Why assertEquals and assertSame in junit return the same result for two instances same class?

Since you didn’t override equals in your class, assertEquals behaves the same as assertSame since the default equals implementation compare references.

150    public boolean equals(Object obj) {
151        return (this == obj);
152    }

If you provide a dumb overriding of equals:

class SomeClass {
    @Override 
    public boolean equals(Object o) {
        return true;
    }
}

you’ll see that assertEquals succeeds.