CS 99

Summer 2001 7.09

Lecture Notes 4

Statements

A statement in Java is basically a line of code terminated by a semicolon. Declarations and statements can be grouped into a block using braces, {}. This is very important, as a block is a compound statement which can be used to group zero or more local declarations and statements; it can be used in any context where a simple statement is permitted.

Statements in Java can be grouped into various categories:

· declaration statements

Examples:

int x; double y;

TokenReader in;

· flow control statements

Examples:

if ( x > y )

x = 2*y;

while ( z < 10 ) {

System.out.println("2 to the power of " + z + " = " + Math.pow( 2, z++ ) );

}

· expression statements
Only certain types of expressions have meaning as statements

· Assignments

Examples:

x = 10;

new_balance = old_balance*(1 + bank_rate);

· Increment and decrement operators

Examples:

x++;

y--;

++z;

· Method calls

Example:

int n = s.length();

· Object creation with the new operator

Examples:

TokenReader in = new TokenReader( System.in );

Triangle t = new Triangle( 3, 4, 5 );

Booleans

The boolean datatype is used to represent logical values that can be either the literal true or the literal false.

Boolean expressions have boolean datatype and can only evaluate the values true or false.

Boolean expressions, when used as conditionals in control statements, allow the control flow in a program to be changed during execution.

Relational Operators <, <=, >, >=

Equality: ==, !=

Boolean Logical Operators: !, ^, &, |

Conditional Operators: &&, ||

Examples of boolean expressions:

Simple if statement

The simple if statement has the following syntax

if ( <conditional expression> )

<statement>

Note that the statement can be a block.

if-else statement

if ( <conditional statement> )

<statement1>

else

<statement2>

The use of block notation {} can be critical to the meaning of if statements. The Java compiler always associates an else with the previous if unless told to do otherwise by braces. This is sometimes known as the dangling-else problem:

if ( x > 5 )

if ( y > 5)

System.out.println( "x and y are > 5" );

else

System.out.println( "x is <=5" );

The person who wrote this clearly wanted the else to be matched with the first if, for two reasons: the program is indented to suggest that that is it's purpose, and the println statement only makes sense if it were that way. However, Java matches an else with the last if if there are no braces to tell it to do otherwise. So to the computer, the code actually looks like this:

if ( x > 5 )

if ( y > 5)

System.out.println( "x and y are > 5" );

else

System.out.println( "x is <=5" );

Surely not what the programmer intended. To fix the segment, the programmer should insert braces around the second if statement:

if ( x > 5 ) {

if ( y > 5)

System.out.println( "x and y are > 5" );

}

else

System.out.println( "x is <=5" );

Now the program segment works properly.

Repetition Structures

Loops allow a block of statements to be executed repeatedly. A boolean condition is commonly used to determine when to terminate the loop, called the loop condition. The statements executed in the loop constitute the loop body. The loop body can be a single statement or a block.

while Statement

while ( <loop condition> )

<loop body>

Note that the loop condition is evaluated before executing the loop body. The while statement executes the loop body as long as the loop condition is true. When the loop condition becomes false, the loop is terminated and execution continues with the statement immediately following the loop.

do-while Statement

do

<loop body>

while ( <loop condition> );

The loop condition in this construct is evaluated after executing the loop body. Note that the loop body is executed at least once.

To illustrate the difference between these statements consider the following code segments:

while ( cat.isAway() ) {

mice.willPlay();

}

do {

mice.willPlay();

} while ( cat.isAway() );

In the first loop, the mice look to see if the cat is away before they go out to play; in the second loop, the mice play before they look to see if the cat is away. The second group of mice get to play at least once, but it might be their last playtime!