2.1 Relational and Logical Operators

Programs do not only run statements in order. They often need to check whether a condition holds, then choose what to do next. In Java, a condition’s result is a boolean: either true or false.

Relational operators

Relational operators compare two values. The result is true or false, not another number.

Common Java relational operators:

• <: less than

• >: greater than

• <=: less than or equal to

• >=: greater than or equal to

• ==: equal to

• !=: not equal to

Important: one equals sign = means assignment. Two equals signs == mean comparison.

int age = 17;

System.out.println(age < 18);     // true
System.out.println(age == 18);    // false
System.out.println(age != 18);    // true

age = 17 stores 17 in the variable age. age == 18 checks whether the current value of age equals 18.

Logical operators

Logical operators combine multiple boolean conditions. Their result is still true or false.

• &&: the whole expression is true only when every condition is true.

• ||: the whole expression is true when at least one condition is true.

• !: turns true into false, and false into true.

Truth tables

A truth table lists every possible input combination to make the rules clear.

&& is true only when both sides are true.

|| is true when at least one side is true.

! acts on a single value and flips the result.

int age = 20;
boolean hasTicket = true;

System.out.println(age >= 18 && hasTicket);    // true
System.out.println(age < 18 || hasTicket);     // true
System.out.println(!hasTicket);                // false

It helps to read conditions like plain language: age >= 18 && hasTicket means “the age is at least 18, and the person has a ticket.”

Combining conditions

Unlike math notation, Java does not support chaining comparisons. 0 < age < 18 is a compile error, so write it as two conditions joined by &&.

int age = 16;

System.out.println(0 < age && age < 18);    // true

When a condition becomes complex, name each smaller condition first, then combine them.

int score = 85;
int attendance = 92;

boolean passed = score >= 60 && attendance >= 80;
boolean excellent = score >= 90 || attendance >= 95;

System.out.println(passed);       // true
System.out.println(excellent);    // false

Do not rush to squeeze every condition into one line. Naming smaller conditions makes the program easier to read and debug.