Thefor
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 thefor
statement can be expressed as follows:When using this version of thefor (initialization; termination; increment) { statement(s) }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 thefor
statement to print the numbers 1 through 10 to standard output:The output of this program is:class ForDemo { public static void main(String[] args){ for(int i=1; i<11; i++){ System.out.println("Count is: " + i); } } }Notice how the code declares a local variable within the initialization expression. The scope of this variable extends from its declaration to the end of the block governed by theCount 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: 10for
statement, so it can be used in the termination and increment expressions as well. If the variable that controls afor
statement is not needed outside of the loop, it's best to declare the variable in the initialization expression. The namesi
,j
, andk
are often used to controlfor
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:for ( ; ; ) { // infinite loop // your code goes here }The
for
statement also has another form designed for iteration through collections (Chapter TBD) 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:
int[] numbers = {1,2,3,4,5,6,7,8,9,10};The following program,
EnhancedForDemo
, uses the enhancedfor
to loop through the array:
In this example, the local variableclass 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); } } }item
holds the current value from the numbers array. The output from this program is the same as before:We recommend using this form of theCount 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: 10for
statement instead of the general form whenever possible.