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 










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 


If the value of age is between 18 and 65 , then the phrase "You can work" is displayed. 

We created an isYoung variable and moved the first part of the expression into it. We simply replaced age >= 18 with age < 18 . 

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 









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 









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 





The operator changes true
to false
and vice versa.
Useful expressions:
Expression  Result 









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