Type boolean
In some programming languages, for example Matlab and C, integers are used to represent the logical values
true and false. Generally, 0 is used for false, and any other integer can be used for true.
Java handles boolean values differently. There is a (primitive) type boolean, whose values are true and
false (that's it). This type has five operations whose operands are booleans:
Type boolean
Values: true, false
Operations: ! (not), && (and, or conjunction), || (or, or disjunction)
== (equality, or equivalence), != (inequality, or inequivalence)
Here is a table that defines the five operations.
b c | !b b && c b || c b == c b != c
false false | true false false true false
false true | true false true false true
true false | false false true false true
true true | false true true true false
We evaluate a few expressions in the interactions pane and discuss the operations.
1. !, which is read "not", is unary logical negation !false is true, and !true is false.
2. && is read and because b && c is true iff both b and c are true.
3. || is read or because b || c is true iff either b or c (or both) is true.
4. == is used for equality: b == c is true iff b and c have the same value.
5. != is used for inequality: b != c is true iff b and c have different values.
Relations
Six relations operate on the numeric types to yield boolean values.
b == c, b != c, b < c, b <= c, b > c, b >= c
You have probably seen these relations in other programming languages, so we don’t go into full details
here. The only strange point is that == is used for equality, and not =. Here are examples.
5 < 6 is true
5 >= 6 is false
5 < true is illegal because one operands is an int and the other a boolean.
These relational operators work for all the number types — int, double, char, etc. For example, we can test
whether 5 < 6.2 is true, or whether 6.0 == 7 is true. If the two operands are not of the same type, one is
converted to the other type so that the operation can be carried out. More on such conversions later.
Short circuit evaluation
Evaluation of
5/0 == 3 && false
results in an error, because of the division by 0. This is to be expected. But evaluation of the same expression
with the operands reversed,
false && 5/0 == 3
does not produce an error message —it yields the value false. This is because evaluation of && is done in short-
circuit mode: as soon as the answer is known, evaluation stops. Since false && b is always false, no matter
what b is, there is no need to evaluate b.
Another way to look at the evaluation of b && c is to say that it is equivalent to an if-expression if b then c else
false, which can actually be written in Java using the expression
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