Loops in Java with examples

In java or any other programming language use three types of looping methods

1.    While loop

2.    Do while loop

3.    For loop

While loop

• This loop is used for iteration purposes in programming.
• First there should be initialization and then check the condition, if condition is true it can go inside the while loop and execute.
• When condition is false it skips the loop.

   Initialization;

   While (condition)

   {

       Body of loop

   }

Try these example for understand about while loops.

public class Mango{ 
     public static void main(String args[]){
         
          int x =1;
          while(x<6){
              System.out.println(x);
              ++x;
          }
     }
}

Illegal while conditions

There should be a condition in while loop

          Ex: while (x==5), while (false), while (x!=3)…etc.

int x=1;
While (x)
{
      Body of loop
}

int x;
While (x=10)
{
Body of loop
}

Legal while conditions

int x;
While (x==10)
{
     Body of loop
}

int x=10;
While (true)
{
     Body of loop
}

Do while loop

• This is like a while loop, but different is this is execute at least one time, and then check the condition.

Initialization;
    do{
         Body of loop
    } While (condition);

Try these example for understand about while loops.

public class Mango{
     public static void main(String args[]){

          int x =1;         
          do{
              System.out.println(x);
              ++x;
          }while(x<6);
     }
}

public class Apple{
     public static void main(String args[]){
       int x=1;
    
       while(x<15){

          int y=1;
          while(y<12){
             
            int res = x*y;
            System.out.println(x+ " * " +y+" = "+ res);
            y++;
          }  
          System.out.println("------------------------");
          ++x;
    
     }
  }
}

For loop

for (initialization; test condition; increment)
    {
          Body of the loop
    }

• This is the structure of for loop, but same thing can be done in several ways. (Look at the examples)
• First you have to declare the variable that you are going to use. This should be the initial state of variable.
• Then semicolon is compulsory.
• After this, condition is there, if condition is true, the body of the loop is executed.
• Then it go to increment process and same thing happen until condition false.

Try this codes to understand about for loop.

public class Apple{

     public static void main(String args[]){
    
          for(int  x=0; x<5; x++){
    
              for(int y=0; y<=x; y++){
                   System.out.print("*");
              }
              System.out.print("\n");
          }
     }
}

public class Apple{

     public static void main(String args[]){

          int mark[] = {10,20,30,40,50,60,};//array in java

          for(int x=0; x<mark.length; x++){
              System.out.println(mark[x]);
          }
     }
}

public class Apple{
    
     public static void main(String args[]){

           for(;;){
                System.out.println("This is endless for loop");
           }
     }
}

public class Apple{
     public static void main(String args[]){

          int x= 10;
          for(;x<15;){
              System.out.println("This is a valid for loop");
              x++;
          }
     }
}

Continue and break

• Break keyword is used to stop entire loop.
• Continue means it skips the current iteration and execute the program.

public class Apple{

     public static void main(String args[]){
    
          for(int  x=0; x<=20; x++){
         
              if(x==5){
                   continue;//this is used to skip a part
              }
         
              if(x==15){
                   break;//this is used to break the loop
              }
          System.out.println(x);
             
          }   
     }
}

Labeled for loop

• You can label for loops as you want.
• If there are nested loops it is better way to identify each loop separately.
• Before “for” keyword you can type the name of loop and add colon after it.

In this example I have labeled outer and inner loops.

public class Apple{ 
     public static void main(String args[]){
         
          outer:
          for(int x=0; x<10; x++){
         
              System.out.println(x);
             
              inner:
              for(int y=0; y<10; y++){
                   System.out.println(y);
             
                   if(y==5){
                        continue outer;
                   }        
              }   
          }
     }
}

Enhanced for loop

• This is available after java 5.
• Syntax is there, first you have to use array to declare data.

          int x[] = {elements};

          for (declaration : expression);

    

Try these codes to understand about enhanced for loop.

public class Apple{
     public static void main(String args[]){

          int x[] = {1,2,3,4,5,6};
         
          //this is normal for loop
          for(int i=0; i<x.length; i++){
            System.out.println("Normal for loop: " + x[i]);
          }
         
          //this is enhanced for loop
          for(int j :x){
            System.out.println("Enhanced for loop: " + j);
          }   
     }
}