The first thing you need to study is the java.util.Set
API.
Here's a small example of how to use its methods:
Set<Integer> numbers = new TreeSet<Integer>();
numbers.add(2);
numbers.add(5);
System.out.println(numbers); // "[2, 5]"
System.out.println(numbers.contains(7)); // "false"
System.out.println(numbers.add(5)); // "false"
System.out.println(numbers.size()); // "2"
int sum = 0;
for (int n : numbers) {
sum += n;
}
System.out.println("Sum = " + sum); // "Sum = 7"
numbers.addAll(Arrays.asList(1,2,3,4,5));
System.out.println(numbers); // "[1, 2, 3, 4, 5]"
numbers.removeAll(Arrays.asList(4,5,6,7));
System.out.println(numbers); // "[1, 2, 3]"
numbers.retainAll(Arrays.asList(2,3,4,5));
System.out.println(numbers); // "[2, 3]"
Once you're familiar with the API, you can use it to contain more interesting objects. If you haven't familiarized yourself with the equals
and hashCode
contract, already, now is a good time to start.
In a nutshell:
@Override
both or none; never just one.
- Be careful with writing
boolean equals(Thing other)
instead; this is not a proper @Override
.
- For non-null references
x, y, z
, equals
must be:
- reflexive:
x.equals(x)
.
- symmetric:
x.equals(y)
if and only if y.equals(x)
- transitive: if
x.equals(y) && y.equals(z)
, then x.equals(z)
- consistent:
x.equals(y)
must not change unless the objects have mutated
x.equals(null) == false
- The general contract for
hashCode
is:
- consistent: return the same number unless mutation happened
- consistent with
equals
: if x.equals(y)
, then x.hashCode() == y.hashCode()
- strictly speaking, object inequality does not require hash code inequality
- but hash code inequality necessarily requires object inequality
- What counts as mutation should be consistent between
equals
and hashCode
.
Next, you may want to impose an ordering of your objects. You can do this by making your type implements Comparable
, or by providing a separate Comparator
.
Having either makes it easy to sort your objects (Arrays.sort
, Collections.sort(List)
). It also allows you to use SortedSet
, such as TreeSet
.
Further readings on stackoverflow: