Control Statements I

A programming language uses control statements to cause the flow of execution to advance and branch based on changes to the state of a program. Java's program control statements can be put into the following categories: selection, iteration, and jump. Selection statements allow your program to choose different paths of execution based upon the outcome of an expression or the state of a variable. Iteration statements enable program execution to repeat one or more statements (that is, iteration statements form loops). Jump statements allow your program to execute in a nonlinear fashion. All of Java's control statements are examined here.

 

If you know C/C++/C#, then Java's control statements will be familiar territory. In fact, Java's control statements are nearly identical to those in those languages. However, there are a few differences—especially in the break and continue 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

The if statement was introduced in. 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) statement1;

else statement2;

 

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 the condition 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;

 

Here, if a is less than b, then a is set to zero. Otherwise, b is set to zero. In no case are they both set to zero. Most often, the expression used to control the if will involve the relational operators. However, this is not technically necessary. It is possible to control the if using a single Boolean variable, as shown in this code fragment:

 

boolean dataAvailable;

// ...

if (dataAvailable)

ProcessData();

else

waitForMoreData();

 

Remember, only one statement can appear directly after the if or the else. If you want to include more statements, you'll need to create a block, as in this fragment:

 

int bytesAvailable;

// ...

if (bytesAvailable > 0) {

ProcessData();

bytesAvailable -= n;

} else

waitForMoreData();

 

Here, both statements within the if block will execute if bytesAvailable is greater than zero. Some programmers find it convenient to include the curly braces when using the if, even when there is only one statement in each clause. This makes it easy to add another statement at a later date, and you don't have to worry about forgetting the braces. In fact, forgetting to define a block when one is needed is a common cause of errors.

 

For example, consider the following code fragment:

 

int bytesAvailable;

// ...

if (bytesAvailable > 0) {

ProcessData();

bytesAvailable -= n;

} else

waitForMoreData();

bytesAvailable = n;

 

It seems clear that the statement bytesAvailable = n; was intended to be executed inside the else clause, because of the indentation level. However, as you recall, whitespace is insignificant to Java, and there is no way for the compiler to know what was intended. This code will compile without complaint, but it will behave incorrectly when run.

 

The preceding example is fixed in the code that follows:

 

int bytesAvailable;

// ...

if (bytesAvailable > 0) {

ProcessData();

bytesAvailable -= n;

} else {

waitForMoreData();

bytesAvailable = n;

}

 

Nested ifs

A nested if is an if statement that is the target of another if or else. Nested ifs are very common in programming. When you nest ifs, 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.

 

Here is a program that uses an if-else-if ladder to determine which season a particular month is in.

 

// Demonstrate if-else-if statements.

class IfElse {

public static void main(String args[]) {

int month = 4; // April

String season;

if(month == 12 || month == 1 || month == 2)

season = "Winter";

else if(month == 3 || month == 4 || month == 5)

season = "Spring";

else if(month == 6 || month == 7 || month == 8)

season = "Summer";

else if(month == 9 || month == 10 || month == 11)

season = "Autumn";

else

season = "Bogus Month";

System.out.println("April is in the " + season + ".");

}

}

 

Here is the output produced by the program:

April is in the Spring.

 

You might want to experiment with this program before moving on. As you will find, no matter what value you give month, one and only one assignment statement within the ladder will be executed.