SELECTIONS - Introduction to Java Programming: Comprehensive Version - page 84

if (radius >= 0 )
area = radius * radius * PI;
System.out.println( "The area "
+ " is " + area);

if (radius >= 0 ){
area = radius * radius * PI;
System.out.println( "The area "
+ " is " + area);
}

(a) Wrong

(b) Correct

Common Error 2: Wrong Semicolon at the if Line

Adding a semicolon at the end of an if line, as shown in (a) below, is a common mistake.

Logic error

Empty block

if (radius >= 0 );
{
area = radius * radius * PI;
System.out.println( "The area "
+ " is " + area);
}

if (radius >= 0 ) { };
{
area = radius * radius * PI;
System.out.println( "The area "
+ " is " + area);
}

Equivalent

(a)

(b)

This mistake is hard to find, because it is neither a compile error nor a runtime error; it is a
logic error. The code in (a) is equivalent to that in (b) with an empty block.
This error often occurs when you use the next-line block style. Using the end-of-line block
style can help prevent this error.

Common Error 3: Redundant Testing of Boolean Values

To test whether a boolean variable is true or false in a test condition, it is redundant to
use the equality testing operator like the code in (a):

if (even == true )
System.out.println(
"It is even." );

if (even)
System.out.println(
"It is even." );

Equivalent

This is better

(a)

(b)

Instead, it is better to test the boolean variable directly, as shown in (b). Another good
reason for doing this is to avoid errors that are difficult to detect. Using the = operator instead
of the == operator to compare the equality of two items in a test condition is a common error.
It could lead to the following erroneous statement:

if (even = true )
System.out.println( "It is even." );

This statement does not have compile errors. It assigns true to even , so that even is
always true .

Common Error 4: Dangling else Ambiguity

The code in (a) below has two if clauses and one else clause. Which if clause is matched
by the else clause? The indentation indicates that the else clause matches the first if clause.