Java Booleans

boolean is the simplest primitive: it has exactly two values, true and false. Every conditional in Java — if, while, the ternary ?:, the operands of && and || — wants a boolean. You don't have to know much about the type, but knowing the difference between boolean and Boolean saves a future bug.

Declaring a boolean

boolean isOpen = true;
boolean ready = false;
boolean canPay = (balance >= amount);

The keywords are true and false — lowercase. There's no True, no TRUE, no 1, no 0 — Java is strict.

Booleans in conditions

Every if, while, do-while, and the middle clause of a for loop expects a boolean:

boolean isAdult = age >= 18;

if (isAdult) {
    System.out.println("Welcome!");
}

while (!stopRequested) {
    process();
}

You cannot use a number where a boolean is required:

int x = 5;
// if (x) { ... }   // compile error: int is not a boolean
if (x != 0) { ... } // OK

That's a feature, not an inconvenience — it eliminates a whole family of C bugs.

Boolean operations

The three logical operators (Logical Operators):

boolean a = true && false;   // false
boolean b = true || false;   // true
boolean c = !true;           // false

&& and || short-circuit. & and | evaluate both sides — almost never what you want on booleans.

Boolean — the wrapper

The wrapper class Boolean lets you put booleans into collections and pass them around as objects:

List<Boolean> answers = new ArrayList<>();
answers.add(true);
answers.add(false);

Map<String, Boolean> flags = new HashMap<>();
flags.put("debug", true);

Autoboxing converts between boolean and Boolean automatically. The one thing to watch out for: a Boolean reference can be null, but a primitive boolean can't.

Boolean maybe = null;
if (maybe) { ... }   // throws NullPointerException — autounbox on null

Use Boolean.TRUE.equals(maybe) or an explicit null check if null is possible.

Parsing booleans

Boolean.parseBoolean(s) returns true if the string is "true" (case-insensitive), and false for everything else — including misspellings and unexpected values:

Boolean.parseBoolean("true");      // true
Boolean.parseBoolean("TRUE");      // true
Boolean.parseBoolean("yes");       // false
Boolean.parseBoolean("1");         // false
Boolean.parseBoolean(null);        // false

If you need strict input validation, write your own check rather than trusting this method's "everything-not-true-is-false" semantics.

boolean[] vs BitSet

For a few booleans, a boolean[] is fine and reads clearly:

boolean[] flags = new boolean[8];
flags[3] = true;

For thousands or millions of flags, BitSet is more memory-efficient — it packs flags into longs internally:

import java.util.BitSet;

BitSet seen = new BitSet();
seen.set(42);
System.out.println(seen.get(42));   // true

A demonstration

What's next

Java Characters (char) — the 16-bit primitive that holds a single Unicode code unit.

Practice