The JavaTM Tutorial
Previous Page Lesson Contents Next Page Start of Tutorial > Start of Trail > Start of Lesson Search
Feedback Form

Trail: Learning the Java Language
Lesson: Language Basics

Summary of Control Flow Statements

For controlling the flow of a program, the Java programming language has three loop constructs, a flexible if-else statement, a switch statement, exception-handling statements, and branching statements.

Loops

Use the while statement to loop over a block of statements while a boolean expression remains true. The expression is evaluated at the top of the loop:
while (boolean expression) {
    statement(s)
}

Use the do-while statement to loop over a block of statements while a boolean expression remains true. The expression is evaluated at the bottom of the loop, so the statements within the do-while block execute at least once:

do {
    statement(s)
} while (expression);

The for statement loops over a block of statements and includes an initialization expression, a termination condition expression, and an increment expression:

for (initialization ; termination ; increment) {
    statement(s)
}

Decision-Making Statements

The Java programming language has two decision-making statements: if-else and switch. The more general-purpose statement is if; use switch to make multiple-choice decisions based on a single integer value.

The following is the most basic if statement whose single statement block is executed if the boolean expression is true:

if (boolean expression) {
    statement(s)
}
Here's an if statement with a companion else statement. The if statement executes the first block if the boolean expression is true; otherwise, it executes the second block:
if (boolean expression) {
    statement(s)
} else {
    statement(s)
}
You can use else if to construct compound if statements:
if (boolean expression) {
    statement(s)
} else if (boolean expression) {
    statement(s)
} else if (boolean expression) {
    statement(s)
} else {
    statement(s)
}
The switch statement evaluates an integer expression or enum and executes the appropriate case statement.
switch (integer expression) {
    case integer expression:
         statement(s)
         break;
    ...
    default:
         statement(s)
         break;
}
switch (expression of enum type) {
    case enum constant:
         statement(s)
         break;
    ...
    default:
         statement(s)
         break;
}

Exception-Handling Statements

Use the try, catch, and finally statements to handle exceptions:
try {
    statement(s)
} catch (exceptiontype name) {
    statement(s)
} catch (exceptiontype name) {
    statement(s)
} finally {
    statement(s)
}
Exception handling is covered in detail in the chapter Handling Errors with Exceptions (in the Learning the Java Language trail).

Branching Statements

Some branching statements change the flow of control in a program to a labeled statement. You label a statement by placing a legal identifier (the label) followed by a colon (:) before the statement:
statementName: someJavaStatement;
Use the unlabeled form of the break statement to terminate the innermost switch, for, while, or do-while statement:
break;
Use the labeled form of the break statement to terminate an outer switch, for, while, or do-while statement with the given label:
break label;
A continue statement terminates the current iteration of the innermost loop and evaluates the boolean expression that controls the loop:
continue;
The labeled form of the continue statement skips the current iteration of the loop with the given label:
continue label;
Use return to terminate the current method:
return;
You can return a value to the method's caller, by using the form of return that takes a value:
return value;

Previous Page Lesson Contents Next Page Start of Tutorial > Start of Trail > Start of Lesson Search
Feedback Form

Copyright 1995-2005 Sun Microsystems, Inc. All rights reserved.