Pitfalls in Java

Mark the pitfalls I fell into while writing Java.

#1

ArrayList<Character> list = new ArrayList<>();
list.add('a');
list.remove('a');  // raise IndexOutOfBoundsException
list.remove((Character) 'a');  // return true

#2

assert new String("123") != new String("123");
assert new String("123").equals(new String("123"));

#3

int[] a = {1, 2, 3};
int[] b = {1, 2, 3};

assert !a.equals(b);
assert java.util.Arrays.equals(a, b);

assert a.hashCode() != b.hashCode();
assert java.util.Arrays.hashCode(a) == java.util.Arrays.hashCode(b);