HashSet
哈希表結構(其實用的是Map),線程不安全,不保證順序,自動去重(必須覆蓋hashCode和equals方法)
類聲明
public class HashSet<E> extends AbstractSet<E> implements Set<E>, Cloneable, java.io.Serializable
當哈希算法算出兩個值相同時,稱之爲哈希衝突
。當發生哈希衝突時需要用equals對內容進行判斷。
重點字段
// 序列化ID
static final long serialVersionUID = -5024744406713321676L;
// Set其實是使用Map的Key進行存儲元素
private transient HashMap<E,Object> map;
// 用於填充Map的Value
private static final Object PRESENT = new Object();
構造函數
// 空參構造函數,創建Map實例,默認16大小的容量和0.75的負載因子
public HashSet() {
map = new HashMap<>();
}
// 構造一個包含指定集合中的元素的新集合
public HashSet(Collection<? extends E> c) {
// 創建一個足夠存儲集合元素的Map
map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
addAll(c);
}
// 根據指定的容量和負載因子創建Map實例
public HashSet(int initialCapacity, float loadFactor) {
map = new HashMap<>(initialCapacity, loadFactor);
}
// 根據指定的容量創建Map實例 默認0.75的負載因子
public HashSet(int initialCapacity) {
map = new HashMap<>(initialCapacity);
}
// 給LinkedHashSet初始化使用, 實例化LinkedHashMap
HashSet(int initialCapacity, float loadFactor, boolean dummy) {
map = new LinkedHashMap<>(initialCapacity, loadFactor);
}
Set 方法實現
// 返回鍵的迭代器
public Iterator<E> iterator() {
return map.keySet().iterator();
}
// 集合大小
public int size() {
return map.size();
}
// 集合是否爲空
public boolean isEmpty() {
return map.isEmpty();
}
// 是否包含指定元素
public boolean contains(Object o) {
return map.containsKey(o);
}
// 調用Map的put方法,put的key如果已經存在則返回舊value,否則返回null,所以插入成功都是返回null
public boolean add(E e) {
return map.put(e, PRESENT)==null;
}
// 調用Map的remove方法,remove返回被刪除key的value,所以刪除成功都是返回PRESENT
public boolean remove(Object o) {
return map.remove(o)==PRESENT;
}
// 清空所有鍵值對映射
public void clear() {
map.clear();
}
public Spliterator<E> spliterator() {
return new HashMap.KeySpliterator<E,Object>(map, 0, -1, 0, 0);
}
序列化
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
s.defaultWriteObject();
s.writeInt(map.capacity());
s.writeFloat(map.loadFactor());
s.writeInt(map.size());
for (E e : map.keySet())
s.writeObject(e);
}
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
int capacity = s.readInt();
if (capacity < 0) {
throw new InvalidObjectException("Illegal capacity: " + capacity);
}
float loadFactor = s.readFloat();
if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
throw new InvalidObjectException("Illegal load factor: " + loadFactor);
}
int size = s.readInt();
if (size < 0) {
throw new InvalidObjectException("Illegal size: " +
size);
}
capacity = (int) Math.min(size * Math.min(1 / loadFactor, 4.0f), HashMap.MAXIMUM_CAPACITY);
SharedSecrets.getJavaOISAccess()
.checkArray(s, Map.Entry[].class, HashMap.tableSizeFor(capacity));
map = (((HashSet<?>)this) instanceof LinkedHashSet ?
new LinkedHashMap<E,Object>(capacity, loadFactor) :
new HashMap<E,Object>(capacity, loadFactor));
for (int i=0; i<size; i++) {
@SuppressWarnings("unchecked")
E e = (E) s.readObject();
map.put(e, PRESENT);
}
}
Object 方法
@SuppressWarnings("unchecked")
public Object clone() {
try {
HashSet<E> newSet = (HashSet<E>) super.clone();
newSet.map = (HashMap<E, Object>) map.clone();
return newSet;
} catch (CloneNotSupportedException e) {
throw new InternalError(e);
}
}
LinkedHashSet
鏈表+哈希表,線程不安全,可以實現有序
LinkedHashSet其實就是走HashSet的三參構造函數,底層調用LinkedHashMap
類聲明
public class LinkedHashSet<E> extends HashSet<E> implements Set<E>, Cloneable, java.io.Serializable
構造器
// 調用父類HashSet的構造器進行初始化,第三個參數true只是HashSet爲了區別於其他構造函數,無實際意義
public LinkedHashSet(int initialCapacity, float loadFactor) {
// 創建LinkedHashMap實例
super(initialCapacity, loadFactor, true);
}
public LinkedHashSet(int initialCapacity) {
super(initialCapacity, .75f, true);
}
public LinkedHashSet() {
super(16, .75f, true);
}
public LinkedHashSet(Collection<? extends E> c) {
super(Math.max(2*c.size(), 11), .75f, true);
addAll(c);
}
HashSet 方法
@Override
public Spliterator<E> spliterator() {
return Spliterators.spliterator(this, Spliterator.DISTINCT | Spliterator.ORDERED);
}