Java 天地·Core Java·Lecture Notes·Unitt 3 - Core Java Programming Concepts

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Content Page # 46

Boolean expressions

A boolean expression is one that evaluates to a result of either true or false.

There are several operators that return a boolean result. These operators are generally from two categories:

comparison operators ?such as those for less than '<' and greater than '>', and test for equality '=='
logical operators ?such as those for logical AND '&&' and logical OR '||'

For example, we might want to define a loop that continues while one number is less than 10, and also another is greater than 100. If the numbers are stored in variables called x and y, then in Java this would be written:

while( (x < 10) && (y > 100) )
{
//... more statements here
}

A question frequently asked is why the symbols used for comparisons are so arcane. For example, it is possible to envisage a programming language where the statement above could be written:

while (x isLessThan 10) and (y isGreaterThan 100)
{
//... more statements here
}

However, this would not make sense to Java. Moreover, in Java the symbols & and && are both read as `and', but they are different kinds of 'and'!

The reason these symbols are used is that the Java developers wanted to use the same symbols as C++, so that C++ programmers could learn Java easily. The reason that C++ uses these symbols is that the C++ developers wanted to use the same symbols as C, so that C programmers could learn C++ easily.

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