DiamondProblem.java

public class DiamondProblem implements IHoge, IPiyo {

    public static void main(String[] args) {
        System.out.println(new DiamondProblem().hoge());
    }
}

IHoge.java

interface IHoge {
    default String hoge() { return "HOGE"; }
}

IPiyo.java

interface IPiyo extends IHoge {
    default String hoge() { return "PIYO"; }
}