The for Statement

Simple for Loop

The statementfor 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 ForDemouses the general form of the for statement to print the numbers 1 through 10 to standard output:

class ForDemo {
    public static void main(String[] args){
         for(int i=1; i<11; i++){
              System.out.println("Count is: " + i);
         }
    }
}

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 statementfor, 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,ij and k often used to control for loop; declaring them within the initialization expression limits their lifespan and reduces errors.

An Infinite for loop

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

// infinite loop
for ( ; ; ) {
    // your code goes here
}

Enhanced for Loop

Since Java 5, we have a second kind of for loop called the enhanced for which makes it easier to iterate over all elements in an array or a collection.

The syntax of the enhanced for loop is:

for (Type item: items){
  statement;
}

Since this loop is simplified in comparison to the standard for loop, we need to declare only two things when initializing a loop:

1. The handle for an element we’re currently iterating over

2. The source array/collection we’re iterating

Therefore, we can say that: For each element in items, assign the element to the item variable and run the body of the loop.

Let’s have a look at the simple example:

int [] intArr = { 0,1,2,3,4}; 
    for (int num : intArr) { 
      System.out.println("Enhanced for-each loop: i = "+ num);
}

We can use it to iterate over various Java’s data structures:

Given a List<String> list object – we can iterate it:

for (String item : list) {
    System.out.println(item);
}

We can similarly iterate over a Set<String> set:

for(String item : set) {
    System.out.println(item);
}

And, given a Map<String, Integer> map we can iterate over it as well:

for (Entry<String, Integer> entry : map.entrySet()) {
    System.out.println("Key: "+ entry.getKey() +" - "+"Value: "+ entry.getValue());
}

Let’s see a concrete example. consider the following array, which holds the numbers 1 through 10:

int[] numbers = {1,2,3,4,5,6,7,8,9,10};

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

class EnhancedForDemo {
    public static void main(String[] args){
         int[] numbers = 
             {1,2,3,4,5,6,7,8,9,10};
         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 statement for instead of the general form whenever possible.