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The if-then and if-then-else Statements

Theif-then Statement

Theif-then statement is the most basic of all the control flow statements. It tells your program to execute a certain section of codeonly if a particular test evaluates totrue. For example, theBicycle class could allow the brakes to decrease the bicycle's speedonly if the bicycle is already in motion. One possible implementation of theapplyBrakes method could be as follows:

void applyBrakes() {    // the "if" clause: bicycle must be moving    if (isMoving){         // the "then" clause: decrease current speed        currentSpeed--;    }}

If this test evaluates tofalse (meaning that the bicycle is not in motion), control jumps to the end of theif-then statement.

In addition, the opening and closing braces are optional, provided that the "then" clause contains only one statement:

void applyBrakes() {    // same as above, but without braces     if (isMoving)        currentSpeed--;}

Deciding when to omit the braces is a matter of personal taste. Omitting them can make the code more brittle. If a second statement is later added to the "then" clause, a common mistake would be forgetting to add the newly required braces. The compiler cannot catch this sort of error; you'll just get the wrong results.

Theif-then-else Statement

Theif-then-else statement provides a secondary path of execution when an "if" clause evaluates tofalse. You could use anif-then-else statement in theapplyBrakes method to take some action if the brakes are applied when the bicycle is not in motion. In this case, the action is to simply print an error message stating that the bicycle has already stopped.

void applyBrakes() {    if (isMoving) {        currentSpeed--;    } else {        System.err.println("The bicycle has already stopped!");    } }

The following program,IfElseDemo, assigns a grade based on the value of a test score: an A for a score of 90% or above, a B for a score of 80% or above, and so on.

class IfElseDemo {    public static void main(String[] args) {        int testscore = 76;        char grade;        if (testscore >= 90) {            grade = 'A';        } else if (testscore >= 80) {            grade = 'B';        } else if (testscore >= 70) {            grade = 'C';        } else if (testscore >= 60) {            grade = 'D';        } else {            grade = 'F';        }        System.out.println("Grade = " + grade);    }}

The output from the program is:

    Grade = C

You may have noticed that the value oftestscore can satisfy more than one expression in the compound statement:76 >= 70 and76 >= 60. However, once a condition is satisfied, the appropriate statements are executed(grade = 'C';) and the remaining conditions are not evaluated.