Consider an interface with a private method, like:

public interface TestInterface {
    
    public default double fact(int n) {
        double f = 1;
        for (int i = 1; i <= n; i++) {
            f *= i;
        }
        return f;
    }
    
    private double zfact(int n) {
        double f = 1;
        for (int i = 1; i <= n; i++) {
            f *= i;
        }
        return -f;
    }
    
    public default double callZFact(int n) {
        return zfact(n);
    }
    
    double afact(int n);
}

and it's implementation:

public class TestImpl implements TestInterface {

    @Override
    public double afact(int n) {
        if (n >= 1) {
            double f = callZFact(n);
            return f;
        }
        return Double.NaN;
    }
}

Then call it like:
 TestImpl ti = new TestImpl();
 ti.afact(10);

Suspend the program on "ti.afact(10);" line and evaluate "ti.zfact(2)" expression.
An java.lang.AbstractMethodError is thrown.