奇怪的assertEquals等于并包含行为

Question

I have a very simple Pair class defined as follows:

public class Pair<L, R> {

    private L left;
    private R right;

    public Pair(L left, R right) {
        this.left = left;
        this.right = right;
    }

    public L getLeft() {
        return this.left;
    }

    public R getRight() {
        return this.right;
    }

    public String toString() {
        return String.format("(%s, %s)", left, right);
    }

    public int hashCode() {
        int hashFirst = left != null ? left.hashCode() : 0;
        int hashSecond = left != null ? right.hashCode() : 0;

        return (hashFirst + hashSecond) * hashSecond + hashFirst;
    }

    public Boolean equals(Pair other) {
        if (other == null) {
            return false;
        }

        return left.equals(other.getLeft()) && right.equals(other.getRight());
    }
}

I then have a Position class defined as follows:

public class Position {

    private Pair<Integer, Integer> pair;

    public Position(Integer x, Integer y) {
        this.pair = new Pair<Integer, Integer>(x, y);
    }

    public Integer getX() {
        return this.pair.getLeft();
    }

    public Integer getY() {
        return this.pair.getRight();
    }

    public boolean equals(Position other) {
        if (other == null) {
            return false;
        }

        boolean b = getX() == other.getX() && getY() == other.getY();
        System.out.println(String.format("%s.equals(%s): %s", this, other, b));
        return b;
    }

    public int hashCode() {
        return pair.hashCode();
    }

    public String toString() {
        return String.format("(%d, %d)", this.pair.getLeft(),
                              this.pair.getRight());
    }

}

That's all nice. However something peculiar happens when I try to test it.

public class PositionTests extends TestCase {

    private Position posOne;
    private Position posTwo;
    private Position posThree;

    public PositionTests() {
        posOne = new Position(7, 6);
        posTwo = new Position(12, 7);
        posThree = new Position(7, 6);
    }

    public void testCreationX() {
        assertEquals(posOne.getX(), (Integer) 7);
    }

    public void testCreationY() {
        assertEquals(posOne.getY(), (Integer) 6);
    }

    public void testEquality() {
        // System.out.println("p1.e(p3): " + posOne.equals(posThree));
        // System.out.println("p3.e(p1): " + posThree.equals(posOne));
        //assertEquals(posOne, posThree);
        assertEquals(posThree, posOne);
    }

    public void testInequality() {
        assertFalse(posOne.equals(posTwo));
    }

    public void testXEquality() {
        assertEquals(posOne.getX(), posThree.getX());
    }

    public void testYEquality() {
        assertEquals(posOne.getY(), posThree.getY());
    }

    public void testSymmTrue() {
        assertTrue(posOne.equals(posThree) == posThree.equals(posOne));
    }

    public void testSymmFalse() {
        assertTrue(posOne.equals(posTwo) == posThree.equals(posTwo));
    }

    public void testHashSetSame() {
        Set<Position> hSet = new HashSet<Position>();
        hSet.add(posOne);
        hSet.add(posThree);
        hSet.add(posTwo);
        hSet.add(posOne);
        hSet.add(posOne);
        hSet.add(posOne);
        assertTrue(hSet.size() == 3);
    }

    public void testListContainsSuccess() {
        List<Position> pList = new ArrayList<Position>();
        pList.add(posOne);
        pList.add(posTwo);
        assertTrue(pList.contains(posOne));
    }

    public void testListContainsSuccessDiff() {
        List<Position> pList = new ArrayList<Position>();
        pList.add(posOne);
        pList.add(posTwo);
        // System.out.println(pList);
        // System.out.println(posThree);
        Boolean b = pList.contains(posThree);
        System.out.println("contains: " + b);
        assertTrue(pList.contains(posThree));
    }

    public void testListContainsFail() {
        List<Position> pList = new ArrayList<Position>();
        pList.add(posOne);
        pList.add(posThree);
        assertFalse(pList.contains(posTwo));
    }

}

The relevant output from the test is as follows:

    [junit] ------------- Standard Output ---------------
    [junit] JUnit version is: 3.8.2
    [junit] contains: false
    [junit] (7, 6).equals((7, 6)): true
    [junit] (7, 6).equals((7, 6)): true
    [junit] (7, 6).equals((12, 7)): false
    [junit] (7, 6).equals((12, 7)): false
    [junit] (7, 6).equals((12, 7)): false
    [junit] (7, 3).equals((7, 4)): false
    [junit] ------------- ---------------- ---------------
    [junit] 
    [junit] Testcase: testListContainsSuccessDiff took 0.005 sec
    [junit]     FAILED
    [junit] null
    [junit] junit.framework.AssertionFailedError
    [junit]     at com.group7.dragonwars.tests.PositionTests.testListContainsSuccessDiff(PositionTests.java:92)
    [junit] 
    [junit] Testcase: testListContainsFail took 0 sec
    [junit] Testcase: testYEquality took 0 sec
    [junit] Testcase: testSymmTrue took 0.014 sec
    [junit] Testcase: testSymmFalse took 0.001 sec
    [junit] Testcase: testHashSetSame took 0 sec
    [junit] Testcase: testListContainsSuccess took 0 sec
    [junit] Testcase: testCreationY took 0 sec
    [junit] Testcase: testEquality took 0.001 sec
    [junit]     FAILED
    [junit] expected:<(7, 6)> but was:<(7, 6)>
    [junit] junit.framework.AssertionFailedError: expected:<(7, 6)> but was:<(7, 6)>
    [junit]     at com.group7.dragonwars.tests.PositionTests.testEquality(PositionTests.java:43)
    [junit] 
    [junit] Testcase: testInequality took 0.001 sec
    [junit] Testcase: testXEquality took 0 sec
    [junit] Testcase: testCreationX took 0 sec
    [junit] Test com.group7.dragonwars.tests.AllTests FAILED

Having now posted all my relevant code and test results, my question is why would assertEquals(posOne, posThree) as well as calls to contains() fail? As you can see at the top of the tests output, .equals() returns true. I'm so confused as to why these fail. Throughout my program I've had to use manual list traversal and manually calling equals() to check for list membership, but why ? The documentation for List says

Returns true if this list contains the specified element. More formally, returns true if and only if this list contains at least one element e such that (o==null ? e==null : o.equals(e)). 

And yet, .contains() fails. What the…?

Answer 1

If you add an @Override annotation to the equals() method you'll see the problem. The parameter type must be Object , not Position .

@Override
public boolean equals(Object other) {

Also, there's a typo in Pair.hashCode() .

    int hashFirst = left != null ? left.hashCode() : 0;
    int hashSecond = left != null ? right.hashCode() : 0;
                     ^^^^^

Answer 2

You want equals on Position and Pair to take an Object -- you're just overloading it with the declaration which takes a Pair.

Also your hashCode implementation on Pair is weird :) Maybe use the Apache EqualsBuilder and HashBuilder classes?

Relevant javadoc: http://docs.oracle.com/javase/1.4.2/docs/api/java/lang/Object.html#equals(java.lang.Object)