Bitwise operators and flags

Key points

• & is AND, | is OR, ^ is XOR, and ~ flips every bit.
• Set with x |= FLAG, clear with x &= ~FLAG, test with (x & FLAG) != 0.
• & and | on booleans do not short-circuit, unlike && and ||. Do not mix them up.
• << doubles, >> halves keeping the sign, and >>> shifts in zeros.
• n & 1 reads the lowest bit, which is the fastest even/odd test there is.
• Print the bits with Integer.toBinaryString — and in new code prefer an EnumSet over a flags int.

Example

public class Main {

    static final int READ = 1;
    static final int WRITE = 2;
    static final int EXEC = 4;

    static String bits(int n) {
        return String.format("%4s", Integer.toBinaryString(n)).replace(' ', '0');
    }

    public static void main(String[] args) {
        int perms = READ | EXEC;
        System.out.println("perms      = " + bits(perms) + "  (" + perms + ")");
        System.out.println("has READ ? " + ((perms & READ) != 0));
        System.out.println("has WRITE? " + ((perms & WRITE) != 0));

        perms |= WRITE;
        System.out.println("set WRITE  = " + bits(perms));
        perms &= ~EXEC;
        System.out.println("clear EXEC = " + bits(perms));
        System.out.println("toggle READ= " + bits(perms ^ READ));

        System.out.println("1 << 5   = " + (1 << 5) + "  (times 32)");
        System.out.println("40 >> 2  = " + (40 >> 2) + "  (divided by 4)");
        System.out.println("-8 >> 1  = " + (-8 >> 1) + " keeps the sign, -8 >>> 28 = " + (-8 >>> 28));
        System.out.println("13 is odd? " + ((13 & 1) == 1));
        System.out.println("& is not &&: it never short-circuits");
    }
}

OR to set, AND-NOT to clear, AND to test — all in a single int.