Group Discussion on Java Topic

Posted by: Instanceofjava Posted date: May 03, 2015 / comment : 0

Hi Friends We are planning to conduct a group discussion on a topic every week So interested candidates can join us based on everybody convenience time we will schedule it.

We will conclude the topic at the end.

Our Skype ID : Instanceofjava.

Update: Based on your interest and time we will schedule the session. its free




Java Interview programs on Strings

Posted by: Instanceofjava Posted date: April 25, 2015 / comment : 0


1.Reverse a String Without using String API?

  1. package com.instaceofjava;
  2.  
  3. public class ReverseString {
  4.  
  5. public static void main(String[] args) {
  6.  
  7. String str="Hello world";
  8. String revstring="";
  9.  
  10. for(int i=str.length()-1;i>=0;--i){
  11. revstring +=str.charAt(i);
  12. }
  13.  
  14. System.out.println(revstring);
  15. }
  16. }


Program:

output: dlrow olleH.

 2)Sorting the String without using String API?


  1. package com.Instanceofjava;
  2.  
  3. public class SortString {
  4.  
  5.  public static void main(String[] args) {
  6.  
  7.   String original = "edcba";
  8.  int j=0;
  9.    char temp=0;
  10.  
  11.      char[] chars = original.toCharArray();
  12.      for (int i = 0; i <chars.length; i++) {
  13.          for ( j = 0; j < chars.length; j++) {
  14.          if(chars[j]>chars[i]){
  15.              temp=chars[i];
  16.              chars[i]=chars[j];
  17.               chars[j]=temp;
  18.           }
  19.      }
  20.   }
  21.  
  22.     for(int k=0;k<chars.length;k++){
  23.     System.out.println(chars[k]);
  24.    }
  25.  
  26.  }
  27. }

program:

output:abcde.

3.Sort the String with using String API?

program:
  1. package com.instanceofjava;
  2.  
  3.  public class SortString {
  4.  
  5.  public static void main(String[] args) {
  6.    String original = "edcba";
  7.  
  8.    char[] chars = original.toCharArray();
  9.    Arrays.sort(chars);
  10.  
  11.     String sorted = new String(chars);
  12.      System.out.println(sorted);
  13.  
  14. }
  15. }




OutPut:abcde

4.Check String is palindrome or not?

program:

 Solution #1:

  1. package com.instaceofjava; 
  2.  
  3. public class PalindromeDemo{ 
  4.  
  5. public static void main(String[] args) {
  6.  
  7. String str="MADAM";
  8. String revstring="";
  9.  
  10. for(int i=str.length()-1;i>=0;--i){
  11. revstring +=str.charAt(i);
  12. }
  13.  
  14. System.out.println(revstring);
  15.  
  16. if(revstring.equalsIgnoreCase(str)){
  17. System.out.println("The string is Palindrome");
  18. }
  19. else{
  20. System.out.println("Not Palindrome");
  21. }
  22.  
  23. }
  24.  
  25. }



  Output:

The string is Palindrome

Solution #2:

  1. package com.instaceofjava;
  2.  
  3.   import java.util.Scanner;
  4.  
  5.  public class Palindrome {
  6.  
  7. public static void main(String[] args)
  8. {
  9.  
  10. Scanner in = new Scanner(System.in);
  11.  
  12.  System.out.println("Enter a string");
  13.  String str=in.nextLine();
  14.  
  15. StringBuffer strone=new StringBuffer(str);
  16.  StringBuffer strtwo=new StringBuffer(strone);
  17.  
  18.   strone.reverse();
  19.  
  20. System.out.println("Orginal String ="+strtwo);
  21. System.out.println("After Reverse ="+strone);
  22.  
  23. if(String.valueOf(strone).compareTo(String.valueOf(strtwo))==0)
  24.    System.out.println("Result:Palindrome");
  25.    else
  26.     System.out.println("Result:Not Palindrome");
  27.  
  28.    }
  29.  
  30. }




 Output:

Enter a string
MOOM
Orginal String =MOOM
After Reverse =MOOM

Result:Palindrome

 5.Program to Check given number is palindrome or not?

Program:

  1.  package com.instaceofjava; 
  2.  
  3. import java.util.Scanner; 
  4.  
  5. public class Palindrome {
  6.  
  7. public static void main(String[] args)
  8. {
  9.  
  10. System.out.println("Please Enter a number : ");
  11.       int givennumber = new Scanner(System.in).nextInt();
  12.       int number=givennumber;
  13.        int reverse=0;
  14.         while (number != 0) {
  15.            int remainder = number % 10;
  16.             reverse = reverse * 10 + remainder;
  17.             number = number / 10;
  18.         }           
  19. if(givennumber == reverse)
  20.    System.out.println("Result:Palindrome");
  21.     else
  22.     System.out.println("Result:Not Palindrome");
  23.     }
  24.  
  25. }

 Output:

Please Enter a number :
535
Result:Palindrome.

 Also Read:


Top 16 Java Inheritance Interview questions for freshers and experienced

Posted by: Instanceofjava Posted date: April 25, 2015 / comment : 3


1.what is inheritance?
  • inheritance is one of the oops concepts in java.inheritance is concept of  getting properties of one class object to another class object.
  • Inheritance represents the IS-A relationship,also known as parent-child relationship.
2.what are the types of inheritance?

1.Multiple inheritance( java doesn't support multiple inheritance).
2.Multilevel inheritance.

3.How Inheritance can be implemented in java?
  • Inheritance can be implemented in JAVA using below two keywords:
1.extends
2.implements
  • extends is used for developing inheritance between two classes and two interfaces.
  • implements keyword is used to developed inheritance between interface and class.
4.Why we need to use Inheritance?

1.For Code Re usability.
2.For Method Overriding.

5.what is syntax of inheritance?

public class subclass extends superclass{

//all methods and variables declare here
}

6.what is multilevel inheritance?
  • Getting the properties from one class object to another class object level wise with different priorities.


6.what is Multiple inheritance?why Java Doesn't Support multiple Inheritance.
  • The concept of Getting the properties from multiple class objects to sub class object with same priorities is known as multiple inheritance.
  • In multiple inheritance there is every chance of multiple properties of multiple objects with  the same name available to the sub class object with same priorities leads for the ambiguity. also known as diamond problem. one class extending two super classes.
  • Because of multiple inheritance there is chance of the root object getting created more than once.
  • Always the root object i.e object of object class hast to be created only once.
  1. Because of above mentioned reasons multiple inheritance would not be supported by java.
  2. Thus in java a class can not extend more than one class simultaneously. At most a class can extend only one class.

 8.How do you implement multiple inheritance in java?
  • Using interfaces java can support multiple inheritance concept in java. in java can not extend more than one classes, but a class can implement more than one interfaces.
Program:

interface A{

}
interface B{
}
class C implements A,B{
}

9.Can a class extend itself?

  • No,A class can't extend itself.

10.What happens if super class and sub class having same field name?


  • Super class field will be hidden in the sub class. You can access hidden super class field in sub class using super keyword.

Static methods in java example

Posted by: Instanceofjava Posted date: April 20, 2015 / comment : 0

  • A method which has static keyword in its definition is called static method.
  1. static void print(){
  2.  
  3. }

  • JVM will not execute static methods by default. They are executed only if they are called explicitly by developer either from main method or from static variable as its assignment statement or from static block.

Static method called from main method:


  1. package com.instanceofjava;
  2. class StaticDemo
  3. {
  4.  
  5. static void show(){
  6. System.out.println("static method"):
  7. }
  8.  
  9. public static void main(String[] args)
  10. {
  11.  
  12. show();
  13.  
  14. }
  15. }
Output:

  1. static method

Static method called from variable assignment:


  1. package com.instanceofjava;
  2. class StaticDemo
  3. {
  4.  static int x=show();
  5. static int show(){
  6. System.out.println("static method called"):
  7. return 10;
  8. }
  9.  
  10. public static void main(String[] args)
  11. {
  12.  
  13. System.our.println(x);
  14.  
  15. }
  16. }
Output:

  1. static method called
  2. 10

Static method called from static block:


  1. package com.instanceofjava;
  2. class StaticDemo
  3. {
  4.  static{
  5.  show();
  6. }

  8. static void show(){
  9. System.out.println("static method called"):
  10. }
  11.  
  12. }
Output:

  1. static method called

Order of Execution:

  •  Static methods are executed in the order of they are called, not in the order of they are defined.
  • All static methods are executed in java stack area by creating separate stack frame.
  • When a method is called from main method, JVM creates stack frame in main thread for that method execution.
  • The stack frame is destroyed immediately after method execution is completed.

Example program on order of execution of static methods:


  1. package com.instanceofjava;
  2. class StaticMehodDemo
  3. {
  4.  
  5. static void show(){
  6.  
  7. System.out.println("show method called");
  8.  
  9. }
  10.  
  11. static void print(){
  12.  
  13. System.out.println("print method called");
  14.  
  15. }
  16.  
  17. static void display(){
  18.  
  19. System.out.println("display method called");
  20.  
  21. }
  22.  
  23.  
  24. public static void main(String[] args)
  25. {
  26.  
  27. System.our.println("main method called");
  28.  
  29. show();
  30. print();
  31. display();
  32.  
  33.  
  34. }
  35. }


Output:

  1. main method called
  2. print method called
  3. show method called
  4. display method called

Variable initialization with same variable:

  •  We can initialize variable with same variable name. this assignment is valid. In this case the variable value is replaced with same value.
  1. int x=20;
  2. x=x; // valid statement
Example program:



  1. package com.instanceofjava;
  2. class StaticDemo
  3. {
  4.  static int a=10;

  6. public static void main(String[] args){
  7.   
  8. int a=20;
  9. a=a;
  10. System.out.println("a="+a);
  11.  System.out.println("StaticDemo.a="+StaticDemo.a);
  12.  
  13. }
  14.  
  15. }
Output:

  1. 20
  2. 10

 

Local preference with parameters:

  • Parameters are also treated as local variables.
  • Hence if parameter declared with same static variable name and if we want to access static variable in presence of parameter or if we want to initialize static variable with parameter name we must refer variable with class name.

Example program:


  1. package com.instanceofjava;
  2. class StaticDemo
  3. {
  4.  static int x=10;
  5.  
  6. static void m1(int x){ 
  7.  
  8.  System.out.println(x);
  9.  System.out.println(StaticDemo.x);

  11. }

  13. public static void main(String[] args){
  14.   
  15.  m1(37);
  16.  System.out.println(x);
  17.  
  18. }
  19.  
  20. }
Output:

  1. 37
  2. 10

 

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Static Members in java

Posted by: Instanceofjava Posted date: April 19, 2015 / comment : 2

  • The class level members which have static keyword in their definition are called static members.

Types of Static Members:

  •  Java supports four types of static members
  1. Static Variables
  2. Static Blocks
  3. Static Methods
  4. Main Method (static method)

  • All static members are identified and get memory location at the time of class loading by default by JVM in Method area.
  • Only static variables get memory location, methods will not have separate memory location like variables.
  • Static Methods are just identified and can be accessed directly without object creation.

1.Static Variable:

  • A class level variable which has static keyword in its creation statement is called static variable.

  1. package com.instanceofjava;
  2. class Demo{
  3.  
  4. static int a=10;
  5. static int b=20;
  6.  
  7. }

  • We can not declare local variables as static it leads to compile time error "illegal start of expression".
  • Because being static variable it must get memory at the time of class loading, which is not possible to provide memory to local variable at the time of class loading.

  1. package com.instanceofjava;
  2. class Demo{
  3.  
  4. static int a=10;
  5. static int b=20;
  6.  public static void main(String [] args){
  7.  
  8.    //local variables should not be static
  9.  static int a=10;// compile time error: illegal start of expression

  11. }


  • All static variables are executed by JVM in the order of they defined from top to bottom.
  • JVM provides individual memory location to each static variable in method area only once in a class life time.

Life time and scope:

  •  Static variable get life as soon as class is loaded into JVM and is available till class is removed from JVM or JVM is shutdown.
  • And its scope is class scope means it is accessible throughout the class.

  1. package com.instanceofjava;
  2. class StaticDemo{
  3.  
  4. static int a=10;
  5. static int b=20;
  6.  
  7.  public static void main(String [] args){
  8.  
  9.    System.out.println("a="+a);
  10.    System.out.println("a="+b);
  11.    show();

  13.  }
  14.  
  15. public static void show(){
  16.  
  17.    System.out.println("a="+a);
  18.    System.out.println("a="+b);
  19. }

  21. }

static methods JVM architecture

Duplicate Variables:

  • If multiple variables are created with same name are considered as duplicate variables.
  • In the same scope we can not create multiple variables with same name.
  • If we create it leads to compile time error "variable is already defined".
  • Even if we change data type or modifier or its assigned value we can not create another variable with same name.

  1. package com.instanceofjava;
  2. class StaticDemo
  3. {
  4.  
  5. static int a=10;
  6. int a=30;// Compile time error 
  7.  
  8. }

  • But it is possible to create multiple variables with same name in different scopes.

  1. package com.instanceofjava;
  2. class StaticDemo
  3. {
  4. static int a=10;
  5. int a=30;// Compile time error
  6.  
  7. public static void main(String [] args){
  8.   
  9. // it is allowed to define "a" in this method.
  10. int a=20;
  11.  
  12. }
  13. }

Shadowing:

  • It is possible to create local variables or parameters with same variable name.
  • The concept is called shadowing . It means local variable is a shadow of class level variable.
  • It means when you access it in side method , you will get local variables value but not  from class level.

  1. package com.instanceofjava;
  2. class StaticDemo
  3. {
  4. static int a=10;

  6.  
  7. public static void main(String [] args){
  8.   

  10.  int a=20;
  11.  System.out.println("a="+a); // prints 20

  13. }
  14. }

 Local preference:

  • When we call a variable compiler and JVM will search for its definition in that method first, if its definition is not found in that method then will search for its definition at class level.
  • If its definition not found at class level also then compiler throws error: can not find symbol.
  • This phenomenon is called local preference.


  1. package com.instanceofjava;
  2. class StaticDemo
  3. {
  4. static int a=10;

  6.  
  7. public static void main(String [] args){
  8.  
  9.  System.out.println("a="+a); // prints 10 : method level no definition so access class level

  11.  int a=20;
  12.  System.out.println("a="+a); // prints 20 :method level variable found

  14. }
  15. }


  • By using class name we can get class level variables value.


  1. package com.instanceofjava;
  2. class StaticDemo
  3. {
  4. static int a=10;

  6.  
  7. public static void main(String [] args){
  8.  
  9.  System.out.println("a="+a); // prints 10 : method level no definition so access class level

  11.  int a=20;
  12.  System.out.println("a="+a); // prints 20 :method level variable found

  14.  System.out.println("a="+StaticDemo.a); // prints 10 : class level variable
  15.  
  16. }
  17. }


Order of execution of static variables and main method:

  • First all static variables are executed in the order they defined from top to bottom then main method is executed.
Example Program:


  1. package com.instanceofjava;
  2. class StaticDemo
  3. {
  4. static int a=m1();
  5.  
  6. static int m1(){
  7.  
  8. System.out.println("variable a is created");
  9. return 10;
  10.  
  11. } 
  12.  
  13. static int b=m2();
  14.  
  15. static int m2(){
  16.  
  17. System.out.println("variable b is created");
  18. return 20;

  20. } 
  21.  
  22. public static void main(String [] args){
  23.   
  24.  System.out.println("in main method");
  25.  System.out.println("a="+a);
  26.  System.out.println("b="+b);


  29.  
  30. }
  31. }

Output:
  1. variable a is created
  2. variable b is created
  3. in main method
  4. 10
  5. 20

  • we can access static fields using objects but static fields should be accessed in a static way because static fields are class level so even if we change field value using particular object that will change the original value 


  1. package com.instanceofjava;
  2.  
  3. public class StaticDemo {
  4.  
  5.  static int x=10;
  6.  
  7. public static void main(String[] args) {
  8.  
  9.  StaticDemo obj=new StaticDemo();
  10.  
  11. System.out.println(x);
  12.  
  13. obj.x=30;
  14.  
  15. System.out.println(x);
  16.  
  17. StaticDemo obj2=new StaticDemo();
  18.  System.out.println(obj2.x); 
  19.  
  20. System.out.println(StaticDemo.x); // Recommended way to access static variables

  22. }
  23. }

Output:

  1. 10
  2. 30
  3. 30
  4. 30

Final Static Variables:

  • Final static variables are constants.
  • Declaration itself we need to assign value to the final static variables otherwise compiler error will come: "The blank final field  may not have been initialized".
  • And we can not change this value if we try then compiler error will come: The final field  cannot be assigned.


  1. package com.instanceofjava;
  2. class Demo{
  3.  
  4.  final static int MAX_VALUE=10;

  6.  public static void main(String [] args){
  7.  
  8.    System.out.println(MAX_VALUE);

  10. }



Java Variables and Types of Variables

Posted by: Instanceofjava Posted date: April 14, 2015 / comment : 0


Variable:

  • Variable is a named memory location used to store data temporarily.
  • During program execution we can modify that data

Limitation of Variable:

  •  It can only store single value at a time.
  • It means , always it returns latest modified value.

How can a variable be created?

  • A variable can be created by using data type.
  • As we have two types of data types  we can create two types of variables.
    1.Primitive Variable
    2.Referenced variable
  • The difference between primitive and referenced variable is "primitive variable stores data directly , where referenced variables stores reference/address of object, not direct values"

  1. package com.instanceofjava;
  2.  
  3. class Example
  4. {
  5.  
  6.  int x=10;
  7.  int y=20;
  8.  
  9. }


  1. package instanceofjava;
  2. class Test
  3. {
  4.  
  5. public int get(){
  6. return 10:
  7. }
  8.  
  9. public static void main(String [] args){
  10.  
  11. //primitive variables
  12. int s=50;
  13. int t=get(); 
  14.  
  15. //reference variables
  16. String str="a";
  17. String str1=new String("a");
  18. Example e= new Example();

  20. }
  21. }



What is an object?

  • Technically object means collection of all non static variables and methods memory locations.
  • Object is created using new keyword

Defining a variable:

  • Variable creation with value is called defining a variable
  • <Accessibility modifier><Modifier><Data type><Variable name>=<Value> ;

  1. package instanceofjava;
  2. public class Demo{
  3. public static void main(String[] args){
  4.   
  5. public static int x=10;
  6. public static Example e= new Example();

  8. }
  9. }

Declaring a variable:

  • Variable creation without value is called declaring a variable.
  • <Accessibility modifier><Modifier><Data type><Variable name>;


  1. package instanceofjava;
  2. public class Demo{
  3. public static void main(String[] args){
  4.   
  5. public static int a;
  6. public static Example e;

  8. }
  9. }


Initializing/Assigning a variable:

  • Storing a value in a variable at the time of its creation is called initializing a variable.
  • Storing a value in a variable after its creation is called assigning a value to a variable.
  • <Variable_Name>=<Value>;

  1. package instanceofjava;
  2. public class Demo{
  3. public static void main(String[] args){
  4.   
  5. //declaring a variable
  6.  int a;
  7.  int x;
  8.  
  9. //assigning a variable
  10.  a=20;
  11.  x=10;
  12.  
  13. // re initialization/ re assignment
  14.  a=30;
  15.  x=20;

  17. }
  18. }


Calling a variable:

  • Reading a value from a variable is called calling a variable.


  1. package instanceofjava;
  2. public class Demo{
  3. public static void main(String[] args){
  4.   
  5.  int x=10;
  6.   
  7.  //calling x variable for printing
  8. System.out.println(x);
  9.  
  10. // calling x variable for initializing y
  11.  int y=x;


  14. }
  15. }



Types of Variables:

  • Local
  • Static
  • Non static
  • Final
  • Transient
  • Volatile

1.Local Variables:

  • The variables created inside a method or block are called local variables.

Rules:

  • While working with local variables , we must follow below 3 rules.

Rule #1:

  •  Local variables can not be accessed from another method. Because its scope is restricted only within its method.
  • Also we can not guarantee that variable creation , because that method may or may not be called. It leads to compile time Exception : can not find symbol.
 Example program:

  1. package com.instanceofjava;
  2. class test
  3. {
  4.  
  5. public void show(){
  6.  System.out.println("a:"+a); Compile time error: can not find symbol a.
  7. }
  8.  
  9. public static void main(String [] args){
  10.  
  11.  int a=10;
  12.  System.out.println("a:"+a);
  13. }
  14. }


Rule #2:

  • Local variable should not be accessed without initialization.

 Example program:

  1. package com.instanceofjava;
  2. class test
  3. {
  4. public static void main(String [] args){
  5.  
  6.  int a=10;
  7.  int b;
  8.  System.out.println("a:"+a);
  9.  
  10.  System.out.println("b:"+b);//Error
  11.  // Above statement throws Compile time Error: variable b might not have been initialized
  12.  
  13.  b=20;
  14. System.out.println("b:"+b);
  15.  
  16. }
  17. }


Rule #3:

  • Local variable must be accessed only after its creation statement. because method execution is sequential execution from top to bottom. If we access before  its creation statement it leads compile time Error : Can not find symbol.


 Example program:

  1. package com.instanceofjava;
  2. class test
  3. {
  4.  
  5.  static void show(){
  6.  System.out.println("a:"+a);// Compile time Error
  7.  int a;
  8.  System.out.println("a:"+a); // Compile time Error
  9.   
  10.  a=10
  11.  
  12.  System.out.println("a:"+a);
  13.  
  14. }
  15. }

2. Static variables :

  • Static variables gets life when class is loaded.
  • It is destroyed either if class is unloaded from JVM or JVM is destroyed.
  • Its scope is throughout the class it means where class is available there static variable is available provided it is public.

 Example program:

  1. package com.instanceofjava;
  2. class test
  3. {
  4.  static int a=10;

  6. public static void show(){
  7.  System.out.println("a:"+a);
  8. }
  9.  
  10. public static void main(String [] args){
  11.  
  12.  System.out.println("a:"+a);
  13.  
  14. }
  15. }

3. Non Static variables:

  • Non static variable gets life when object is created. It is destroyed when object is destroyed.
  • Object is destroyed when it is unreferenced.
  • Its scope is the scope of the object, object is available only if its referenced variable is available

 Example program:

  1. package com.instanceofjava;
  2. class test
  3. {
  4.  
  5.  int a=10;

  7. public void show(){
  8.   test t= new test();
  9.  System.out.println("a:"+t.a);
  10. }
  11.  
  12. public static void main(String [] args){
  13.  
  14.   test t= new test();
  15.  System.out.println("a:"+t.a);
  16.  
  17. }
  18. }


4.Final variables:

  • The class level or local variable that has final keyword in its definition is called final variable.

Rule:

  •  Once it is initialized by developer its value can not be changed. If we try to change its value it leads to Compile tile error.


 Example program:

  1. package com.instanceofjava;
  2. class test
  3. {
  4.  static final int a=10;
  5. static final int b=20;

  7. public static void main(String [] args){
  8.  
  9.  a=20;//Error : variable might be already have been assigned
  10.  b=30; //Error : variable might be already have been assigned
  11.  
  12. }
  13. }


5. Transient variable:

  • The class level variable that has transient keyword in its definition is called transient variable.

Rule:

  • Local variable can not be declared as transient.
  • It leads to Compile time error: Illegal start expression


 Example program:

  1. package com.instanceofjava;
  2. class test
  3. {
  4.  static transient int a=10;
  5. static transient  int b=20;

  7. public static void main(String [] args){
  8.  
  9.  transient int x=10; // compile time error : illegal start expression
  10.  
  11. }
  12. }

  • We declare variable as transient to tell to JVM that we do not want to store variable value in a file in object serialization. Since local variable is not part of object , declaring it as transient is illegal.
  • Refer IOStreams for more details

6.Volatile Variable:

  • The class level variable that has volatile keyword in its definition. 

Rule:

  • Local variable can not be declared as transient.
  • It leads to Compile time error: Illegal start expression


 Example program:

  1. package com.instanceofjava;
  2. class test
  3. {
  4.  static volatile int a=10;
  5. volatile int b=20;

  7. public static void main(String [] args){
  8.  
  9.  volatile int x=10; // compile time error : illegal start expression
  10.  
  11. }
  12. }

  • We declare variable as volatile to tell to JVM that we do not want to modify variable value concurrently by multiple threads.
  • If we declare variable as volatile multiple threads are allowed to change its value in sequence one after another one.


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