Java 天地·Core Java·Lecture Notes·Unitt4 - Control of execution

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Content Page # 3

Selection: IF...ELSE

An if...else statement works in a similar way to an if statement, except that it also provides a statement to be executed if the test is false.

Using pseudocode code we can illustrate how an if..else statement works. The general form of an if...else statement is as follows:

if( test )
statement to execute if test was true
else
statement to execute if test was false

Let us extend our pseudocode design for the fire alarm from Unit 3, making the device flash a green light if all is well (i.e. if the temperature is not too high):

if ( temperature to high )
sound the alarm
else
display a flashing green light to show alarm is working

This might be implemented in Java as follows:

if (temperature > 78)
soundAlarm();
else
displayFlashingLight( green );

The way an if...else statement can cause deviation from the default sequential flow of execution can be illustrated as follows. Consider this pseudocode design:

statementA;
if( age > 60 )
statementB;
else
statementC;
statementD;

If the variable age has a value over 60, say 65, then the test ( age > 60 ) will be true, and the therefore statementB will be executed. So the statements to be executed will be:

statementA
statementB
statementD

However, if the variable age has a value of 60 or less, say 21, then the test ( age > 60 ) will be false, and statementB will not be executed. However, since the test is false the statement after else will be executed. So the statements executed will be:

statementA
statementC
statementD

Thus the result of an if...else selection statement means that either one or some other statement(s) will be executed, but not both.

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