Creating size method for list implementation

Jakub Szarubka :

I need to implement my own List.size() method. I create such a method :

    public  class MyArrayList<Long> implements List<Long> {

 List<Long> list;

    @Override
    public int size() {
        if(list == null) {
            throw new NullPointerException("List is null");
        }
        int size = 0;
        for(Long element : list) {
            size++;
        }
        return size;

} I write an abstract test class to do the test for the existing method of size and mine.

public abstract class AbstractArrayListTest {
    protected abstract List<Long> getEmptyList();
    protected abstract List<Long> getSizeOfTheList();

    @Test
    public void shouldReturnFalseIfListContainElement() {
        assertTrue(getEmptyList().isEmpty());
    }
    @Test
    public void shouldReturnSizeOfList() {
        int size = getSizeOfTheList().size();
        assertEquals(size, getEmptyList().size());
    }
}

Every time I try to test it a get null E. Any thought how I can write it ? Worth to mention the original implementation of List is passing this test.

Andronicus :

You're not initializing the list you're wrapping over. When you're trying to iterate over it, you're getting a NPE. Please, try the following class definition:

public  class MyArrayList<Long> implements List<Long> {

    List<Long> list = new ArrayList<>();

    // rest of the code

}