Java 集合，也叫作容器，主要是由两大接口派生而来：一个是 Collection接口，主要用于存放单一元素；另一个是 Map 接口，主要用于存放键值对。对于Collection 接口，下面又有三个主要的子接口：List、Set 、 Queue。本文将详细介绍这些接口及其实现类，并通过源码解读和实例来帮助你更好地理解它们。
Collection 接口
Collection 是 Java 集合框架的根接口，抽象了集合对象的基本操作。常用的方法有：
- add(E e)：添加元素到集合中。
- remove(Object o)：从集合中移除指定元素。
- size()：返回集合中元素的数量。
- contains(Object o)：判断集合中是否包含指定元素。
- iterator()：返回集合的迭代器。
java
public interface Collection<E> extends Iterable<E> {
boolean add(E e);
boolean remove(Object o);
int size();
boolean contains(Object o);
Iterator<E> iterator();
// 其他方法省略
}List 接口
List 接口继承自 Collection 接口，表示一个有序的元素集合。常用的实现类有 ArrayList、LinkedList、Vector 和 Stack。
java
public interface List<E> extends Collection<E> {
void add(int index, E element);
E get(int index);
E remove(int index);
int indexOf(Object o);
// 其他方法省略
}ArrayList
ArrayList 是基于动态数组实现的 List，支持快速随机访问。它的内部实现是一个数组，当数组容量不足时，自动扩容为原来的1.5倍。
java
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable {
private transient Object[] elementData; // 存放元素的数组
private int size; // 元素数量
public ArrayList() {
this.elementData = new Object[10]; // 默认初始容量为10
}
public boolean add(E e) {
ensureCapacity(size + 1); // 检查容量
elementData[size++] = e;
return true;
}
private void ensureCapacity(int minCapacity) {
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
private void grow(int minCapacity) {
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1); // 扩容为1.5倍
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
elementData = Arrays.copyOf(elementData, newCapacity);
}
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
// 其他方法省略
}LinkedList
LinkedList 是基于双向链表实现的 List，适合频繁的插入和删除操作。
java
public class LinkedList<E> extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable {
private static class Node<E> {
E item;
Node<E> next;
Node<E> prev;
Node(Node<E> prev, E element, Node<E> next) {
this.item = element;
this.next = next;
this.prev = prev;
}
}
private transient Node<E> first;
private transient Node<E> last;
private int size;
public LinkedList() {
}
public boolean add(E e) {
linkLast(e);
return true;
}
void linkLast(E e) {
final Node<E> l = last;
final Node<E> newNode = new Node<>(l, e, null);
last = newNode;
if (l == null)
first = newNode;
else
l.next = newNode;
size++;
}
public E get(int index) {
checkElementIndex(index);
return node(index).item;
}
Node<E> node(int index) {
if (index < (size >> 1)) {
Node<E> x = first;
for (int i = 0; i < index; i++)
x = x.next;
return x;
} else {
Node<E> x = last;
for (int i = size - 1; i > index; i--)
x = x.prev;
return x;
}
}
private void checkElementIndex(int index) {
if (!isElementIndex(index))
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
private boolean isElementIndex(int index) {
return index >= 0 && index < size;
}
// 其他方法省略
}Set 接口
Set 接口继承自 Collection 接口，表示一个不包含重复元素的集合。常用的实现类有 HashSet、LinkedHashSet 和 TreeSet。
java
public interface Set<E> extends Collection<E> {
// 继承了 Collection 的所有方法
}HashSet
HashSet 是基于哈希表实现的 Set，不保证元素的顺序。
java
public class HashSet<E> extends AbstractSet<E> implements Set<E>, Cloneable, java.io.Serializable {
private transient HashMap<E, Object> map;
// Dummy value to associate with an Object in the backing Map
private static final Object PRESENT = new Object();
public HashSet() {
map = new HashMap<>();
}
public boolean add(E e) {
return map.put(e, PRESENT) == null;
}
public boolean contains(Object o) {
return map.containsKey(o);
}
public boolean remove(Object o) {
return map.remove(o) == PRESENT;
}
public int size() {
return map.size();
}
// 其他方法省略
}Queue 接口
Queue 接口继承自 Collection 接口，表示一个先进先出的队列。常用的实现类有 LinkedList 和 PriorityQueue。
java
public interface Queue<E> extends Collection<E> {
boolean offer(E e);
E poll();
E peek();
// 其他方法省略
}PriorityQueue
PriorityQueue 是一个基于优先级堆实现的队列，元素按优先级顺序进行排序。
java
public class PriorityQueue<E> extends AbstractQueue<E> implements java.io.Serializable {
private transient Object[] queue;
private int size;
private final Comparator<? super E> comparator;
public PriorityQueue() {
this.queue = new Object[11];
this.comparator = null;
}
public boolean offer(E e) {
if (e == null)
throw new NullPointerException();
int i = size;
if (i >= queue.length)
grow(i + 1);
size = i + 1;
if (i == 0)
queue[0] = e;
else
siftUp(i, e);
return true;
}
private void siftUp(int k, E x) {
if (comparator != null)
siftUpUsingComparator(k, x);
else
siftUpComparable(k, x);
}
private void siftUpComparable(int k, E x) {
Comparable<? super E> key = (Comparable<? super E>) x;
while (k > 0) {
int parent = (k - 1) >>> 1;
Object e = queue[parent];
if (key.compareTo((E) e) >= 0)
break;
queue[k] = e;
k = parent;
}
queue[k] = key;
}
private void grow(int minCapacity) {
int oldCapacity = queue.length;
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
queue = Arrays.copyOf(queue, newCapacity);
}
public E poll() {
if (size == 0)
return null;
int s = --size;
E result = (E) queue[0];
E x = (E) queue[s];
queue[s] = null;
if (s != 0)
siftDown(0, x);
return result;
}
private void siftDown(int k, E x) {
if (comparator != null)
siftDownUsingComparator(k, x);
else
siftDownComparable(k, x);
}
private void siftDownComparable(int k, E x) {
Comparable<? super E> key = (Comparable<? super E>) x;
int half = size >>> 1;
while (k < half) {
int child = (k << 1) + 1;
Object c = queue[child];
int right = child + 1;
if (right < size && ((Comparable<? super E>) c).compareTo((E) queue[right]) > 0)
c = queue[child = right];
if (key.compareTo((E) c) <= 0)
break;
queue[k] = c;
k = child;
}
queue[k] = key;
}
public E peek() {
return (size == 0) ? null : (E) queue[0];
}
// 其他方法省略
}Map 接口
Map 接口表示一个键值对集合，每个键最多只能映射到一个值。常用的实现类有 HashMap、LinkedHashMap 和 TreeMap。
java
public interface Map<K, V> {
V put(K key, V value);
V get(Object key);
V remove(Object key);
boolean containsKey(Object key);
int size();
Set<K> keySet();
Collection<V> values();
// 其他方法省略
}HashMap
HashMap 是基于哈希表实现的 Map，允许使用 null 键和 null 值，不保证元素的顺序。
java
public class HashMap<K, V> extends AbstractMap<K, V> implements Map<K, V>, Cloneable, Serializable {
static final int DEFAULT_INITIAL_CAPACITY = 16;
static final float DEFAULT_LOAD_FACTOR = 0.75f;
transient Node<K, V>[] table;
transient int size;
int threshold;
final float loadFactor;
static class Node<K, V> implements Map.Entry<K, V> {
final int hash;
final K key;
V value;
Node<K, V> next;
Node(int hash, K key, V value, Node<K, V> next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}
public final K getKey() { return key; }
public final V getValue() { return value; }
public final String toString() { return key + "=" + value; }
}
public HashMap(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
this.loadFactor = loadFactor;
this.threshold = tableSizeFor(initialCapacity);
}
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K, V>[] tab; Node<K, V> p; int n, i;
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<K, V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
else if (p instanceof TreeNode)
e = ((TreeNode<K, V>)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1)
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
if (e != null) {
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
final Node<K, V>[] resize() {
Node<K, V>[] oldTab = table;
int oldCap = (oldTab == null) ? 0 : oldTab.length;
int oldThr = threshold;
int newCap, newThr = 0;
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1;
}
else if (oldThr > 0)
newCap = oldThr;
else {
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
if (newThr == 0) {
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
threshold = newThr;
@SuppressWarnings({"rawtypes","unchecked"})
Node<K, V>[] newTab = (Node<K, V>[])new Node[newCap];
table = newTab;
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K, V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e;
else if (e instanceof TreeNode)
((TreeNode<K, V>)e).split(this, newTab, j, oldCap);
else {
Node<K, V> loHead = null, loTail = null;
Node<K, V> hiHead = null, hiTail = null;
Node<K, V> next;
do {
next = e.next;
if ((e.hash & oldCap) == 0) {
if (loTail == null)
loHead = e;
else
loTail.next = e;
loTail = e;
}
else {
if (hiTail == null)
hiHead = e;
else
hiTail.next = e;
hiTail = e;
}
} while ((e = next) != null);
if (loTail != null) {
loTail.next = null;
newTab[j] = loHead;
}
if (hiTail != null) {
hiTail.next = null;
newTab[j + oldCap] = hiHead;
}
}
}
}
}
return newTab;
}
final Node<K, V> newNode(int hash, K key, V value, Node<K, V> next) {
return new Node<>(hash, key, value, next);
}
// 其他方法省略
}Java 集合框架提供了丰富的数据结构和算法，能够满足各种常见的编程需求。在实际开发中，选择合适的集合类型可以显著提高程序的性能和可读性。
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