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:
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 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:
// infinite loop
for ( ; ; ) {
// your code goes here
}
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:
int[] numbers = {1,2,3,4,5,6,7,8,9,10};
The following program, EnhancedForDemo, 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 for statement instead of
the general form whenever possible.
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