55. Taking on the hard way to discover equals()

Check out the following code:

Integer x1 = 14; Integer y1 = 14; Integer x2 = 129; Integer y2 = 129; List<Integer> listOfInt1 = new ArrayList<>( Arrays.asList(x1, y1, x2, y2)); listOfInt1.removeIf(t -> t == x1 || t == x2); List<Integer> listOfInt2 = new ArrayList<>( Arrays.asList(x1, y1, x2, y2)); listOfInt2.removeIf(t -> t.equals(x1) || t.equals(x2)); 

So, initially, listOfInt1 and listOfInt2 have the same items, [x1=14, y1=14, x2=129, y2=129]. But, what will contain listOfInt1/listOfInt2 after executing the code based on removeIf() and ==, respectively equals()?

The first list will remain with a single item, [129]. When t is x1, we know that x1 == x1, so 14 is removed. But, why is x2 removed? When t is y1, we know that y1 == x1 should be false since, via ==, we compare the object’s references in memory, not their values. Obviously, y1 and x1 should have different references in the memory… or shouldn’t they ? Actually, Java has an internal rule to cache integers in -127 … 128. Since x1=14 is cached, y1=14 uses the cache so no new Integer is created. This is why y1 == x1 and y1 is removed as well. Next, t is x2, and x2 == x2, so x2 is removed. Finally, t is y2, but y2 == x2 returns false, since 129 > 128 is not cached, so x2 and y2 have different references in memory.

On the other hand, when we use equals(), which is the recommended approach for comparing the object’s values, the resulting list is empty. When t is x1, x1 =x1, so 14 is removed. When t is y1, y1 =x1, so y1 is removed as well. Next, t is x2, and x2= x2, so x2 is removed. Finally, t is y2, and y2 =x2, so y2 is removed as well.