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Syntax of a while statement
The general syntax of a while statement is shown in Figure 2 .
| Figure 2 . Syntax of a while statement. |
|---|
while (conditional expression)
statement or compound statement; |
Behavior of a while statement
The three pillars of procedural programming are
The while statement is commonly used to create a loop structure, often referred to as a while loop .
Once the while statement is encountered in the sequence of code, the program will continue to executethe statement or compound statement shown in Figure 2 for as long as the conditional expression evaluates to true. (Note that a compound statement is created by enclosing two or more statements inside a pair of matching curly brackets, thus creating ablock of code as the body of the while statement or loop.)
Sample Java while statement
The while statement shown in Listing 1 was extracted from a Java program in an earlier module.
| Listing 1 . Sample Java while statement. |
|---|
while( (ch1 = System.in.read() ) != '#')
ch2 = ch1; |
The in variable of the System class
The System class defines a class variable named in . Because it is a class variable, it can be accessed using the name of the System class without the requirement to instantiate an object of the System class.
What the in variable contains
The in variable refers to an instance of a class that provides a read method that returns a character from the standard input device (typically the keyboard) .
Therefore, the expression System.in.read() in Listing 1 constitutes a call to the read method of the object referred to by the in variable of the System class.
A while loop is an entry condition loop
The while statement is used to form an entry condition loop. The significance of an entry condition loop is that the conditional expression is tested before the statements in the loop areexecuted. If it tests false initially, the statements in the loop are never executed.
The while loop shown in Listing 1 will continue reading characters from the keyboard for as long as the characterentered is not the # character. (Recall the not equal (!=) operator from an earlier module.)
The general syntax of an if-else statement is shown in Figure 3 .
| Figure 3 . Syntax of an if-else statement. |
|---|
if(conditional expression)
statement or compound statement;else //optional
statement or compound statement; //optional |
The if-else statement is the most basic of the statements used to control the logical flow of a Java program. It is used to satisfy the selection pillar mentioned earlier .
This statement will execute a specified block of code if some particular condition is true, and optionally, will execute a different block of code if thecondition is not true.
The else clause shown in Figure 3 is optional. If it is not provided and the condition is not true, control simply passes to the next statement following the If statement with none of the code in the body of the if statement being executed. If the condition is true, the code in the body of the if statement is executed.
If the else clause is provided and the condition is true, the code in the body of the if clause is executed and the code in the body of the else clause is ignored.
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