来自Effective Java的第9项（等于合同）：示例是否正确？

Question

Bloch's wonderful book "Effective Java" points out that if equals is not symmetric then the behavior of Collections contains is indeterminate.

In the example he gives (reproduced with small modifications below), Bloch says that he sees a "false", but could just as well have seen a true or an Exception.

You could see a "true" if the standard does not specify whether contains(Object o) checks e.equals(o) or o.equals(e) for each item in the collection, and the former is implemented. However, the Collections Javadoc clearly states that it has to be the latter (and it's what I observed).

So the only possibilities I see are "false" or possibly an exception (but the String Javadoc seems to preclude the latter).

I understand the broader point, it's likely that an asymmetric equals is will lead to problems in code outside of Collections, but I don't see it for the example he quotes.

Am I missing something?

import java.util.List;
import java.util.ArrayList;

class CIString {
  private final String s;

  public CIString(String s) {
    this.s = s;
  }

  @Override public boolean equals( Object o ) {
    System.out.println("Calling CIString.equals from " + this.s );
    if ( o instanceof CIString) 
      return s.equalsIgnoreCase( ( (CIString) o).s);
    if ( o instanceof String) 
      return s.equalsIgnoreCase( (String) o );
    return false;
  }
  // Always override hashCode when you override equals
  // This is an awful hash function (everything collides -> performance is terrible!)
  // but it is semantically sound.  See Item 10 from Effective Java for more details.
  @Override public int hashCode() { return 42; }
}

public class CIS {
  public static void main(String[] args) {
   CIString a = new CIString("Polish");
   String s = "polish";

   List<CIString> list = new ArrayList<CIString>();
   list.add(a);
   System.out.println("list contains s:" + list.contains(s));
 }
}

Answer 1

It is early in the morning, so perhaps I am missing the true point of your question, this code will fail:

public class CIS 
{
    public static void main(String[] args) 
    {
        CIString a = new CIString("Polish");
        String s = "polish";

        List<String> list = new ArrayList<String>();
        list.add(s);
        System.out.println("list contains a:" + list.contains(a));
    }
}

At the very least it is odd that your code finds it and my code does not (from the point of view of sanity, not that that is clearly how your code is written :-)

Edit:

public class CIS {
  public static void main(String[] args) {
   CIString a = new CIString("Polish");
   String s = "polish";

   List<CIString> list = new ArrayList<CIString>();
   list.add(a);
   System.out.println("list contains s:" + list.contains(s));

   List<String> list2 = new ArrayList<String>();
   list2.add(s);
   System.out.println("list contains a:" + list2.contains(a));
 }
}

Now the code prints out:

list contains s:false
Calling CIString.equals from Polish
list contains a:true

Which still doesn't make sense... and is very fragile. If two objects are equal like a.equals(b) then they must also be equal like b.equal(a), that is not the case with your code.

From the javadoc :

It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.

So, yes, the example in the book may be contrary to the Javadoc of the collections API, but the principle is correct. One should not create an equals method that behaves oddly or eventually problems will arise.

Edit 2:

The key point of the text is:

In Sun's current implementation, it happens to return false, but that's just an implementation artifact. In another implementation, it could just as easily return true or throw a run-time exception. Once you've violated the equals contract, you simply don't know how other objects will behave when confronted with your object.

However, given that the Javadoc says what it says it would seem that the behaviour is fixed not an implementation artifact.

If it wasn't in the javadoc, or if the javadoc is not meant to be part of the specification, then it could change at a later date and the code would no longer work.

Answer 2

In the copy of the book I look at now (2nd Edition), item number is 8 and the whole section about symmetry requirement is presented pretty poorly.

Particular issue you mention seem to be caused by usage code being too close to implementation, obscuring the point author is trying to make. I mean, I look at list.contains(s) and I see ArrayList and String through it and all the reasoning about returning true or throwing exception makes zero sense to me, really.

• I had to move the "usage code" further away from implementation to get the idea of how it may be:

   void test(List<CIString> list, Object s) { if (list != null && list.size() > 0) { if (list.get(0).equals(s)) { // unsymmetric equality in CIString assert !list.contains(s); // "usage code": list.contain(s) } } } 

Above looks weird but as long as list is our ArrayList and s is our String, test passes.

Now, what will happen if we use something else instead of String? say, what happens if we pass new CIString("polish") as the second argument?

Look, despite passing through first equals check , assertion fails at the next line - because contains will return true for this object.

---

Similar reasoning applies for the part where Bloch mentions exception. This time, I kept the second parameter as String, but for first one, imagined an List implementation other than ArrayList (that's legal isn't it).

• You see, List implementations are generally allowed to throw ClassCastException from contains , we just need to get one that does exactly that and use it for our test. One that comes to mind could be based on TreeSet wrapped around our original list with appropriate comparator.

   List<CIString> wrapperWithCce(List<CIString> original, Comparator<CIString> comparator) { final TreeSet<CIString> treeSet = new TreeSet<CIString>(comparator); treeSet.addAll(original); return new ArrayList<CIString>() { { addAll(treeSet); } @Override public boolean contains(Object o) { return treeSet.contains(o); // if o is String, will throw CCE } }; } 

What happens if we pass list like above and String "polish" to test ? list.get(0).equals(s) will still pass the check but list.contains(s) will throw ClassCastException from TreeSet.contains() .

This seem to be like the case Bloch had in mind when he mentioned that list.contains(s) may throw an exception - again, despite passing through first equals check .