Comment on page
20.3 Contains & Duplicate Items
Suppose the
equals()
method for ColoredNumber is as below, i.e. two ColoredNumbers are equal if they have the same num.@Override
public boolean equals(Object o) {
if (o instanceof ColoredNumber otherCn) {
return this.num == otherCn.num;
}
return false;
}
Suppose the
equals()
method for ColoredNumber is on the previous slide, i.e. two ColoredNumbers are equal if they have the same num. int N = 20;
HashSet<ColoredNumber> hs = new HashSet<>();
for (int i = 0; i < N; i += 1) {
hs.add(new ColoredNumber(i));
}
Suppose we now check whether 12 is in the hash table.
ColoredNumber twelve = new ColoredNumber(12);
hs.contains(twelve); //returns true
What do we expect to be returned by
contains
?Suppose we are using the default hash function (uses memory address):
int N = 20;
HashSet<ColoredNumber> hs = new HashSet<>();
for (int i = 0; i < N; i += 1) {
hs.add(new ColoredNumber(i));
}
ColoredNumber twelve = new ColoredNumber(12);
hs.contains(twelve); // returns ??
which yields the table below:
Suppose equals returns true if two ColoredNumbers have the same num (as we've defined previously).
What is actually returned by
contains
?Returns false with probability 5/6ths.
Default
hashCode()
is based on memory address. equals is based on num
.There are two ColoredNumber objects with
num = 12
. One of them is in the HashSet and one of them was created by the code above.Each memory address is random, with only a 1/6th chance they modulo to the same bucket.
Example: If the
ColoredNumber
object twelve
created by the code above is in memory location 6000000, its hashCode % 6 is 0. HashSet looks in bucket zero, and doesn't find 12.Hard Question: If the default hash code achieves a good spread, why do we even bother to create custom hash functions?
Basic rule (also definition of deterministic property of a valid hashcode): If two objects are equal, they must have the same hash code so the hash table can find it.
Suppose we have the same
equals()
method (comparing num
), but we do not override hashCode()
.public boolean equals(Object o) {
... return this.num == otherCn.num; ...
}
The result of adding 0 through 19 is shown below:

ColoredNumber zero = new ColoredNumber(0);
hs.add(zero); // does another zero appear?
Which can happen when we call add(zero)?
Answer Choices:
- 1.We get a 0 to bin zero.
- 2.We add another 0 to bin one.
- 3.We add a 0 to some other bin.
- 4.We do not get a duplicate zero
Bottom line: If your class override equals, you should also override hashCode in a consistent manner.
- If two objects are equal, they must always have the same hash code.
If you don’t, everything breaks:
Contains
can’t find objects (unless it gets lucky).Add
results in duplicates.