Java Booleans

Declaring a Boolean

You declare a boolean with the keyword boolean (lowercase) and assign it either true or false. These two literals are not Strings, so they never appear in quotes. A boolean variable is often used to remember a state, such as whether a user is logged in.

boolean isJavaFun = true;
boolean isFishTasty = false;
System.out.println(isJavaFun);   // true
System.out.println(isFishTasty); // false

Boolean Expressions

Most booleans are not typed by hand; they come from comparisons. A boolean expression is any piece of code that Java evaluates to true or false, such as comparing two numbers with the greater-than or equal-to operators.

int age = 20;
System.out.println(age > 18);  // true
System.out.println(age == 21); // false
System.out.println(age <= 20); // true

Comparison Operators

The operators below each return a boolean result. Notice that equality uses two equals signs (==); a single equals sign is assignment and would not compare anything.

Using a Boolean to Decide

The real value of booleans appears inside if statements. The condition in the parentheses must be a boolean, and Java runs the block only when that condition is true.

int temperature = 30;
boolean isWarm = temperature > 25;

if (isWarm) {
  System.out.println("It is a warm day.");
} else {
  System.out.println("Bring a jacket.");
}

• A boolean holds only true or false, never anything else.
• Comparison operators such as > and == evaluate to a boolean.
• Use == to compare, and a single = only to assign a value.
• Booleans are the fuel for if statements, while loops, and other logic.