Quick introduction to Java Programming language - Part 1

In some situation code need to be execute severaltimes to achieve this by using loop.

Looping Statements:

There are four types

• for
• while
• do…while
• for-each

For Loop:

This loop allows usto execute a block of code for a specific number of times.

Syntax:

for(initialization; condition; increment/decrement) {
       …
}

Various parts of above for loop are explained below

Initialization: This is beginning step allows us to declare and initialize loop control.It execute only once.

Condition: This step evaluates each time when iteration starts/at beginning of each iteration

Increment/Decrement: This step executes at the end of each and every iteration

public classForDemo
{
 public static void main(String[] args) 
 {
   int sum=0;
   for(int i=1;i<=10;i++)
   {
     sum+=i;
   }
   System.out.println("Sum of numbers from 1 to 10 is: "+sum);
   System.out.println("Program ends");
   }
} 

Output of above program is shown below

Nested For Loop:

In nested, where for within for loop.

Example

public class NestedForDemo
{
	public static void main(String[] args) 
		{
			for(int i=1;i<=3;i++)
			{
				for(int j=1;j<=3;j++)
				{
					System.out.println(i+" "+j);
				}
			}
			System.out.println("Program ends");
		}
   }

 Above Program will output

Break:

is a keyword used in “for” statements with if condition. Break is used to terminate the “for” loop iteration.

Syntax:

break;

Example: 

public class BreakDemo
{
	public static void main(String[] args) 
	{
		for(inti=1;i<=10;i++)
		{
			System.out.println("i is: " + i );
			if(i==3)
			   {
				break; // program will exit this loop when condition is true
			   }
		}
		System.out.println("Program ends");
	}
}

 Above program will output

Continue:

This is keyword used to skip the current iteration and execute the next iteration.

Syntax:

   continue;

Example: 

public class Account 
 {
	public static void main(String[] args) 
    {
		for(inti=0;i<=5;i++)
		{
			if(i==3)
			   {
				continue;
			   }
			System.out.println("i: "+i);
		} 
		System.out.println("Program ends");
    }
}

Output 

As you can see that value 3 was not printed because of continue; 

The return statement:

The return is a keyword to return the data from method and alsoto exit from the current method and jump back to the statement within the calling method that follows the original method call.

• Statements exist after return statement i.e called unreachable.
• If java has unreachable state, it will throw error.

Two types of return: One that returns a value and one that doesn't.

Syntax:

return variable;  or return;

public class ReturnDemo
{
	public static void main(String[] args) 
	{
		int totSum=0;
		for(int i=5;i<=10;i++)
		   {
			totSum+=sum(i);
		   }
		System.out.println(totSum);
	}
	static int sum(int a)
	{
		if(a%3==0)
		{
			return a;
		}
		else
		{
			return 0;
		}
	}
}

Output for above program will be 15 

While Loop:

This loop provides condition, to iteratespecific statements based on condition.

If condition fails/not met then statements will not execute inside the while loop.

Syntax:

while(condition) {
   //statements
}

Example:

public class WhileLoopDemo
{
	public static void main(String[] args) 
	{
		int i=1;
		while(i<=10)
		{
			System.out.println("i is: "+i);
			i++;
		}
		System.out.println("Program ends");
	}
}

Output of above program is shown below

Do…while:

This loop to iterate specific statements based on condition.But here

if condition is true/false,the statements will execute atleast once.

Syntax:

do 
{
 //statements
}
while(condition);

Example:

public class DoWhileDemo
 {
	public static void main(String[] args) 
	{
		int i=1;
		do
		{
			System.out.println("i is: "+i);
			i=i+2;
			System.out.println("After addition i value is: "+i);
		}
		while(i==1);
			System.out.println("Program ends");
	}
}

Output:

For each: (Advanced / Enhanced for)

For-each is used to iterate elements in a arrays/collections

• It is added from java1.5/5.0 release
• It works with array and collections

Syntax:

for(declaration: expression)   
{
 //Statements
}

Main method Example: 

public static void  main(String[] args)
 {
	int [] marks = {92, 80, 90, 88, 50};
	for(int i : marks )
	{
		System.out.print( i );
		System.out.print(",");
	}
	System.out.print("\n");
	String [] subjects ={"Maths", "Science", "English", "History"};
	for( String subName : subjects )
	{
		System.out.print( subName );
		System.out.print(",");
	}
}

Output is shown below