Loops in Java

In programming languages, loops are used to execute a set of instructions/functions repeatedly when some conditions become true. There are three types of loops in Java.

Loops in Java

Java For Loop vs While Loop vs Do While Loop

Comparison for loop while loop do while loop
Introduction The Java for loop is a control flow statement that iterates a part of the programs multiple times. The Java while loop is a control flow statement that executes a part of the programs repeatedly on the basis of given boolean condition. The Java do while loop is a control flow statement that executes a part of the programs at least once and the further execution depends upon the given boolean condition.
When to use If the number of iteration is fixed, it is recommended to use for loop. If the number of iteration is not fixed, it is recommended to use while loop. If the number of iteration is not fixed and you must have to execute the loop at least once, it is recommended to use the do-while loop.
Syntax
for(init;condition;incr/decr){  
// code to be executed 
}
while(condition){  
//code to be executed 
}
do{  
//code to be executed  
}while(condition); 
Example
//for loop  
for(int i=1;i<=10;i++){  
System.out.println(i);  
}  
//while loop  
int i=1;  
while(i<=10){  
System.out.println(i);  
i++;  
}  
//do-while loop  
int i=1;  
do{  
System.out.println(i);  
i++;  
}while(i<=10); 
Syntax for infinitive loop
for(;;){  
//code to be executed  
}
while(true){  
//code to be executed  
}
do{  
//code to be executed  
}while(true);  

Java For Loop

The Java for loop is used to iterate a part of the program several times. If the number of iteration is fixed, it is recommended to use for loop.

There are three types of for loops in java.

  • Simple For Loop
  • For-each or Enhanced For Loop
  • Labeled For Loop

Java Simple For Loop

A simple for loop is the same as C/C++. We can initialize the variable, check condition and increment/decrement value. It consists of four parts:

  1. Initialization: It is the initial condition which is executed once when the loop starts. Here, we can initialize the variable, or we can use an already initialized variable. It is an optional condition.
  2. Condition: It is the second condition which is executed each time to test the condition of the loop. It continues execution until the condition is false. It must return boolean value either true or false. It is an optional condition.
  3. Statement: The statement of the loop is executed each time until the second condition is false.
  4. Increment/Decrement: It increments or decrements the variable value. It is an optional condition.

Syntax:

snippet
for(initialization;condition;incr/decr){
//statement or code to be executed
}

Flowchart:

for loop in java flowchart

Example:

snippet
//Java Program to demonstrate the example of for loop
//which prints table of 1
public class ForExample {
public static void main(String[] args) {
    //Code of Java for loop
	for(int i=1;i<=10;i++){
		System.out.println(i);
	}
}
}

Output:

Output
1 2 3 4 5 6 7 8 9 10

Java Nested For Loop

If we have a for loop inside the another loop, it is known as nested for loop. The inner loop executes completely whenever outer loop executes.

Example:

snippet
public class NestedForExample {
public static void main(String[] args) {
//loop of i
for(int i=1;i<=3;i++){
//loop of j
for(int j=1;j<=3;j++){
		System.out.println(i+" "+j);
}//end of i
}//end of j
}
}

Output:

Output
1 1 1 2 1 3 2 1 2 2 2 3 3 1 3 2 3 3

Pyramid Example 1:

snippet
public class PyramidExample {
public static void main(String[] args) {
for(int i=1;i<=5;i++){
for(int j=1;j<=i;j++){
		System.out.print("* ");
}
System.out.println();//new line
}
}
}

Output:

Output
* * * * * * * * * * * * * * *

Pyramid Example 2:

snippet
public class PyramidExample2 {
public static void main(String[] args) {
int term=6;
for(int i=1;i<=term;i++){
for(int j=term;j>=i;j--){
		System.out.print("* ");
}
System.out.println();//new line
}
}
}

Output:

Output
* * * * * * * * * * * * * * * * * * * * *

Java for-each Loop

The for-each loop is used to traverse array or collection in java. It is easier to use than simple for loop because we don't need to increment value and use subscript notation.

It works on elements basis not index. It returns element one by one in the defined variable.

Syntax:

snippet
for(Type var:array){
//code to be executed
}

Example:

snippet
//Java For-each loop example which prints the
//elements of the array
public class ForEachExample {
public static void main(String[] args) {
    //Declaring an array
	int arr[]={12,23,44,56,78};
	//Printing array using for-each loop
	for(int i:arr){
		System.out.println(i);
	}
}
}

Output:

Output
12 23 44 56 78

Java Labeled For Loop

We can have a name of each Java for loop. To do so, we use label before the for loop. It is useful if we have nested for loop so that we can break/continue specific for loop.

Usually, break and continue keywords breaks/continues the innermost for loop only.

Syntax:

snippet
labelname:
for(initialization;condition;incr/decr){
//code to be executed
}

Example:

snippet
//A Java program to demonstrate the use of labeled for loop
public class LabeledForExample {
public static void main(String[] args) {
    //Using Label for outer and for loop
	aa:
		for(int i=1;i<=3;i++){
			bb:
				for(int j=1;j<=3;j++){
					if(i==2&&j==2){
						break aa;
					}
					System.out.println(i+" "+j);
				}
		}
}
}

Output:

Output
1 1 1 2 1 3 2 1

If you use break bb;, it will break inner loop only which is the default behavior of any loop.

snippet
public class LabeledForExample2 {
public static void main(String[] args) {
	aa:
		for(int i=1;i<=3;i++){
			bb:
				for(int j=1;j<=3;j++){
					if(i==2&&j==2){
						break bb;
					}
					System.out.println(i+" "+j);
				}
		}
}
}

Output:

Output
1 1 1 2 1 3 2 1 3 1 3 2 3 3

Java Infinitive For Loop

If you use two semicolons ;; in the for loop, it will be infinitive for loop.

Syntax:

snippet
for(;;){
//code to be executed
}

Example:

snippet
//Java program to demonstrate the use of infinite for loop
//which prints an statement
public class ForExample {
public static void main(String[] args) {
    //Using no condition in for loop
	for(;;){
		System.out.println("infinitive loop");
	}
}
}

Output:

Output
infinitive loop infinitive loop infinitive loop infinitive loop infinitive loop ctrl+c

Now, you need to press ctrl+c to exit from the program.

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