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Another powerful tool in programming is the loop, which provides for repetitive execution of a section of code. In Java there are three such loop statements:

• for statement
• while statement
• do/while statement

for Statement

The for loop provides a neat and readable way to execute a loop with initialization, an update action for every repetition step, and a test for ending the loop all in one line:

    for(initialize ; boolean test ; step update ) statement ;

or

    for(initialize ; boolean test ; step update )
    {
       statement 1;
       statement 2;
        ...
    }

where

• initialize represents whatever initial setup actions you want to do before the looping begins.
• step update - after each loop this expression is executed.
• boolean test - after the initialization, after each loop and the step update, this Boolean expression is executed. If it returns false, the looping ends, if true the looping continues.
• statement - this statement executes once for each loop as long as the Boolean test is true. Can be a single statement or a compound statement containing a code block within parentheses.

A typical for loop:

    int i, j=1;
    for(i=0; i<10; i++) j = j +1;

which follows these steps

1. Assigns 0 to integer i
2. Checks if i still less than 10
3. Executes j = j + 1
4. Executes the i++ operation
5. If so then repeats from step 2,
6. Otherwise, it ends the loop and proceeds to the next statement

This is the most common type for loop but note that you can put any expressions you desire within the for loop parentheses as long as the Boolean test returns a Boolean value.

Here are some variations:

• Countdown:
  
     int k, m;
     for( k=10, m =1; k > 0; k--) m = m*k;

• Multiple expressions in initialization using comma to separate expressions:
  
      for(int i=0, j=0; i<10; i++ )
      {
        j = j*i;
        j++;
      }
  
  
• Multiple expressions in step update using comma to separate expressions:
  
     int k,m,n;
     for( k=10; k > 0; k--, m++) n = m*k;
  
  
• Empty loop expressions:
  
     for(; i<limit ;)
     { i = a.test();}
  

while Statement

Suppose you want to do an indefinite number of loops, stopping only when a particular condition occurs. It is possible to do this with a for loop and a break statement (see flow control). For example,

   for (;;)
   {
      ...
      if( i < 2) break;
   }
   next statement;

here the process will jump to the next statement following the loop if the i<2 test is true.

However, this does not make for very elegant code. The while statement provides a more readable option:

   while( test ) statement;

where statement will repeatedly execute as long as the Boolean test expression returns true. The statement can be a single statement or a compound statement of multiple statements enclosed by parentheses:

   int i = 5, j=1;
   while ( i >= 2 )
   {
       j = i*j;
       i--;
   }

do/while Statement

The test for the while statement occurs before the first loop through the code segment. So if the test returns false the first time, the statement will not execute.

Perhaps a situation occurs where you want the code to execute at least once. Then you can use the do-while statement:

   do
      statement;
   while( test );

where statement ( which, as usual, can can consist of a compound statement with multiple statements in parentheses) will execute at least once.

Note: The semicolon at the end of the while(test); is required.

For example,

   int i = 3;
   do{
       i = i - 2;
   } while ( i < 2);

Enhanced for loop (added with Java 5.0)

    for (type value : container) statement

This new feature of J2SE5.0 allows you to loop through a Collections container or an array with the short compact statement as shown. Here statement is executed for each element in the container. (The enhanced loop is also referred to as the "for-each loop.)

So the the following snippet:

   int [] an_array = {5, 6, 8, 10};
   int cnt = 0;
   for (int j : an_array) {
       System.out.println ("an_array[" + cnt +"] = " + j);
       cnt++;
   }

would give the following output:

  an_array[0] = 5
  an_array[1] = 6
  an_array[2] = 8
  an_array[3] = 10

The enhanced for loop works nicely with the generics feature discussed in Chapter 10 since all elements of a container are guaranteed in that case to be of a single type.

See also the Chapter 10: Collections Framework and Chapter 10: Generics for more about these.

References & Web Resources

• For-Each Loop - Java Programming Guide - J2SE5.0
• The Enhanced For Loop - Core Java Technical Tips - May 5, 2005.
• The For-Each Loop - Java Language Guide

Latest update: Oct. 19, 2005

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