CONTROL STATEMENTS

Java’s Selection Statements

Java supports two selection statements: if and switch . These statements allow you to control the flow of your program’s execution based upon conditions known only during run time. You will be pleasantly surprised by the power and flexibility contained in these two statements.

if

It is examined in detail here. The if statement is Java’s conditional branch statement. It can be used to route program execution through two different paths. Here is the general form of the if statement:

if (condition ) statement 1 ;

else statement 2 ;

Here, each statement may be a single statement or a compound statement enclosed in curly braces (that is, a block ). The condition is any expression that returns a boolean value.

The else clause is optional.

The if works like this: If thecondition is true, then statement1 is executed. Otherwise, statement2 (if it exists) is executed. In no case will both statements be executed. For example, consider the following:

int a, b;

// ...

if(a < b) a = 0;

else b = 0;

Nested ifs

A nested if is an if statement that is the target of another if or else . Nested if s are very

common in programming. When you nest if s, the main thing to remember is that an

else statement always refers to the nearest if statement that is within the same block

as the else and that is not already associated with an else . Here is an example:

if(i == 10) {

if(j < 20) a = b;

if(k > 100) c = d; // this if is

else a = c;        // associated with this else

}

else a = d;          // this else refers to if(i == 10)

As the comments indicate, the final else is not associated with if(j<20), because it is not in the same block (even though it is the nearest if without an else ). Rather, the final else is associated with if(i==10) . The inner else refers to if(k>100), because it is the closest if within the same block.

The if-else-if Ladder

A common programming construct that is based upon a sequence of nested ifs is the if-else-if ladder . It looks like this:

if( condition )

statement;

else if (condition)

statement ;

else if( condition )

statement ;

.

.

else

statement ;

The if statements are executed from the top down. As soon as one of the conditions controlling the if is true , the statement associated with that if is executed, and the rest of the ladder is bypassed. If none of the conditions is true, then the final else statement will be executed. The final else acts as a default condition; that is, if all other conditional tests fail, then the last else statement is performed. If there is no final else and all other conditions are false, then no action will take place.

switch

The switch statement is Java’s multiway branch statement. It provides an easy way to dispatch execution to different parts of your code based on the value of an expression.As such, it often provides a better alternative than a large series ofif-else-if statements.

Here is the general form of a switch statement:

switch ( expression) {

case value1 :

// statement sequence

break;

case value2 :

// statement sequence

break;

.

.

case valueN :

// statement sequence

break;

default:

// default statement sequence

}

The expression must be of type byte, short, int, or char ; each of the values specified in the case statements must be of a type compatible with the expression. Each case value must be a unique literal (that is, it must be a constant, not a variable). Duplicate case values are not allowed.

// A simple example of the switch.

class SampleSwitch {

public static void main(String args[]) {

for(int i=0; i<6; i++)

switch(i) {

case 0:

System.out.println("i is zero.");

break;

case 1:

System.out.println("i is one.");

break;

case 2:

System.out.println("i is two.");

break;

case 3:

System.out.println("i is three.");

break;

default:

System.out.println("i is greater than 3.");

}

}

}

The output produced by this program is shown here:

i is zero.

i is one.

i is two.

i is three.

i is greater than 3.

i is greater than 3.

Nested switch Statements

You can use a switch as part of the statement sequence of an outerswitch . This is called a nested switch . Since a switch statement defines its own block, no conflicts arise between the case constants in the inner switch and those in the outer switch . For example, the following fragment is perfectly valid:

switch(count) {

case 1:

switch(target) { // nested switch

case 0:

System.out.println("target is zero");

break;

case 1: // no conflicts with outer switch

System.out.println("target is one");

break;

}

break;

case 2: // ...

Here, the case 1: statement in the inner switch does not conflict with the case 1: statement in the outer switch. The count variable is only compared with the list of cases at the outer level. If count is 1, then target is compared with the inner list cases.