Logical operators

1. Boolean logic

In Java, you can't write the expression 18 < age <65. That is incorrect syntax and the program will not compile.

But you can write it like this:

            (18 < age) AND (age < 65)
        

Of course, instead of the word AND, there would be a logical operator. We'll talk about them in more detail now.

There are three logical operators in Java: AND (&&), OR (||) and NOT (!).

The good news is that you can use parentheses to construct logical expressions of any complexity.

The bad news is that Java developers decided to use notation from the C language instead of the words and, or and not.

Look at the screen:

Here are some examples of using logical operators in Java:

(0 < a) && (a < 100)

(0 < a) and (a < 100)

(0 < a) AND (a < 100)

(!a) && (!b)

(not a) and (not b)

(NOT a) AND (NOT b)

!(!a || !b)

not((not a) or (not b))

NOT((NOT a) OR (NOT b))

2. Examples of using comparison operators and boolean variables

Wherever you can write a logical expression, you can write a logical variable.

Example:

int age = 35;
if (age >= 18 && age <= 65)
   System.out.println("You can work");

int age = 35;
boolean isYoung = (age < 18);
if (!isYoung && age <= 65)
   System.out.println("You can work");

int age = 35;
boolean isYoung = (age < 18);
boolean isOld = (age > 65);
if (!isYoung && !isOld)
   System.out.println("You can work");

These three examples are equivalent. Only in the second example did we move part of the expression from the if statement into a separate boolean variable (isYoung). In the third example, we moved the second part of the expression into a second variable (isOld).

3. Logical arithmetic

Let's briefly go through logical operations.

The AND operator is &&, also known as conjunction.

true && true

true

true && false

false

false && true

false

false && false

false

In other words, the result of an expression is true only if both values that make up the expression are true. Otherwise, it is always false.

The OR operator is ||, also known as disjunction.

true || true

true

true || false

true

false || true

true

false || false

false

In other words, the result of an expression is always true if at least one term in the expression is true. If both are false, then the result is false.

The NOT operator is !, also known as the logical inverse.

!true

false

!false

true

The operator changes true to false and vice versa.

Useful expressions:

m && !m

false

m || !m

true

!(a && b)

!a || !b

!(a || b)

!a && !b