Decisions - Java Concepts - page 287

A !A

true false

false true

C OMMON E RROR 5.3: Multiple Relational Operators

Consider the expression

if (0 < amount < 1000) . . . // Error

This looks just like the mathematical notation for Ðamount is between 0 and
1000ȓ. But in Java, it is a syntax error.

Let us dissect the condition. The first half, 0 < amount , is a test with outcome
true or false . The outcome of that test ( true or false ) is then compared
against 1000. This seems to make no sense. Is true larger than 1000 or not?
Can one compare truth values and numbers? In Java, you cannot. The Java
compiler rejects this statement.

Instead, use && to combine two separate tests:

if (0 > amount && amount > 1000) . . .

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Another common error, along the same lines, is to write

if (ch == 'S' || 'M') . . . // Error

to test whether ch is ÒSÓ or ÒMÓ . Again, the Java compiler flags this construct
as an error. You cannot apply the || operator to characters. You need to write
two Boolean expressions and join them with the || operator:

if (ch == 'S' || ch == 'M') . . .

C OMMON E RROR 5.4: Confusing && and || Conditions

It is a surprisingly common error to confuse and and or conditions. A value lies
between 0 and 100 if it is at least 0 and at most 100. It lies outside that range if it
is less than 0 or greater than 100. There is no golden rule; you just have to think
carefully.