The boolean type and logical operations

1. Boolean data type: boolean

As we have already seen, Java has a very useful operator—if-else. It executes one block of statements if the condition in parentheses is true, and a second block of statements if the condition is false.

To conveniently work with expressions that can be true or false, Java added a special type—boolean. Its key feature is that variables of this type can take only two values: true (true) and false (false).

It is impossible to assign any other values to variables of type boolean. The compiler will not allow it.

Examples of boolean variables:

boolean isAdult = true;
boolean hasMoney = false;

A variable of type boolean has only two values—true and false. That’s it! Almost like a test result—either “passed” or “failed.”

2. Comparison operations

So what is such a primitive type for? The real power of boolean shows when we build logical expressions. Usually they are formed using comparison operators.

a == b

a != b

a > b

a < b

a >= b

a <= b

Example:

int age = 20;
boolean isAdult = age >= 18;   // true (20 is greater than or equal to 18)
boolean isTeenager = age >= 13 && age < 18; // false (20 is not less than 18)
boolean isEven = age % 2 == 0; // true (20 is divisible by 2 with no remainder)

isAdult will be true here because 20 is indeed an “adult” age.

Important point 1:

Operators consisting of two characters must not be split.

a < = 10
speed > = max
age = = 18
time ! = 0

Important point 2:

Note that the operators => and =< do not exist: there are only <= and >=. If you write a =< 3, your code simply won’t compile.

Important point 3:

In Java you cannot write an expression like 18 < age < 65. The expression 18 < age evaluates to true or false. And you cannot compare true with a number (65)—they are different types.

3. Logical operations

In Java you cannot write the expression 18 < age < 65—it is syntactically incorrect and the program won’t compile.

But you can write it like this:

(18 < age) AND (age < 65)

Of course, instead of the word AND we use logical operators. Java has three logical operators: AND (and), OR (or), and NOT (not).

Good news: using parentheses, you can build logical expressions of any complexity.
Bad news: in Java instead of the words and, or, and not, you use C-family symbols: &&, ||, !.

Here are a few examples of using logical operators in Java:

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

(!a) && (!b)

!(!a || !b)

4. Truth tables

Let’s briefly go over the logical operations.

The AND operator

The AND operator, aka &&, also called conjunction or logical multiplication.

The OR operator

The OR operator, aka ||, also called disjunction or logical addition.

The NOT operator

The NOT operator, aka !, also called inversion (negation).

Useful expressions:

5. Examples of logical operations: and, or, not

The “AND” operation is written as &&

The result is true only when both operands are true.

int age = 16;
boolean hasTicket = true;
boolean canAttend = age >= 14 && hasTicket; // true (16 is greater than 14 AND there is a ticket)

If you’re at least 14 AND you have a ticket—come on in!

The “OR” operation is written as (||)

The result is true if at least one condition is true.

boolean isAdmin = false;
boolean isModerator = true;
boolean canEdit = isAdmin || isModerator; // true (editing is allowed if at least one of these is true)

If you’re an admin OR a moderator—go ahead and press the buttons!

The “NOT” operation is written as (!)

The inverter turns true into false and vice versa.

boolean isWeekend = false;
boolean shouldGoToWork = !isWeekend; // true (if it’s not the weekend, we work)

If it’s NOT the weekend—it’s time to get up!

Combining: build conditions of any complexity

You can combine as many logical operations as you like (preferably without overdoing it). Parentheses can (and should!) be used for clarity.

Example:

int age = 17;
boolean hasTicket = false;
boolean isVip = true;

boolean canGo = (age >= 18 && hasTicket) || isVip; // true, because isVip = true

Logic: if a person is an adult and has a ticket, or if they are a VIP—let them in. If they are not a VIP—only if they are an adult and have a ticket.

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