Flow Control and Exception Handling - for Statement

Syntax (JLS §14.13)

    for( initialization; boolean expression; iteration ) {
        statement(s);
    }

• executes some initialization code, then repeatedly executes a boolean expression and some iteration code until the expression is false
• all three parts are optional ie the following examples are legal

    for( intialization; ; )
    for( ; expression; iteration )
    for( ; ; iteration)
    for( ; ; )                 // endless loop    

Initialization

• initializes variables used within the loop
• if variables are declared within the loop, they are discarded after the loop completes
• For example, in the following code the initialization variable i is declared outside the for loop; so it's value is still available once the loop completes

    int i;
    for ( i=0; i<10 ; i++ ) {
        // do something
    }
    System.out.println("value of i: " + i );

• In the following code, x is declared and initialized inside the for-loop and is therefore only accessible within the loop

    for ( int x=0; x<10 ; x++ ) {
        // do something
    }
    // compile-error, cannot resolve symbol: x
    System.out.println("value of i: " + x );

• can be more than one initialization statement but the variables must either be declared outside the for-loop or the type for the variables must be declared at the beginning

Following compiles and runs ok:

    for( int x=10, y=0; x>y; x--, y++){
        System.out.println( x + "\t" + y);
    }
    
Following produces compile error    

    int x;
    
    for( x=10, int y=0; x>y; x--, y++){
        System.out.println( x + "\t" + y);
    }    

Boolean expression

• if the expression evaluates to true the loop continues; otherwise, the loop is exited and execution continues after the loop statment block

Iteration

• if the expression evaluates to true, the block statements are executed and then the iteration occurs
• if the expression evaluates to false, iteration does not occur

Break statement

• you can use a break statement to exit a for-loop at any time
• the break forces processing to the line following the for-loop statement block

    for( i=0; i<10; i++ ){
        if( i==5 ) break;
    }
    // process continues here after the break    

Continue statement

• you can use continue to force processing to the next loop iteration

    for( i=0; i<10; i++){
        if( i==5)
            continue;   // skip printing 5
        else
            System.out.println(i);
    }

Example Code

• TestFor.java
• Jaworski Exam Question 7, Chapter 5
• Jaworski Exam Question 8, Chapter 5

Tips

• all sections of the for() loop are optional

Traps

• attempting to access a variable declared in the initialization outside of the for-loop
• incorrect initialization expression
• non-boolean expression