== vs equals()

Examples

For primitives == compares values, for objects it asks "is this the same object":

void main() {
    int a = 1000;
    int b = 1000;
    IO.println(a == b);
 
    String s1 = "hi";
    String s2 = new String("hi");
    IO.println(s1 == s2);
    IO.println(s1.equals(s2));
}

Output:

true
false
true

The Integer wrapper trap: small numbers are cached, large ones are not:

void main() {
    Integer x = 127, y = 127;
    Integer p = 1000, q = 1000;
    IO.println(x == y);
    IO.println(p == q);
    IO.println(p.equals(q));
}

Output:

true
false
true

Notes

The rule: compare primitives (int, double, char, boolean) with == and objects (String, Integer, records, lists) with equals(). The exception is enum values, compared with ==. For equals() to work in your own class you must override it (together with hashCode()); records do this automatically.

Because of rounding errors, double values are rarely exactly equal: 0.1 + 0.2 == 0.3 is false. Compare with a tolerance: Math.abs(a - b) < 1e-9.