Java Control Flow Statements

Java Control statements control the order of execution in a java program, based on data values and conditional logic. There are three main categories of control flow statements;

• Selection statements: if, if-else and switch.
• Loop statements: while, do-while and for.
• Transfer statements: break, continue, return, try-catch-finally and assert.

We use control statements when we want to change the default sequential order of execution

Selection Statements

The If Statement

The if statement executes a block of code only if the specified expression is true. If the value is false, then the if block is skipped and execution continues with the rest of the program. You can either have a single statement or a block of code within an if statement. Note that the conditional expression must be a Boolean expression.

The simple if statement has the following syntax:

if ()

The If-else Statement

The if/else statement is an extension of the if statement. If the statements in the if statement fails, the statements in the else block are executed. You can either have a single statement or a block of code within if-else blocks. Note that the conditional expression must be a Boolean expression.

The if-else statement has the following syntax:
if (
)
else

Switch Case Statement:

The switch case statement, also called a case statement is a multi-way branch with several choices. A switch is easier to implement than a series of if/else statements. The switch statement begins with a keyword, followed by an expression that equates to a no long integral value. Following the controlling expression is a code block that contains zero or more labeled cases. Each label must equate to an integer constant and each must be unique. When the switch statement executes, it compares the value of the controlling expression to the values of each case label. The program will select the value of the case label that equals the value of the controlling expression and branch down that path to the end of the code block. If none of the case label values match, then none of the codes within the switch statement code block will be executed. Java includes a default label to use in cases where there are no matches. We can have a nested switch within a case block of an outer switch. Its general form is as follows:

switch () {
case label1:
case label2:
…
case labeln:
default:
} // end switch

When executing a switch statement, the program falls through to the next case. Therefore, if you want to exit in the middle of the switch statement code block, you must insert a break statement, which causes the program to continue executing after the current code block.

Iteration Statements

While Statement

The while statement is a looping construct control statement that executes a block of code while a condition is true. You can either have a single statement or a block of code within the while loop. The loop will never be executed if the testing expression evaluates to false. The loop condition must be a boolean expression.
The syntax of the while loop is
while ()



Do-while Loop Statement

The do-while loop is similar to the while loop, except that the test is performed at the end of the loop instead of at the beginning. This ensures that the loop will be executed at least once. A do-while loop begins with the keyword do, followed by the statements that make up the body of the loop. Finally, the keyword while and the test expression completes the do-while loop. When the loop condition becomes false, the loop is terminated and execution continues with the statement immediately following the loop. You can either have a single statement or a block of code within the do-while loop.

The syntax of the do-while loop is
do

while ();

For Loops

The for loop is a looping construct which can execute a set of instructions a specified number of times. It’s a counter controlled loop.

The syntax of the loop is as follows:

for (; ; )


The first part of a for statement is a starting initialization, which executes once before the loop begins. The section can also be a comma-separated list of expression statements. The second part of a for statement is a test expression. As long as the expression is true, the loop will continue. If this expression is evaluated as false the first time, the loop will never be executed. The third part of the for statement is the body of the loop. These are the instructions that are repeated each time the program executes the loop. The final part of the for statement is an increment expression that automatically executes after each repetition of the loop body. Typically, this statement changes the value of the counter, which is then tested to see if the loop should continue.All the sections in the for-header are optional. Any one of them can be left empty, but the two semicolons are mandatory. In particular, leaving out the signifies that the loop condition is true. The (;;) form of for loop is commonly used to construct an infinite loop.

Continue Statement

A continue statement stops the iteration of a loop (while, do or for) and causes execution to resume at the top of the nearest enclosing loop. You use a continue statement when you do not want to execute the remaining statements in the loop, but you do not want to exit the loop itself.

The syntax of the continue statement is
continue; // the unlabeled form
continue