For Loop in Java

October 22, 2022

The for loop in Java is the loop that is typically most used in programming. For loops are great when we know how many times we want the loop to execute. For example when looping through an entire array, we can use the array's size to tell us how many times to loop.

For Loop Concepts

As with the while and do-while loops, for loops consist of the same structure, just written in a different syntax. In a for loop, we write the initialization, terminating condition, and update all on one line. Here's an example of printing the word "Hello" 3 times.

for (int i = 0; i < 3; i++) {
  System.out.println("Hello");
}

We can see that the variable i is initialized with a value of 0. Then i is evaluated by the terminating condition to see whether we should execute the loop body. After the loop body runs, then the update where we increase the value of i by 1 is executed.

Array Example

Consider this example of printing all the items in an array:

int[] items = new int[]{ 1, 2, 3, 4, 5 };
for (int i = 0; i < items.length; i++) {
  System.out.println(items[i]);
}

Since we know that the number of times the loop should execute is the amount of items in the array, the for loop is useful.

Why Use a For Loop Over a While or Do-While Loop?

In the while and do-while loop, we have to create an initialization variable before the loop block. This means if we have multiple while or do-while loops, we may end up reusing the initialization variable, which could make the code harder to read. With for loops, once the loop is done executing we no longer have access to the initialization variable. This means we can redeclare an initialization variable with the same name if we were to have multiple for loops in a function.

In a while loop, we can access the counter variable still when the loop is done executing:

int counter = 0;
while (counter < 3) {
  System.out.println("Hello");
  counter++;
}

System.out.println(counter);

Same for a do-while loop:

int counter = 0;
do {
  System.out.println("Hello");
  counter++;
} while (counter < 3);

System.out.println(counter);

However, outside a for loop, we do not have access to the variable outside the loop block:

for (int counter = 0; counter < 3; counter++) {
  System.out.println("Hello");
}

// Will cause an error!
System.out.println(counter);

The advantage of this is that the initialization variable is only accessible within the for loop body and cannot be called outside it. This makes the code easier to read and prevent bugs that can happen when we try to reuse initialization variables as with the while and do-while loops.

Conclusion

The for loop is very handy and an important concept to learn in programming languages as they are probably the most common loop you will write.