Controlling the Flow of Your Program - Beginning Java Programming: The Object-Oriented Approach - page 131

table 5-1: Control Operators for Primitive Data Types

oper ator Java synta x english eQuivalent

x == y
x != y

Equality Is x equal to y ?

Is x not equal to y ?

Relational x < y

x <= y
x > y
x >= y

Is x less than y ?
Is x less than or equal to y ?
Is x greater than y ?
Is x greater than or equal to y ?

Logical x && y

x || y
!x

Are x and y both true?
Is x , y , or both true?
Is x false?

Logical operators, on the other hand, are specific to Booleans, and are used to combine or negate
one or more conditions. There are three logical operators: AND ( && ), OR ( || ), and NOT ( ! ). If two
or more Boolean operands are joined using the AND operator, all must evaluate to true for the
overall expression to evaluate to true . If they are joined using the OR operator, at least one of them
must evaluate to true in order for the overall expression to evaluate to true . Finally, if the NOT
operator precedes a Boolean operand that evaluates to true , the overall expression will evaluate to
false and vice versa. It evaluates as the opposite of the original expression.
Truth tables are used in logic to show the outcome of Boolean operators on pairs of statements. In
the first two columns are two statements that can be true or false . In the columns that follow,
operators are listed with their results based on whether the statements are true or false . Table 5-2
is a truth table demonstrating the Boolean operators discussed here—namely NOT, AND, and OR.

table 5-2: Boolean Operator Truth Table

p Q !p

not p

p && Q
p and Q

p || Q
p or Q

TRUE TRUE ! TRUE =FALSE True AND True = TRUE True OR True = TRUE

TRUE FALSE True AND False = FALSE True OR False = TRUE

FALSE TRUE ! FALSE =TRUE False AND True = FALSE False OR True = TRUE

FALSE FALSE False AND False = FALSE False OR False = FALSE

In fact, you probably encounter these kinds of logical operators in your everyday life. For example, a
public transit system might follow a certain schedule if the day is Saturday OR a holiday and another
schedule otherwise. In other words, if (day == Saturday || day == holiday) , then follow the
weekend schedule. Then it is understood that if it is Saturday or a public holiday or both, the entire
statement is true . On the other hand, you might have a rule that states if a person is over 60 years old
AND they possess a bus card, then their fare is reduced, or if (age > 60 && busCard == true) ,