1. Boolean logic

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

Logical operator Expectation Reality
AND (∧) and &&
OR (∨) or ||
NOT (¬) not !

Here are some examples of using logical operators in Java:

Expression Interpretation Explanation
(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:

Code Explanation
int age = 35;
if (age >= 18 && age <= 65)
   System.out.println("You can work");
If the value of age is between 18 and 65, then the phrase "You can work" is displayed.
int age = 35;
boolean isYoung = (age < 18);
if (!isYoung && age <= 65)
   System.out.println("You can work");
We created an isYoung variable and moved the first part of the expression into it. We simply replaced age >= 18 with age < 18.
int age = 35;
boolean isYoung = (age < 18);
boolean isOld = (age > 65);
if (!isYoung && !isOld)
   System.out.println("You can work");
We created an isOld variable and moved the second part of the expression into it. Additionally, we replaced age <= 65 with age > 65.

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.

Expression Result
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.

Expression Result
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.

Expression Result
!true
false
!false
true

The operator changes true to false and vice versa.

Useful expressions:

Expression Result
m && !m
false
m || !m
true
!(a && b)
!a || !b
!(a || b)
!a && !b