Question

I was surprised to see that equals() is apparently overridden for ArrayList<String>. Because contains() in Collection<> apparently compares values, not references. Of course, for Collection<Object>, references would be compared. In the program below, shouldn’t I get false on the second line?

public static void main(String[] args) {
    ArrayList<String> al = new ArrayList<String>();
    al.add("Obama");
    al.add("Reagan");
    al.add("Bush");
    al.add("Nyquist");
    StringBuffer sb = new StringBuffer();
    sb.append("Bush");

    System.out.println("compares values? using constants " + al.contains("Bush"));
    System.out.println("compares values? using local variable " + al.contains(sb.toString()));
}

run:
compares values? using constants true
compares values? using local variable true

Answer 1

This is exactly the output you should expect, and Collection<Object> would be no different. All Collection types, unless specified otherwise, use .equals(Object), and differing implementations violate the Collection contract. (And to be clear, upcasting a String to an Object does not change the behavior of its equals method.)

There is some precedent — see e.g. the TreeSet implementations, which use comparison-based equality, and IdentityHashSet, which uses reference equality — but these should usually be used only when the two notions of equality match, or for significant and unusual need.