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Core Java Interview Questions and Answers

Core Java Interview Questions: Java is a general purpose, strongly typed, object-oriented Programming Language. Java is designed by - James Gosling.
Core Java Interview Questions
Q:- What is Java?
  1. Java is a general purpose, high level, object oriented programming language and java is also platform independent.
  2. Java was originally developed by James Gosling at Sun Microsystems (which has since been acquired by Oracle Corporation) and released in 1995.
Q:- How to write very first simple program in Java?
/* This is a very first simple Java program.
   FileName : "FirstProgram.java". */
public class FirstProgram
{
    // Your program begins with a call to main().
    // Prints "Java Interview Questions" to the terminal window.
    public static void main(String args[])
    {
        System.out.println("Core Java Interview Questions");
    }
}

//Output: Core Java Interview Questions
Q:- What is the most important features of Java?
1. Simple

Java is a very popular and simple language. The Syntax of java is just like c++, so it is very easy to learn.

2. Object oriented
  1. Java is a pure Object oriented
  2. Every thing in java is object
  3. All programs and data reside inside objects and classes
3. Distributed
  1. Java facilitates it's users to create distributed applications in Java
  2. RMI and EJB are used for creating distributed applications
4. Robust
Java gives importance to memory management by using the technique called Garbage Collection and Exception handling etc.
5. Secure

Java is secure becasue of

  1. Byte code concept
  2. Java is secured because it doesn't use explicit pointers
  3. Exception handling concept
  4. Garbage collection mechanism
  5. Type-safe reference casting in JVM
6. Platform Independent
Java compiler generates an platform independent code called bytecode.
7. Portable
The Bytecode generated by java can be used on any machine. So it can be portable.
8. Compiled and Interpreted
Generally computer languages are either complied or interpreted. but java combines both compiler and interpreted.
9. High Performance
The use of bytecode makes the performance high. the speed is also high with comparing c, c++.
10. Multi Threading
Multithreading means handling more than one job at a time. Java supports Multithreading.
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Q:- What is a Java Virtual Machine(JAM)?

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.

Q:-What is the difference between a JDK, JVM and JRE?
JDK: Java Development Kit:

JDK is Java Development Kit which is used for development purpose and it includes execution environment also.

JVM: Java Virtual Machine:

JVM is an abstract machine. It is called a virtual machine because it doesn't physically exist. JVM is a specification that provides a runtime environment in which Java bytecode can be executed.

JRE: Java Runtime Environment:

It is used to provide the runtime environment.

Q:-What is pointer? does Java support pointers?

A pointer is a reference handle to a memory location.

No, Java supports pointer internally. It means java has restricted pointer support in java.

Q:-Does Java support multiple inheritance?

When one class extends more than one classes then this is called multiple inheritance.

C++ and few other languages supports multiple inheritance while java doesn't support multiple inheritance. Java doesn't allow multiple inheritance to avoid the ambiguity caused by it.

Q:-What are instance variables?
Instance variables are those which are defined at the class level. Instance variables need not be initialized before using them as they are automatically initialized to their default values.
Q:-How to define a constant variable in Java?

Java does not directly support constants. However, a static final variable is effectively a constant.

The variable should be declared as static final.

So only one copy of the variable exists for all instances of the class and the value can't be changed also.

//Example:
static final int MAX_LENGTH = 50;
Q:-Should a main() method be compulsorily declared in all java classes?
No, main() method should be defined only if the source class is a java application.
Q:-What is the return type of the main() method?
Main() method doesn't return anything hence declared as void.
Q:-Why main() method declared static in Java?
main() method is called by the JVM even before the instantiation of the class hence it is declared as static.
Q:-What is the arguement of main() method?
main() method accepts an array of String object as arguement.
Q:-Can a main() method be overloaded?
Yes. You can have any number of main() methods with different method signature and implementation in the class.
Q:-Can a main() method be declared final?
Yes. Any inheriting class will not be able to have it's own default main() method.
Q:-Does the order of public and static declaration matter in main() method?
No. It doesn't matter but void should always come before main().
Q:-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.
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Q:-Can a class be declared as protected?
The protected access modifier cannot be applied to class and interfaces. Methods, fields can be declaredprotected, however methods and fields in a interface cannot be declared protected.
Q:-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.
Q:-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.
Q:-What is the impact of declaring a method as final?
A method declared as final can't be overridden. A sub-class can't have the same method signature with a different implementation.
Q:-I don't want my class to be inherited by any other class. What should i do?
You should declared your class as final. But you can't define your class as final, if it is an abstractclass. A class declared as final can't be extended by any other class.
Q:-Can you give few examples of final classes defined in Java API?
java.lang.String, java.lang.Math are final classes.
Q:-How is final different from finally and finalize()?
Final:
final is a modifier which can be applied to a class or a method or a variable. final class can't be inherited, final method can't be overridden and final variable can't be changed.
Finally:
finally is an exception handling code section which gets executed whether an exception is raised or not by the try block code segment.
Finalize:
finalize() is a method of Object class which will be executed by the JVM just before garbage collecting object to give a final chance for resource releasing activity.
Q:-Can a class be declared as static?

Yes, We can not declare top-level class as static, but the only inner class can be declared static.

Q:-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.
Q:-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 and cannot use "this" operator to refer to the instance.
Q:-Print "Java Interview Questions" even before main() is executed. How will you achieve 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. And it will be executed only once.

public class PrintMessage {
    static {
        System.out.println("Core Java Interview Questions");
        System.exit(0);
    } 
}

//Note - 
//1. The System.exit(0) exits the program before the JVM starts to look for main()
//2. This will not work with JDK7 - Because, In JDK7 the code would not execute as it looks for the main method before any other thing.

//Output: Core Java Interview Questions
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Q:-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 changes gets reflected in all the other objects.

Q:-Can we declare a static variable inside a method?

Since, Static variables are class level variables so they can't be declared inside a method, If you declared, then the class will not compile.

Q:-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.

Q:-Can a abstract class be declared final?

No, Not possible. An abstract class without being inherited is of no use and hence will result in compile time error.

Q:-What is use of a abstract variable?

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

Q:-Can you create an object of an abstract class?
Not possible. Abstract classes can't be instantiated.
Q:-Can a abstract class be defined without any abstract methods?
Yes, it's possible. This is basically to avoid instance creation of the class.
Q:-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.
Q:-Can an Interface implement another Interface?
Interfaces doesn't provide implementation hence a interface cannot implement another interface.
Q:-Can an Interface extend another Interface?
Yes, an Interface can inherit another Interface, for that matter, an Interface can extend more than one Interface.
Q:-Can a Class extend more than one Class?
Not possible. A Class can extend only one class but can implement any number of Interfaces.
Q:-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 inheritances, 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.

Q:-Can an Interface be final?
Not possible. Doing so so will result in compilation error.
Q:-Can a class be defined inside an Interface?
Yes it's possible.
Q:-Can an Interface be defined inside a class?
Yes it's possible.
Q:-What is a Marker Interface?
An Interface which doesn't have any declaration inside but still enforces a mechanism.
Q:-Which object oriented Concept is achieved by using overloading and overriding?
Polymorphism.
Q:-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.
Q:-Can we define private and protected modifiers for variables in interfaces?
No.
Q:-What is Externalizable?
Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format.

It has two methods

// to read object from stream
void readExternal(ObjectInput in) 

// to write object into stream
void writeExternal(ObjectOutput out) 
Q:-What modifiers are allowed for methods in an Interface?
Only public and abstract modifiers are allowed for methods in interfaces.
Q:-What is a local, member and a class variable?
Local Variable:
Variables declared within a method are "local" variables.
Member Variable:
Variables declared within the class i.e not within any methods are "member" variables (global variables).
Class Variable:
Variables declared within the class i.e not within any methods and are defined as "static" are class variables.
Q:-What is an abstract method?
An abstract method is a method whose implementation is deferred to a subclass.
Q:-What value does read() return when it has reached the end of a file?
The read() method returns -1 when it has reached the end of a file.
Q:-Can a Byte object be cast to a double value?
No, an object cannot be cast to a primitive value.
Q:-What is the difference between a static and a non-static inner class?
A non-static inner class may have object instances that are associated with instances of the class's outer class. A static inner class does not have any object instances.
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Q:-What is an object's lock and which object's have locks?
An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object's lock. All objects and classes have locks. A class's lock is acquired on the class's Class object.
Q:-What is the % operator?
It is referred to as the modulo or remainder operator. It returns the remainder of dividing the first operand by the second operand.
Q:-When can an object reference is cast to an interface reference?
An object reference be cast to an interface reference when the object implements the referenced interface.
Q:-Which class is extended by all other classes?
The Object class is extended by all other classes.
Q:-Which non-Unicode letter characters may be used as the first character of an identifier?
The non-Unicode letter characters $ and _ may appear as the first character of an identifier
Q:-What restrictions are placed on method overloading?
Two methods may not have the same name and argument list but different return types.
Q:-What is the return type of a program's main() method?
void.
Q:-If a variable is declared as private, where may the variable be accessed?
A private variable may only be accessed within the class in which it is declared.
Q:-What is casting?
There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.
Q:-What is Downcasting ?
Downcasting is the casting from a general to a more specific type, i.e. casting down the hierarchy
Q:-What modifiers may be used with an inner class that is a member of an outer class?
A (non-local) inner class may be declared as public, protected, private, static, final, or abstract.
Q:-How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters?
Unicode:
Unicode requires 16 bits and ASCII require 7 bits Although the ASCII character set uses only 7 bits, it is usually represented as 8 bits.
UTF-8:
UTF-8 represents characters using 8, 16, and 18-bit patterns.
UTF-16:
UTF-16 uses 16-bit and larger bit patterns.
Q:-What is a native method?
A native method is a method that is implemented in a language other than Java.
Q:-What are order of precedence and associativity, and how are they used?
Order of precedence determines the order in which operators are evaluated in expressions. Associativity determines whether an expression is evaluated left-to-right or right-to-left.
Q:-Can an anonymous class be declared as implementing an interface and extending a class?
An anonymous class may implement an interface or extend a superclass, but may not be declared to do both.
Q:-What is the range of the char type?
The range of the char type is 0 to 216 - 1 (i.e. 0 to 65535.)
Q:-What is the range of the short type?
The range of the short type is -(215) to 215 - 1. (i.e. -32,768 to 32,767)
Q:-Why isn't there operator overloading?
Because C++ has proven by example that operator overloading makes code almost impossible to maintain.
Q:-What does it mean that a method or field is "static"?

Static variables and methods are instantiated only once per class.

In other words, they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class.

Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That's how library methods like System.out.println() work. out is a static field in the java.lang.System class.

Q:-Is null a keyword?

The null value is not a keyword.

Q:-Does a class inherit the constructors of its superclass?

A class does not inherit constructors from any of its superclasses.

Q:-Name the eight primitive data types in Java?

The eight primitive types are byte, char, short, int, long, float, double, and Boolean.

Q:-When does the compiler supply a default constructor for a class?
The compiler supplies a default constructor for a class if no other constructors are provided.
Q:-If a method is declared as protected, where may the method be accessed?

A protected method may only be accessed by classes or interfaces of the same package or by subclasses of the class in which it is declared.

Q:-What is the difference between inner class and nested class?

When a class is defined within a scope of another class, then it becomes an inner class. If the access modifier of the inner class is static, then it becomes a nested class.

Q:-What modifiers can be used with a local inner class?
A local inner class may be final or abstract.
Q:-What is numeric promotion?

Numeric promotion is the conversion of a smaller numeric type to a larger numeric type, so that integer and floating-point operations may take place.

#Example:

In numerical promotion, byte, char, and short values are converted to int values. The int values are also converted to long values, if necessary. The long and float values are converted to double values, as required.

Q:-What is the difference between a public and a non-public class?

A public class may be accessed outside of its package. A non-public class may not be accessed outside of its package.

Q:-What restrictions are placed on method overriding?
  1. Overridden methods must have the same name, argument list, and return type.
  2. The overriding method may not limit the access of the method it overrides.
  3. The overriding method may not throw any exceptions that may not be thrown by the overridden method.
Q:-Can a class declared as private be accessed outside it's package?
Not possible.
Q:-What is a Java package and how is it used?

A Java package is a naming context for classes and interfaces. A package is used to create a separate namespace for groups of classes and interfaces. Packages are also used to organize related classes and interfaces into a single API unit and to control access to these classes and interfaces.

OR

In simple words, A java package is a group of similar types of classes, interfaces and sub packages.

Type of Packages in Java:
  1. Built-in package: javax, lang, awt, swing, io, util, sql etc
  2. User-defined package
Q:-Which package is imported by default?
java.lang package is imported by default even without a package declaration.
Q:-What is the base class of all classes?
java.lang.Object

java.lang.Object class is the super base class of all Java classes. Every other Java classes descends from Object.

Q:-What modifiers may be used with top-level class in Java?
A top-level class may be:
  1. public
  2. abstract
  3. final
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