HashSet in Java

After we have learned HashMap, it is very easy to look at HashSet. Because HashSet is implemented based on HashMap.

Open the source code of HashSet, you can see that a HashMap is maintained

1. Member variables

// The element
 used to store HashSet private transient HashMap<E , Object> map ;
 // This Object is used to fill the value of HashMap .
private static final Object PRESENT = new Object() ;

Second, the construction method

public HashSet() {
    map = new HashMap<>();
}

As you can see, the default construction is to instantiate the HashMap of member variables.

3. Methods

public int size() {
    return map.size();
}

size is the size of HashMap.

public boolean contains(Object o) {
    return map.containsKey(o);
}

Because HashSet only uses the keys of HashMap, use the containsKey method of HashMap

public boolean add(E e) {
    return map.put(e, PRESENT)==null;
}

When adding, just add the element to be added and the Object object that just filled the Value to the HashMap

public boolean remove(Object o) {
    return map.remove(o)==PRESENT;
}

Remove remove method using HashMap

Is not it simple. As long as you know that HashSet maintains a HashMap, then the operation of HashSet is the operation of the key of HashMap that you are familiar with.