Creating loop in java

For loop in Java with example

For loop is used to execute a set of statements repeatedly until a particular condition returns false. In Java we have three types of basic loops: for, while and do-while. In this tutorial you will learn about for loop in Java. You will also learn nested for loop, enhanced for loop and infinite for loop with examples.

Syntax of for loop:

for(initialization; condition ; increment/decrement)

Initialization: In the initialization part, variables like loop counter (you will generally see i and j in loops, these are the loop counters) are initialized. This is an optional part of the loop as the variables can be initialized before the loop. This executes only once when the loop starts.

Condition: This is one of the important part of the loop. This condition determines till when the loop should keep repeating. The loop keeps repeating until the condition becomes false.

Increment/Decrement: In this part of the loop declaration, you can specify the increment or decrement of loop counter. This is to modify the loop counter value so that at one point condition becomes false and the loop ends.

Statement: The statements inside the loop body keeps executing for each iteration of the loop until the loop stops.

Flow of Execution of the for Loop

As a program executes, the interpreter always keeps track of which statement is about to be executed. We call this the control flow, or the flow of execution of the program.

First step: In for loop, initialization happens first and only one time, which means that the initialization part of for loop only executes once.

Second step: Condition in for loop is evaluated on each iteration, if the condition is true then the statements inside for loop body gets executed. Once the condition returns false, the statements in for loop does not execute and the control gets transferred to the next statement in the program after for loop.

Third step: After every execution of for loop’s body, the increment/decrement part of for loop executes that updates the loop counter.

Fourth step: After third step, the control jumps to second step and condition is re-evaluated.

Example: Simple for loop

This is a simple example of for loop. Here we are displaying the value of variable i inside the loop body. We are using decrement operator in the loop so the value of i decreases by one after each iteration of the loop and at some point the condition i>1 returns false, this is when the loop stops.

The output of this program is:

The value of i is: 10 The value of i is: 9 The value of i is: 8 The value of i is: 7 The value of i is: 6 The value of i is: 5 The value of i is: 4 The value of i is: 3 The value of i is: 2

In the above program:
int i=1 is initialization expression
i>1 is condition(Boolean expression)
i– Decrement operation

Java Nested for loop

A for loop inside another for loop is called nested for loop. Let’s take an example to understand the concept of nested for loop. In this example, we are printing a pattern using nested for loop.

public class JavaExample < public static void main(String[] args) < //outer loop for(int i=1;i<=6;i++)< //inner loop for(int j=1;j<=i;j++)< System.out.print("* "); >// this is to move the cursor to new line // to print the next row of the pattern System.out.println(); > > >

Output:

Infinite for loop

A loop that never stops executing is called infinite loop. This happens when the condition expression defined in loop never returns false. The following example will help you understand the importance of Boolean expression and increment/decrement operation co-ordination:

class ForLoopExample2 < public static void main(String args[])< for(int i=1; i>=1; i++) < System.out.println("The value of i is: "+i); >> >

This is an infinite loop as the condition would never return false. The initialization step is setting up the value of variable i to 1, since we are incrementing the value of i, it would always be greater than 1 (the Boolean expression: i>1) so it would never return false. This would eventually lead to the infinite loop condition. Thus it is important to see the co-ordination between Boolean expression and increment/decrement operation to determine whether the loop would terminate at some point of time or not.

Here is another example of infinite for loop:

Example: Iterate an array using for loop

Let’s see another example of for loop. Here we are iterating and displaying array elements using the for loop.

class ForLoopExample3 < public static void main(String args[])< int arr[]=; //i starts with 0 as array index starts with 0 too for(int i=0; i > >

Enhanced For loop

Enhanced for loop is another way of defining a loop. This is especially useful when you want to iterate array, ArrayList and other collections classes. Tt is easy to read and write.

Let’s take the same example that we have seen above. We are rewriting it using enhanced for loop.

class ForLoopExample3 < public static void main(String args[])< int arr[]=; for (int num : arr) < System.out.println(num); >> >

Note: In the above example, I have declared the num as int in the enhanced for loop. This will change depending on the data type of array. For example, the enhanced for loop for string type would look like this:

String arr[]=; for (String str : arr)

Check out these java programming examples related to for loop:

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About the Author

I have 15 years of experience in the IT industry, working with renowned multinational corporations. Additionally, I have dedicated over a decade to teaching, allowing me to refine my skills in delivering information in a simple and easily understandable manner.

Comments

I really appreciate and recommend this website to all the beginners and experienced as well. As i find it very ease in learning java, i’ve been in touch with the same for past 1yr but never been so comfortable with it. But after reading the few tutorials on this website i realized that, my concepts weren’t that much clear which i feel have been cleared after hitting these tutorials.
Thanks –

Very Helpful for beginners…As I am from a Non-IT background in my graduation, this site has helped me a lot…Add more sample & simple programs so that we can know more.
Great going guys…keep it up 🙂

I think this array declaration is wrong.it can’t be like this.
int arr[4]=;
It should be like this
int arr[]=;

Both ways are correct. He declared and FIXED the size of array to 4 and if I come to your point, you can declare any number of members in your array!. Your concept is good for Multidimensional Array but for Single array.. Always declare the size of array!.

This is one of the best sites for learning java. Concepts are made very clear and explained in such a simple way. Wonderful way of teaching. Thank you so much!

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The for Statement

The for statement provides a compact way to iterate over a range of values. Programmers often refer to it as the «for loop» because of the way in which it repeatedly loops until a particular condition is satisfied. The general form of the for statement can be expressed as follows:

for (initialization; termination; increment) < statement(s) >

When using this version of the for statement, keep in mind that:

• The initialization expression initializes the loop; it’s executed once, as the loop begins.
• When the termination expression evaluates to false , the loop terminates.
• The increment expression is invoked after each iteration through the loop; it is perfectly acceptable for this expression to increment or decrement a value.

The following program, ForDemo , uses the general form of the for statement to print the numbers 1 through 10 to standard output:

The output of this program is:

Count is: 1 Count is: 2 Count is: 3 Count is: 4 Count is: 5 Count is: 6 Count is: 7 Count is: 8 Count is: 9 Count is: 10

Notice how the code declares a variable within the initialization expression. The scope of this variable extends from its declaration to the end of the block governed by the for statement, so it can be used in the termination and increment expressions as well. If the variable that controls a for statement is not needed outside of the loop, it’s best to declare the variable in the initialization expression. The names i , j , and k are often used to control for loops; declaring them within the initialization expression limits their life span and reduces errors.

The three expressions of the for loop are optional; an infinite loop can be created as follows:

The for statement also has another form designed for iteration through Collections and arrays This form is sometimes referred to as the enhanced for statement, and can be used to make your loops more compact and easy to read. To demonstrate, consider the following array, which holds the numbers 1 through 10:

The following program, EnhancedForDemo , uses the enhanced for to loop through the array:

class EnhancedForDemo < public static void main(String[] args)< int[] numbers = ; for (int item : numbers) < System.out.println("Count is: " + item); >> >

In this example, the variable item holds the current value from the numbers array. The output from this program is the same as before:

Count is: 1 Count is: 2 Count is: 3 Count is: 4 Count is: 5 Count is: 6 Count is: 7 Count is: 8 Count is: 9 Count is: 10

We recommend using this form of the for statement instead of the general form whenever possible.

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Loops in Java

Looping in programming languages is a feature which facilitates the execution of a set of instructions/functions repeatedly while some condition evaluates to true. Java provides three ways for executing the loops. While all the ways provide similar basic functionality, they differ in their syntax and condition checking time.

java provides Three types of Conditional statements this second type is loop statement .

• while loop: A while loop is a control flow statement that allows code to be executed repeatedly based on a given Boolean condition. The while loop can be thought of as a repeating if statement.

Java

• Flowchart:
  • While loop starts with the checking of Boolean condition. If it evaluated to true, then the loop body statements are executed otherwise first statement following the loop is executed. For this reason it is also called Entry control loop
  • Once the condition is evaluated to true, the statements in the loop body are executed. Normally the statements contain an update value for the variable being processed for the next iteration.
  • When the condition becomes false, the loop terminates which marks the end of its life cycle.
• for loop: for loop provides a concise way of writing the loop structure. Unlike a while loop, a for statement consumes the initialization, condition and increment/decrement in one line thereby providing a shorter, easy to debug structure of looping.

for (initialization condition; testing condition;increment/decrement)

Java

• Flowchart:
  • Initialization condition: Here, we initialize the variable in use. It marks the start of a for loop. An already declared variable can be used or a variable can be declared, local to loop only.
  • Testing Condition: It is used for testing the exit condition for a loop. It must return a boolean value. It is also an Entry Control Loop as the condition is checked prior to the execution of the loop statements.
  • Statement execution: Once the condition is evaluated to true, the statements in the loop body are executed.
  • Increment/ Decrement: It is used for updating the variable for next iteration.
  • Loop termination:When the condition becomes false, the loop terminates marking the end of its life cycle.
• do while: do while loop is similar to while loop with only difference that it checks for condition after executing the statements, and therefore is an example of Exit Control Loop.

Java

• Flowchart:
  • do while loop starts with the execution of the statement(s). There is no checking of any condition for the first time.
  • After the execution of the statements, and update of the variable value, the condition is checked for true or false value. If it is evaluated to true, next iteration of loop starts.
  • When the condition becomes false, the loop terminates which marks the end of its life cycle.
  • It is important to note that the do-while loop will execute its statements atleast once before any condition is checked, and therefore is an example of exit control loop.

Pitfalls of Loops

• Infinite loop: One of the most common mistakes while implementing any sort of looping is that it may not ever exit, that is the loop runs for infinite time. This happens when the condition fails for some reason. Examples:
• Infinite for loop :

Java

infinite while loop:

Java

Java

Another pitfall is that you might be adding something into you collection object through loop and you can run out of memory. If you try and execute the below program, after some time, out of memory exception will be thrown.

Java

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space at java.util.Arrays.copyOf(Unknown Source) at java.util.Arrays.copyOf(Unknown Source) at java.util.ArrayList.grow(Unknown Source) at java.util.ArrayList.ensureCapacityInternal(Unknown Source) at java.util.ArrayList.add(Unknown Source) at article.Integer1.main(Integer1.java:9)

Nested Loop:

Nested loop means a loop statement inside another loop statement.

There are different combinations of loop using for loop, while loop, do-while loop.

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