List.contains() fails while .equals() works

Question

I have an ArrayList of Test objects, which use a string as the equivalency check. I want to be able to use List.contains() to check whether or not the list contains an object that uses a certain string.

Simply:

Test a = new Test("a");
a.equals("a"); // True

List<Test> test = new ArrayList<Test>();
test.add(a);
test.contains("a"); // False!

Equals and Hash function:

@Override
public boolean equals(Object o) {
    if (o == null) return false;
    if (o == this) return true;
    if (!(o instanceof Test)) {
        return (o instanceof String) && (name.equals(o));
    }
    Test t = (Test)o;
    return name.equals(t.GetName());
}

@Override
public int hashCode() {
    return name.hashCode();
}

I read that to make sure contains works for a custom class, it needs to override equals. Thus it's super strange to me that while equals returns true, contains returns false.

How can I make this work?

Full code

Solution 1

Just because your Test's equals may return true when you pass a String to it doesn't mean that String's equals will ever return true when you pass a Test instance to it. In fact, String's equals can only return true when the instance passed to it is another String :

public boolean equals(Object anObject) {
    if (this == anObject) {
        return true;
    }
    if (anObject instanceof String) { // the passed instance must be a String
        String anotherString = (String)anObject;
        int n = value.length;
        if (n == anotherString.value.length) {
            char v1[] = value;
            char v2[] = anotherString.value;
            int i = 0;
            while (n-- != 0) {
                if (v1[i] != v2[i])
                    return false;
                i++;
            }
            return true;
        }
    }
    return false;
}

ArrayList's contains calls indexOf which uses the equals method of the searched instance (the String "a" in your example), not the element type of the List (which is Test in your case) :

public int indexOf(Object o) {
    if (o == null) {
        for (int i = 0; i < size; i++)
            if (elementData[i]==null)
                return i;
    } else {
        for (int i = 0; i < size; i++)
            if (o.equals(elementData[i])) // o in your case is a String while
                                          // elementData[i] is a Test
                                          // so String's equals returns false
                return i;
    }
    return -1;
}

Solution 2

equals() should always be commutative, i.e. a.equals(b) and b.equals(a) should always return the same value. Or symmetric, as the javadoc of equals() calls it:

The equals method implements an equivalence relation on non-null object references:

• It is reflexive: for any non-null reference value x, x.equals(x) should return true.
• 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.
• It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
• It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.
• For any non-null reference value x, x.equals(null) should return false.

Unfortunately, even the Java Runtime Library gets this one wrong. Date.equals(Timestamp) will compare the millisecond values, ignoring the nanoseconds present in the Timestamp, while Timestamp.equals(Date) returns false.

Solution 3

The problem is that List<E>.contains(object o) is documented to return true:

if and only if this list contains at least one element e such that (o==null ? e==null : o.equals(e)).

(From https://docs.oracle.com/javase/8/docs/api/java/util/List.html#contains-java.lang.Object-)

Note that it doesn't perform the test as e.equals(o) which is what would be necessary for your test to work. Your equals method fails to work commutatively ('symmetrically' using the terms from the Java docs).

Java documents that the equals() method for a class must follow these rules:

The equals method implements an equivalence relation on non-null object references:

• It is reflexive: for any non-null reference value x, x.equals(x) should return true.
• 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.
• It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
• It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.
• For any non-null reference value x, x.equals(null) should return false.

Solution 4

If you write

test.contains(new Test("a")); 

then it will surely return true. You are checking for string object in list of Test.

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