Java源碼分析之HashSet

成員變量

//HashSet的本質，其實就是HashMap
private transient HashMap<E,Object> map;
//HashMap是鍵值對，而HashSet是單值，所以需要一個值來充當鍵值對中的值
private static final Object PRESENT = new Object();

構造函數

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

public HashSet(Collection<? extends E> c) {
    map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
    addAll(c);
}

public HashSet(int initialCapacity, float loadFactor) {
    map = new HashMap<>(initialCapacity, loadFactor);
}

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

基本的方法

size() isEmpty()

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

public boolean isEmpty() {
    return map.isEmpty();
}

contains()

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

add

//注意put進去時候，值是PRESENT
public boolean add(E e) {
    return map.put(e, PRESENT)==null;
}

remove

//爲什麼要判相等呢？因爲map.remove返回的是值，而set返回的是boolean
public boolean remove(Object o) {
    return map.remove(o)==PRESENT;
}

方法大概就這麼多，基本都是用map的方法的，很機智