100+ Core Java Interview Question And Answers

Ans :Object-Oriented Programming System (OOps) is model in programming languages based on concepts of objects and classes.

OOps concepts are:

• Abstraction
• Inheritance
• Polymorphism
• Encapsulation

Ans: Abstraction refers to the act of representing essential features without including the background details(complexities).

In simple terms hiding complexities and displaying necessary details only.

In real world take an example of online banking transaction where end user are shown only User interface of Payments gateways and complexities that is how actual transaction and operations is been performed are kept hidden from end user. This what abstraction is.

/*
code for Abstraction */
abstract class OnlineBank{
abstract int displaybalance();
}
class SavingAccount extends OnlineBank{
int displaybalance(){
return 50000;
}
class CurrentAccount extends Onlinebank{
int displaybalance(){
return 150000;
}
class main{
OnlineBank b =new SavingAccount();
System.out.println("Balance in Saving Account="+b.displaybalance()+"Rs");
OnlineBank b =new CurrentgAccount();
System.out.println("Balance in Current Account="+b.displaybalance()+"Rs");
}
}

Output:
Balance in Saving Account= 50000 Rs
Balance in Current Account= 150000 Rs

Ans:No, if there is a abstract method they must be declare in abstract class.

Ans: Inheritance is the process by which one class acquires properties of other class. or mechanism of deriving new class from old class is called as Inheritance.

Types of Inheritance-

• Single
• Multilevel
• Hierarchical
• Hybrid
• Multiple

No, Java not supports Multiple Inheritance. Because it leads to the diamond problem.

Since multiple inheritance is not supported in java classes, but with the help of interface we can achieve multiple inheritance.

/* Code To Achieve Multiple Inheritance */
interface A{
void showa();
}
interface B{
void showb();
}
class C implements A,B{
public void showa()
{
System.out.println("I Am In A");
}
public void showb()
{
System.out.println("I Am In B");
}
}
class multiple{
Public static void main(String args[]){
C c= new C();
c.showa();
c.showb();
}
}

Output:
I Am In A
I Am In B

Ans: Polymorphism in java is a concept by which we can perform single action by different means/ways. Polymorphism is the Greek word in which poly means many and morph means form hence many forms.

There Are two Types of Polymorphism:-

1. Compile time- Achieved by Method Overloading.
2. Run Time- Achieved by Method Overriding.

/*
Java Program to implement Method Overloading */
 class Area{
int area(int side){
return(side*side);
}
double area(double circle){
return(3.14*radius*radius);
}
int area(int len,int wid){
return(len*wid);
}
class Shape{
public static void main(String args[]){
Area s=new Area();
System.out.println("Area of Square:"+s.area(5));
System.out.println("Area of circle:"+s.area(10.2));
System.out.println("Area of Rectangle:"+s.area(10,2));
}
}

Output:
Area of Square: 25
Area of circle: 326.685
Area of Square: 20

/*Code to implement Method Overiding */
class A{
public void show(){
System.out.println("I am in A");
}
}
class B extends A
{
public void show()
{
super.show();
System.out.println("I am in B");
}
}
class Inherits{
public static void main(String args[]){
B b1= new B();
b1.show();
}
}

Output:
I am in A
I am in B

i. When a reference variable of parent class refers to the object of child class,it is known as Upcasting.Converting a subclass to super class type.

reference variable of parent class—–>object of child class

ii.Converting Super class type to sub class is know as downcasting.

Ans: Static Variables are the variables, we assign memory only once at the time of class loading. Using a Static variable it makes your program efficient as it saves memory.

/* 
Java Static Variable */
class StaticExample{
int EmployeeNo;
String EmployeeName;
static string department="Testing";
StaticExample(int eno,String n)
EmployeeNo=eno;
EmployeeName=n;
}
void display(){
System.out.println(EmployeeNo+""+EmployeeName+""+department);
}
public static void main(String args[]){
StaticExample S1= new StaticExample(1000,Manthan);
StaticExample S2= new StaticExample(1001,Nived);
S1.display();
S2.display();
}

Output:
1000 Manthan Testing
1001 Nived   Testing

Ans: Static methods can be called without using objects.Static methods can only call other static method and static variables defined in the class.

Syntax: static return_type method_name(parameter_list)

/* 
Java Program To demonstrate the use of Static method */

class example
{
static void show(){
System.out.println("I am in Static Method");
}
public static void main(String args[]){
System.out.println(I am in main);
show();
}
}

Output:
I am in main
I am in Static Meethod

Ans: We know that Constructors invokes as soon as we create objects. And Static (method, block, or variable) belongs to class not the object. Hence that is why there no sense in making Constructor as static. If you try to make Constructor Static Compiler will throw an error.

Ans: Yes,We can execute Program without main method also.

Ans: If we make the Abstract method as static in java then they became part of the class. Therefore we can call them from anywhere, then what is the use of abstraction, Hence this is not allowed in java.

Ans:Many times it is necessary to refers to its own object in a method or constructor.To allow this Java defines ‘this’ keyword.

this is used inside the method or constructor to refers to its own object.

/* Java Program To Demonstrate The use of This Keyword */
class Data{
int a ,b;
void setdata(){
a=10;
b=20;
void setdata(int a,int b){
this.a=a;
yhis.b=b;
}
void showdata(){
System.out.println("A="+a);
System.out.println("B="+b);
}
}
class Mydata{
public static void main(String arga[]){
Data d1= new Data();
d1.setdata(100,200);
d1.showdata();
}
}

Output:
A=100
B=200

Ans: Constructor Chaining is the process of calling one constructor from another constructor of the class with the help of the current class object.

This keyword is used to perform constructor chaining within same class.

/* Java Program for Constructor Chaining */
class student{
int rollno,age;
String name;
public student(int age){
this.age=age;
}
public student(int rollno,int age){
this(age);
this.rollno=rollno;
}
public student(int rollno,String name){
this(rollno,age);
this.name=name;
}
public static void main(String args[]){
student std=new student(100,17,"Shyam");
System.out.println("RollNo"+std.rollno+"Age:+age +"Name:"+name);
}
}

Output:
Rollno:100 Age:17 Name:Shyam

Ans: The Keyword “Super” is a special keyword in java use for inheritance. whenever the subclass needs to refer to its immediate superclass, the “super” keyword is used.

There are two main uses of Super keyword:

1. Accessing members from super class that has been hidden by members of the subclass.
2. Calling super class constructor.

/* Java Program to Implement Super Keyword */
Class Shape{
String colour="Black";
Shape(){System.out.println("Shape is Created");}
}
class Square extends Shape{
String colour="Red";
void colourprint(){
System.out.println(colour);
System.out.println(super.colour);
}
class test{
public static void main(String args[]){
Square s= new Square();
s.colourprint();
}

}

Black
Red

Ans:It is a process of creating exact copy of an object.

Ans: Following Are the Uses of Final Keyword

1. One use of the final keyword is to make constant variables,i.e we cannot change the value of the variable.
2. It is used for preventing method overriding,i.e is method is declared as the final method cannot be overridden.
3. Used to prevent Inheritance,i.e we cannot extends the parent class

/* 
Java Program To demonstrate Use of Final Keyword */
class A{
final void show(){System.out.printlb("I am in A");}
}
class B extends A{
void show(){System.out.printlb("I am in B");}
}
class Example(){
public static void main(String args[])
B b1 = new B();
b1.show();
}

show() in B cannot override show() in A; overridden method is final
void show()
1 error

Ans: String is a class in java, In Java.lang package. It is not a primitive datatype like: int, float, and double.

String is an important class in java which is used to perform various operations like:

Split, trim, String to uppercase and lowercase and vice versa, concatenation and various other complex operation converting other types to string type, etc.

Ans: There are two ways of creating String in Java:

1. Creating String using new Keyword.
   • String str= new String(”Noteshacker’);
2. Simple declaration using double quotes
   • String str=”NotesHacker”;

When we create a String using double quotes, JVM looks in the String pool to find if there is any other String with the same value/name. If same string found it refers to that String object else create a new object in the String pool.

When we use the new operator, Java Virtual Machine will create the new String Object, but initially it is not stored in string pool.

Ans: First of all Immutable means: Once something is created it cannot be changed.

Yes, String is Immutable that is once we create its object, its value cannot be changed.

String s1="NotesHacker";
String s2=new String("NotesHacker");

In the above example Two object will be created,Object created using string literal s1 is stored in string constant pool.

Object created using new operator is stored in the heap section s2.

Ans: It is one of the favorite Question of Interview and mostly asked:

Click here to view Solution

Ans: All the three classes are final,this type of question are asked to confuse.

Ans: String Interning is a process of creating only one unique copy of distinct string value that must be immutable.

For example: if a name lets say-Noteshacker appears 10 times by interning you must ensure that only one Noteshacker is allocated with memory.

public class Intern{
public static void main(String args[]){
String s1=new String("Noteshacker");
String s2="Noteshacker";
String s3=s1.intern();

System.out.println(s2==s3);// return true as reference variable are pointing to sameinstance
}
}

Output: true

Ans: Java.lang.Interger.toString method is used to return string objects representing this integer value.

public class test{
public static void main(String args[]){
int s=5;
System.out.println(s.toString());
}
}

Output: 5

Please practice these following programs because these kinds of question are asked in interview for sure 100% assurity.

Ans:An Array is a Structure that is collection of similar types of elements.An Array is a homogeneous data type where it can hold only elements of one data type.

Ans: no we cannot change the size of array at runtime.

Ans: No we cannot declare array without assigning it size.if we try to do so it will throw compile time error

When new Array is initialized the default value is as follows:

• For byte,short,int,long-default value is 0.
• Float,double-default value is 0.0.
• Boolean-default value is false.
• Object -default value is null.

Ans: Jagged Array are multidimensional Array,i.e Array having different length.

Ans: In Java,Memory for arrays is always allocated on heap as array in java are considered as object.

Here is the list of most important Java Array Programs.

Ans: An Exception is a problem that arises during the execution of a program.An Exception is a condition that is cause by run -time error in program.

Ans: There are basically two types of exceptions:

1. Checked Exceptions
2. UnChecked Exceptions.

Checked Exceptions: Exceptions that are handled at compile-time are called as checked Exceptions.example-ClassNotFoundException

UnChecked Exceptions: Exceptions that are handled at runtime are called as unchecked exceptions.example-NullPointerException

Ans:The Process of handling run-time errors thrown by an error causing condition and then try to catch error conditions and display appropriate message for taking corrective action,This Task is called as Exception Handling.

Ans: Try :The Program statement that you want to monitor for exceptions are contained within try block.if an exception occurs within try block,it is thrown.

The try is the keyword used for the block of code that causes an error condition and throws exceptions.

Ans: Your Code can catch the exceptions using catch and then throw it in some rational manner.

The catch keyword is used for the block of code that catches the exception thrown by the try block and handles it.

Catch Block is added immediately after try block.

Ans: Any code that must be executed before method return are written in finally block.

A finally block is always executed.finally block can be used to handle any exception generated at try.

Ans: A Thread is the smallest unit of executable code or single task is also called as thread.

Each thread has its own local variables, program counter and lifetime.

Ans:A thread can be in various stages of execution:

1. A thread can be running.
2. Thread can be ready to run.
3. Running thread can be suspended.
4. It can be blocked.
5. Suspended thread can resume.
6. At any time thread can be terminated.

Ans: In Java, each thread is assigned a priority and according to their priority its is schedule for running. The same priority threads are given equal treatment by java scheduler and therefore they share processor on first come first serve basis (fifs).

Ans: Three are the two ways of creating threads in java.

1. Implement the runnable interface(java.lang.runnable)
   • define a class that implements the runnable interface.The Runnable interface has only one method,run() that is to be defined in the method with code to be executed by a thread.
2. By extending Thread class(java.lang.Thread)
   • User-specified Thread class is created by extending the class Thread and overriding its run(), method.