HashSet contains not working for Integer

Question

I don't understand why the contains is not working (in fact if I had passed in custom class I could have revisited my hascode and equals method but this is Integer). So instead of contains what can I use? Please help.

Set<Integer> st = new HashSet<>();
st.add(12);
Set<Integer> st1 = new HashSet<>();
st1.add(12);
System.out.println(st.contains(st1));

Answer 1

st.contains(st1) returns false , because the type of st1 ( Set<Integer> ) is not the same as the type of the elements in st (which is Integer ).

You can, however, use the Set#containsAll(Collection<?>) method:

System.out.println(st.containsAll(st1));

which will check if the elements of st1 are present in st .

Answer 2

st1 is a HashSet not an Integer .

try it with that code and you will see it works:

Set<Integer> st = new HashSet<>();
st.add(12);
System.out.println(st.contains(12));

or

public static void main(String[] args) {
    Set<Integer> st = new HashSet<>();
    st.add(12);
    Set<Integer> st1 = new HashSet<>();
    st1.add(12);
    System.out.println(st.containsAll(st1));
  }