What does comparison being consistent with equals mean ? What can possibly happen if my class doesn’t follow this principle?

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Here’s a simple but realistic example of what can happen if a comparison method is inconsistent with equals. In the JDK, BigDecimal implements Comparable but its comparison method is inconsistent with equals. For example:

> BigDecimal z = new BigDecimal("0.0")
> BigDecimal zz = new BigDecimal("0.00")
> z.compareTo(zz)
0
> z.equals(zz)
false

This is because the comparison method of BigDecimal considers only the numeric value, but equals also considers the precision. Since 0.0 and 0.00 have different precisions, they are unequal even though they have the same numeric value. (For an explanation, see
this answer.)

Here’s an example of what it means for a TreeSet to violate the general contract of Set. (It’s the same situation with TreeMap and Map but it’s a bit easier to demonstrate using sets.) Let’s compare the results of contains to the result of getting the element out of the set and calling equals:

> TreeSet<BigDecimal> ts = new TreeSet<>()
> ts.add(z)
> ts.contains(z)
true
> z.equals(ts.iterator().next())
true
> ts.contains(zz)
true
> zz.equals(ts.iterator().next())
false

The surprising thing here is that the TreeSet says it contains the object zz, but it’s unequal to the element that’s actually contained in the set. The reason is that TreeSet uses its comparison method (BigDecimal.compareTo) to determine set membership, not equals.

Now let’s compare TreeSet to HashSet:

> HashSet<BigDecimal> hs = new HashSet<>(ts)
> hs.equals(ts)
true
> ts.contains(zz)
true
> hs.contains(zz)
false

This is strange. We have two sets that are equal, but one set says that it contains an object but another set says that it doesn’t contain the same object. Again, this reflects the fact that TreeSet is using the comparison method whereas HashSet is using equals.

Now let’s add the other object to a HashSet and see what happens:

> HashSet<BigDecimal> hs2 = new HashSet<>()
> hs2.add(zz)
> ts.equals(hs2)
true
> hs2.equals(ts)
false

Now that’s weird. One set says it’s equal to the other, but the other set says it’s not equal to the first! To understand this, you need to understand how equality of sets is determined. Two sets are considered equal if a) they are of the same size, and b) each element in the other set is also contained in this set. That is, if you have

set1.equals(set2)

then the equality algorithm looks at the sizes and then it iterates over set2, and for each element it checks whether that element is contained in set1. That’s where the asymmetry comes in. When we do

ts.equals(hs2)

both sets are of size 1, so we proceed to the iteration step. We iterate over hs2 and use then call the TreeSet.contains method — which uses the comparison method. As far as the TreeSet is concerned, it’s equal to the HashSet hs2.

Now when we do

hs2.equals(ts)

the comparison goes the other way. We iterate over the TreeSet and get its element, and ask hs2 whether it contains that element. Since the HashSet.contains uses equals, it returns false, and the overall result is false.

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