Lambda_using default methods

public class Test3 {
    public static void main(String[] args) {
        Formula<Formula> formula = a -> a.sqrt(100);
        
        System.out.println(formula.calculate(new ImplFormula(100)));
    }
}

@FunctionalInterface
interface Formula<T> {
    
    double calculate(T a);
    
    default double sqrt(int a) {
        return Math.sqrt(a);
    }
}

class ImplFormula implements Formula {
    private int val;
    
    public ImplFormula(int val) {
        this.val = val;
    }
    
    @Override
    public double calculate(Object a) {
        return 0;
    }
}