with a operating system hence java program also loses OS independence.

156.What is the purpose of transient modifier?

Only variables can be marked as transient. Variables marked as transient will not be persisted during object persistence.

157.What is the purpose of volatile modifier?

Only variables can be marked as volatile. Volatile variables might be modified asynchronously.

158.What is an Interface?

Interfaces say what a class must do but does not say how a class must do it. Interfaces are 100% abstract.

159.Class C implements Interface I containing method m1 and m2 declarations. Class C has provided implementation for method m2. Can i create an object of Class C?

No not possible. Class C should provide implementation for all the methods in the Interface I. Since Class C didn't provide implementation for m1 method, it has to be declared as abstract. Abstract classes can't be instantiated.

160.Can a method inside a Interface be declared as final?

No not possible. Doing so will result in compilation error. public and abstract are the only applicable modifiers for method declaration in an interface.

161.Can an Interface implement another Interface?

Intefaces doesn't provide implementation hence a interface cannot implement another interface.

162.Can an Interface extend another Interface?

Yes an Interface can inherit another Interface, for that matter an Interface can extend more than one Interface.

163.Can a Class extend more than one Class?

Not possible. A Class can extend only one class but can implement any number of Interfaces.

164.Why is an Interface be able to extend more than one Interface but a Class can't extend more than one Class?

Basically Java doesn't allow multiple inheritance, so a Class is restricted to extend only one Class. But an Interface is a pure abstraction model and doesn't have inheritance hierarchy like classes(do remember that the base class of all classes is Object). So an Interface is allowed to extend more than one Interface.

165.Can an Interface be final?

Not possible. Doing so so will result in compilation error.

166.Can a class be defined inside an Interface?

Yes it's possible.

167.Can an Interface be defined inside a class?

Yes it's possible.

168.What is a Marker Interface?

An Interface which doesn't have any declaration inside but still enforces a mechanism.

169.Can we define private and protected modifiers for variables in interfaces?

No. Always all variables declared inside a interface are of public access.

170.What modifiers are allowed for methods in an Interface?

Only public and abstract modifiers are allowed for methods in interfaces.

171.When can an object reference be cast to an interface reference?

An object reference can be cast to an interface reference when the object implements the referenced interface.

172.What are Inner Classes?

Inner classes are classes which are defined inside another class.

173.What are Nested Classes?

Static Inner classes are called sometimes referred as nested classes because these classes can exist without any relationship with the containing class.

174.What is the super class of all Inner Classes?

Inner class is just a concept and can be applied to any class, hence there is no common super class for inner classes.

175.What are the disadvantages of Inner classes?

1. Inner classes are not reusable hence defeats one of the fundamental feature of Java.2. Highly confusing and difficult syntax which leads poor code maintainability.

176.Name the different types of Inner Classes?

The following are the different types of Inner classes:Regular Inner ClassMethod Local Inner ClassStatic Inner ClassAnonymous Inner Class

177.What are Regular Inner Classes?

A Regular inner class is declared inside the curly braces of another class outside any method or other code block. This is the simplest form of inner classes.

178.Can a regular inner class access a private member of the enclosing class?

Yes. Since inner classes are treated as a member of the outer class they can access private members of the outer class.

179.How will you instantiate a regular inner class from outside the enclosing class?

Outer out=new Outer();Outer.Inner in=out.new Inner();

180.What are Local Inner Classes or Method Local Inner Classes?

A method-local inner class is defined within a method of the enclosing class.

181.What are the constraints on Method Local Inner Classes?

The following are the restrictions for Method Inner Classes:Method Local Inner classes cannot acccess local variables but can access final variables.Only abstract and final modifiers can be applied to Method Local Inner classesMethod Local Inner classes can be instantiated only within the method in which it is contained that too after the class definition.

182.What are Anonymous Inner Classes? Name the various forms of Anonymous Inner Classes.

Anonymous Inner Classes have no name, and their type must be either a subclass of the named type or an implementer of the named interface. The following are the different forms of inner classes:Anonymous subclass(i.e. extends a class)Anonymous implementer (i.e. implements an interface)Argument-Defined Anonymous Inner Classes

183.How many classes can an Anonymous Inner classes inherit from?One.

184.How many Interfaces can an Anonymous Inner classes implement?

One. Normal classes and other inner classes can implement more than one interface whereas anonymous inner classes can either implement a single interface or extend a single class.

185.What are Static Inner Classes?

Static Inner Classes are inner classes which marked with a static modifier. These classes need not have any relationship with the outer class. These can be instantiated even without the existence of the outer class object.

186.Can you instantiate the static Inner Class without the existence of the outer class object?

If Yes, Write a sample statement. Yes. It can be instantiated as follows by referencing the Outer class.Outer.Inner in = new Outer.Inner();

187.What are the constraints on Static Inner Classes?

It cannot access non-static members of the outer class.It cannot use this reference to the outer class.

188.How many class files are produced for source file having one Outer class and one Inner class?

Two class files will be produced as follows:Outer.classOuter$Inner.class