Java 9: Private methods in interfaces

Interfaces can hold private helper methods to share code between default methods.

Code

interface Calc {
    default int doubled(int n) { return scale(n, 2); }
    default int tripled(int n) { return scale(n, 3); }
    private int scale(int n, int by) { return n * by; }
}
Calc c = new Calc() {};
System.out.println(c.doubled(5) + " " + c.tripled(5));

Output

10 15

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Published 2026-07-26