Объясните плиз )

private void method0() {
        Double d = method3();
    }

    protected void method1(String param1) {
        Solution solution = new Solution();
        solution.method0();
    }

    public void method2(int param1) {
        param1++;
    }
protected void method5(String param2) {
        new StringBuffer().append(param2).append(param2).append(param2);
    }
String method7(double param2) {
        return "" + param2;
    }