Error: method X must override or implement a supertype method

I'm using JUnit 5, and copied the code from "Software Testing" book in order to create a mock object for testing. part of the tester code is:

 @Test
 public void rangesOKTestWithoutDependency() {
    // This is an anonymous class
    SimpleDate simpleDate = new SimpleDate(1, 1, 2000) {
         @Override
         public boolean isLeap(int year) {               
             if(2000 == year) return true;
             else if(2001 == year) return false;
            else throw new IllegalArgumentException("No Mock for year " + year);
         }
 };
 assertTrue(simpleDate.rangesOK(2, 29, 2000)); // Valid due to leap year
 assertFalse(simpleDate.rangesOK(2, 29, 2001)); // Valid due to leap year
 }

I have a compiler error, which says "Method isLeap(int) must override or implement a supertype method". This error is reported in the line that I override isLeap() method. (The line below @override)

Well, amazingly this is what I have done. So I don't know what is this complain about. Here is the isLeap() method in class simpleDate:

private boolean isLeap(int year) {
           boolean isLeapYear = true;
           if(year % 4 != 0)
                 isLeapYear = false;
           else if(year % 100 != 0)
                 isLeapYear = true;
           else if(year % 400 != 0)
                 isLeapYear = false;
           return isLeapYear;
    }

As you see the method in the tester is an overridden version of the original method but still I get an error. Any thoughts?

ps: I'm using eclipse.