Welcome back everyone. In today’s article we will be looking at while loop and for loop in Java. We will explore while loop, do-while loop, for loop, nested for loop and for-each loop. We will also practice some real world problems. So, let’s get started!

Loops

Loops can execute a block of code as long as a specified condition is reached. Loops are handy because they save time, reduce errors, and they make code more readable.

Java While Loop

The while loop loops through a block of code as long as a specified condition is true:

while (condition) {
  // code block to be executed
}

int i = 0;
while (i < 5) {
  System.out.println(i);
  i++;
}

Countdown Example

This example counts down from 3 to 1 and then displays “Happy New Year!!” at the end:

Example

int countdown = 3;

while (countdown > 0) {
  System.out.println(countdown);
  countdown--;
}

System.out.println("Happy New Year!!");

do {
  // code block to be executed
}
while (condition);

int i = 0;
do {
  System.out.println(i);
  i++;
}
while (i < 5);

Condition is False from the Start

In the example above, the condition i < 5 was true at the beginning, so the loop executed multiple times. But what if the condition is false right from the start?

In the example below, the variable i starts at 10, so the condition i < 5 is false immediately – yet the do/while loop still runs once:

Example

int i = 10;
do {
  System.out.println("i is " + i);
  i++;
} while (i < 5);

Summary

The do/while loop always runs at least once, even if the condition is already false. This is different from a regular while loop, which would skip the loop entirely if the condition is false at the start.

This behavior makes do/while useful when you want to ensure something happens at least once, like showing a message or asking for user input.

Real-Life Examples

To demonstrate a practical example of the while loop, we have created a simple “countdown” program:

int countdown = 3;

while (countdown > 0) {
  System.out.println(countdown);
  countdown--;
}

System.out.println("Happy New Year!!");

int dice = 1;

while (dice <= 6) {
  if (dice < 6) {
    System.out.println("No Yatzy.");
  } else {
    System.out.println("Yatzy!");
  }
  dice = dice + 1;
}

If the loop passes the values ranging from 1 to 5, it prints “No Yatzy”. Whenever it passes the value 6, it prints “Yatzy!”.

Java For Loop

When you know exactly how many times you want to loop through a block of code, use the for loop instead of a while loop:

Syntax

for (statement 1; statement 2; statement 3) {
  // code block to be executed
}

Statement 1 is executed (one time) before the execution of the code block.

Statement 2 defines the condition for executing the code block.

Statement 3 is executed (every time) after the code block has been executed.

Print Numbers

The example below will print the numbers 0 to 4:

Example

for (int i = 0; i < 5; i++) {
  System.out.println(i);
}

Example Explained

• Statement 1 sets a variable before the loop starts: int i = 0
• Statement 2 defines the condition for the loop to run: i < 5. If the condition is true, the loop will run again; if it is false, the loop ends.
• Statement 3 increases a value each time the code block has run: i++

Print Even Numbers

This example prints even values between 0 and 10:

Example

for (int i = 0; i <= 10; i = i + 2) {
  System.out.println(i);
}

Sum of Numbers

This example calculates the sum of numbers from 1 to 5:

Example

int sum = 0;
for (int i = 1; i <= 5; i++) {
  sum = sum + i;
}
System.out.println("Sum is " + sum);

Countdown

This example prints a countdown from 5 to 1:

Example

for (int i = 5; i > 0; i--) {
  System.out.println(i);
}

Nested Loops

It is also possible to place a loop inside another loop. This is called a nested loop.

The “inner loop” will be executed one time for each iteration of the “outer loop”:

Example

// Outer loop
for (int i = 1; i <= 2; i++) {
  System.out.println("Outer: " + i); // Executes 2 times
  
  // Inner loop
  for (int j = 1; j <= 3; j++) {
    System.out.println(" Inner: " + j); // Executes 6 times (2 * 3)
  }
} 

Multiplication Table Example

This example uses nested loops to print a simple multiplication table (1 to 3):

Example

for (int i = 1; i <= 3; i++) {
  for (int j = 1; j <= 3; j++) {
    System.out.print(i * j + " ");
  }
  System.out.println();
}

1 2 3
2 4 6
3 6 9

Nested loops are useful when working with tables, matrices, or multi-dimensional data structures.

The for-each Loop

There is also a “for-each” loop, which is used exclusively to loop through elements in an array (or other data structures):

Syntax

for (type variableName : arrayName) {
  // code block to be executed
}

The following example outputs all elements in the cars array, using a “for-each” loop:

Example

String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
for (String i : cars) {
  System.out.println(i);
}

Real Life Examples

To demonstrate a practical example of the for loop, let’s create a program that counts to 100 by tens:

Example

for (int i = 0; i <= 100; i += 10) {
  System.out.println(i);
} 

In this example, we create a program that only print even values between 0 and 10:

Example

for (int i = 0; i <= 10; i = i + 2) {
  System.out.println(i);
}

And in this example, we create a program that prints the multiplication table for a specified number:

Example

int number = 2;

// Print the multiplication table for the number 2
for (int i = 1; i <= 10; i++) {
  System.out.println(number + " x " + i + " = " + (number * i));
} 

Blogroll

---

Florida Politics – Campaigns u0026amp; Elections. Lobbying u0026amp; Government.

---

Social, Cultural u0026amp; Behavioral Issues in PHC u0026amp; Global Health

Social, Cultural and Behavioral Foundations of Primary Health Care (JHSPH)

---

Penny Zeller

Random thoughts from a day in the life of a wife, mom, and author