Java 集合

Posted 2020-06-13 菜鸟-翡青

Java 集合

标签 ： Java基础

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集合/容器

Java集合由Collection Map两个接口派生而出,Collection代表序列式容器,Map代表关联式容器.

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Collection

Collection作为List Queue Set等序列式容器的父接口, 提供了一些公共基础方法:

• update相关方法:
  boolean add(E e)
boolean addAll(Collection<? extends E> c)
void clear()
boolean remove(Object o)
boolean removeAll(Collection<?> c)
boolean retainAll(Collection<?> c)(取交集)

• select相关方法
  boolean contains(Object o)
boolean containsAll(Collection<?> c)
Iterator<E> iterator()
Object[] toArray()
<T> T[] toArray(T[] a)
boolean isEmpty()
int size()

详细可参考JDK文档

---

Iterator

iterator()方法返回一个迭代器Iterator.与其他容器主要用于存储数据不同,Iterator主要用于遍历容器.
Iterator隐藏了各类容器的底层实现细节,向应用程序提供了一个遍历容器的统一接口:

方法	释义
boolean hasNext()	Returns true if the iteration has more elements.
E next()	Returns the next element in the iteration.
void remove()	Removes from the underlying collection the last element returned by this iterator (optional operation).

注意: 当遍历Collection时不要使用Collection自带的remove方法删除数据,确实需要删除时,需要使用Iterator提供的remove.

/**
 * @author jifang
 * @since 16/1/25 上午9:59.
 */
public class RemoveClient {

    Collection<Integer> collection = new ArrayList<>();

    @Before
    public void setUp() {
        Random random = new Random();
        for (int i = 0; i < 10; ++i) {
            collection.add(random.nextInt(i + 1));
        }
    }

    @Test
    public void client() {
        System.out.print("before:");
        for (Iterator<Integer> iterator = collection.iterator(); iterator.hasNext(); ) {
            Integer integer = iterator.next();
            System.out.printf(" %d", integer);
            if (integer == 0) {
                //collection.remove(i);
                iterator.remove();
            }
        }
        System.out.printf("%n after:");
        for (Integer integer : collection) {
            System.out.printf(" %d", integer);
        }
    }
}

Java 1.5提供foreach循环使得代码更加简洁,但实际foreach迭代容器元素底层也是用的Iterator,这一点可以在调试时看得很清楚.

---

List

List代表有序/可重复集合,因此List在Collection的基础上添加了根据索引来操作元素的方法:

方法	描述
void add(int index, E element)	Inserts the specified element at the specified position in this list (optional operation).
E get(int index)	Returns the element at the specified position in this list.
int indexOf(Object o)	Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element.
int lastIndexOf(Object o)	Returns the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element.
E remove(int index)	Removes the element at the specified position in this list (optional operation).
E set(int index, E element)	Replaces the element at the specified position in this list with the specified element (optional operation).
List<E> subList(int fromIndex, int toIndex)	Returns a view of the portion of this list between the specified fromIndex, inclusive, and toIndex, exclusive.

List判断两个元素是否相等是通过equals()方法.

public int indexOf(Object o) {
    if (o == null) {
        for (int i = 0; i < size; i++)
            if (elementData[i]==null)
                return i;
    } else {
        for (int i = 0; i < size; i++)
            if (o.equals(elementData[i]))
                return i;
    }
    return -1;
}

---

ListIterator

List增加了返回ListIterator的方法:

方法	描述
ListIterator<E> listIterator()	Returns a list iterator over the elements in this list (in proper sequence).
ListIterator<E> listIterator(int index)	Returns a list iterator over the elements in this list (in proper sequence), starting at the specified position in the list.

ListIterator继承自Iterator,专门用于操作List, 在Iterator的基础上增加了如下方法:

方法	描述
void add(E e)	Inserts the specified element into the list (optional operation).
void set(E e)	Replaces the last element returned by next() or previous() with the specified element (optional operation).
boolean hasPrevious()	Returns true if this list iterator has more elements when traversing the list in the reverse direction.
E previous()	Returns the previous element in the list and moves the cursor position backwards.
int previousIndex()	Returns the index of the element that would be returned by a subsequent call to previous().
int nextIndex()	Returns the index of the element that would be returned by a subsequent call to next().

与Iterator相比增加了前向迭代 获取迭代元素index 以及add set的功能.

---

ArrayList

ArrayList是List基于数组的实现,它封装了一个动态自增长/允许再分配的Object[]数组:

/**
 * The array buffer into which the elements of the ArrayList are stored.
 * The capacity of the ArrayList is the length of this array buffer. Any
 * empty ArrayList with elementData == EMPTY_ELEMENTDATA will be expanded to
 * DEFAULT_CAPACITY when the first element is added.
 */
private transient Object[] elementData;

ArrayList可以使用initialCapacity参数来设置该数组的初始长度ArrayList(int initialCapacity),或者使用默认长度DEFAULT_CAPACITY = 10; 当添加元素超过elementData数组容量时,ArrayList会重新分配数组, 以容纳新元素:

    /**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }

    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

    /**
     * Increases the capacity to ensure that it can hold at least the
     * number of elements specified by the minimum capacity argument.
     *
     * @param minCapacity the desired minimum capacity
     */
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

如果在创建时就知道ArrayList的容量,最好同时指定initialCapacity的大小,以避免重新分配数组,耗费性能.

ArrayList还提供如下方法来调整initialCapacity大小:

方法	描述
void ensureCapacity(int minCapacity)	Increases the capacity of this ArrayList instance, if necessary, to ensure that it can hold at least the number of elements specified by the minimum capacity argument.
void trimToSize()	Trims the capacity of this ArrayList instance to be the list’s current size.

工具类Arrays还提供了一个static方法List<T> asList(T... a), 该方法可以把数组或N个对象转换成一个List集合, 这个List集合并不是普通的ArrayList,而是Arrays内部实现的一个Arrays.ArrayList(一个固定长度的List,不可对该集合做add/remove操作).
关于ArrayList实现原理还可以参考ArrayList源码解析

---

LinkedList

LinkedList是基于双向链表实现的List,虽然可以根据索引来访问集合中的元素,但性能不高(平均时间复杂度为O(N)),但其插入/删除操作非常迅速(尤其是在头尾,平均时间复杂度为O(1));除此之外,LinkedList还实现了Deque接口,因此还可以当成[双端]队列/栈来使用.
关于LinkedList的实现原理还可以参考 [1.LinkedList源码解析, 2. 双向循环链表的设计与实现]

/**
 * @author jifang
 * @since 16/1/23 下午9:07.
 */
public class ListClient {

    private Random random = new Random();

    @Test
    public void client() {
        List<Integer> list = new LinkedList<>();
        for (int i = 0; i < 10; ++i) {
            list.add(random.nextInt(i + 1));
        }

        for (ListIterator<Integer> i = list.listIterator(); i.hasNext(); ) {
            if (i.next() == 0) {
                i.set(188);
                i.add(-1);
            }
        }

        System.out.println(list);
    }
}

---

Queue

Queue用于模拟队列,队列是一种先进先出/FIFO容器,新元素插到队尾(offer), 访问操作会返回队首元素(poll). 通常, 队列不允许随机访问队列中的元素:

方法	描述
boolean add(E e)	Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning true upon success and throwing an IllegalStateException if no space is currently available.
boolean offer(E e)	Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions.
E element()	Retrieves, but does not remove, the head of this queue.
E peek()	Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.
E poll()	Retrieves and removes the head of this queue, or returns null if this queue is empty.
E remove()	Retrieves and removes the head of this queue.

Queue有一个PriorityQueue实现类,另外Queue还有一个Deque子接口,代表可以从两端存取数据的队列(因此Deque可以当成Stack使用),Java为Deque提供了ArrayDeque和LinkedList两个实现类.

---

PriorityQueue

PriorityQueue并不是按照插入队列顺序进行排序,而是按照队列元素的大小(权重)进行排序, 因此element/peek/poll/remove返回的并不是最早进入队列的元素,而是队列中[权重]最小的元素:

/**
 * @author jifang
 * @since 16/1/28 下午6:20.
 */
public class QueueClient {

    @Test
    public void clientPriorityQueue() {

        Random random = new Random();

        Queue<Integer> queue = new PriorityQueue<>();
        for (int i = 0; i < 10; ++i) {
            queue.add(random.nextInt(100));
        }

        // 无序
        System.out.print("iterator:");
        for (Integer i : queue) {
            System.out.printf(" %d", i);
        }
        System.out.println();

        // 有序
        System.out.print("pool:");
        while (!queue.isEmpty()) {
            System.out.printf(" %d", queue.poll());
        }
        System.out.println();
    }
}

可以看到遍历PriorityQueue得到的并不是有序序列, 因为PriorityQueue内部并不是一个按照顺序排序的数组, 而是一个二叉堆(详细可以参考[1. PriorityQueue源码解析, 2. 堆与堆排序 ]).

由于需要排序,PriorityQueue不允许插入null;

PriorityQueue的元素有两种排序方式

• 自然排序: 采用自然排序的元素必须实现了Comparable接口.
• 定制排序: 创建PriorityQueue时,传入一个Comparator实例,该对象负责对元素进行排序,采用定制排序时不要求队列元素实现Comparable接口.

关于两种排序的详细内容可以参考下面关于TreeMap的讨论.

---

Deque-ArrayDeque

Deque接口代表一个双端队列,提供了如下方法从队列的两端存取数据:

Java为Deque提供了两个实现类ArrayDeque(基于数组)与LinkedList(基于链表);由于ArrayDeque底层基于数组E[] elements实现,因此创建时可以指定一个numElements参数设置elements数组初始长度,如果不指定numElements参数,默认数组长度为16(关于ArrayDeque的实现原理可参考ArrayDeque源码解析).

Deque还可以作为栈stack使用, 他提供了如下方法:

@Test
public void asStack() {
    Deque<Integer> stack = new ArrayDeque<>();

    for (int i = 0; i < 10; ++i) {
        stack.push(i);
    }

    while (!stack.isEmpty()) {
        System.out.println(stack.pop());
    }
}

此外, LinkedList也实现了Deque接口,因此也可以作为Queue/Deque的实现类.

---

Map

Map用于保存具有映射关系的key-value数据,key和value之间存在单向一对一关系,通过指定的key,总能找到唯一确定的value.

• update相关
  V put(K key, V value)
void putAll(Map<? extends K,? extends V> m)
V remove(Object key)
void clear()

• select相关
  V get(Object key)
Set<K> keySet()
Collection<V> values()
Set<Map.Entry<K,V>> entrySet()
boolean containsKey(Object key)
boolean containsValue(Object value)
boolean isEmpty()
int size()

Map内部定义一个Map.Entry<K,V>接口,封装key-value对,Entry提供如下方法:

方法	描述
K getKey()	Returns the key corresponding to this entry.
V getValue()	Returns the value corresponding to this entry.
V setValue(V value)	Replaces the value corresponding to this entry with the specified value (optional operation).

---

HashMap

HashMap是基于hash算法的Map实现(用它代替Hashtable),针对key-value的插入/检索,这种形式具有最稳定的性能(O(1)),还可通过构造器对这一性能进行调整.
为了成功在HashMap中存取数据,key对象必须实现hashCode()与equals()方法,HashMap先通过key的hashCode()定位到元素所在桶,如果两个元素在同一个桶,再用equals()进行判断是否相等.如果两个对象的hashCode()相同,但equals()不同, 则将两个对象放在同一个桶的不同链表位置(这样会导致hash效率下降).如果两个对象通过equals()返回true, 但这hashCode()不同,则非常有可能导致HashMap将这两个对象分配在不同桶中,从而使这两个对象都添加成功,这就与Map规则冲突了.(关于HashMap详细原理可以参考: [1. 哈希表的设计与实现, 2.HashMap源码解析]).

建议: 如果两个对象通过equals()方法比较返回true, 则两个对象的hashCode()值也相同.

hashCode()重写规则:

• 运行过程中, 同一个对象多次调用hashCode()应具有相同返回值;
• 当两个对象通过equals()比较返回true时, hashCode()应具有相同返回值;
• 对象中用作equals()比较标准的实例变量, 都应该用于计算hashCode().

hashCode()重写方法:

将每个有意义的实例变量都计算出一个 int的 hashcode值.

类型	计算方式
boolean	hashCode = (true ? 1 : 0);
float	hashCode = Float.floatToIntBits(f);
double	long value = Double.doubleToLongBits(f);
	hashCode = (int)(value^(value>>>32));
int/short/byte	hashCode = (int)i;
long	hashCode = (int)(l^(l>>>32));
引用类型	hashCode = object.hashCode();

用上面计算出来的多个 hashcode组合计算成一个最终的 hashcode,为了避免直接相加产生偶然相等,可以为各个 hashcode乘以任意一个质数再相加:

• String实现

public int hashCode() {
    int h = hash;
    if (h == 0 && value.length > 0) {
        char val[] = value;

        for (int i = 0; i < value.length; i++) {
            h = 31 * h + val[i];
        }
        hash = h;
    }
    return h;
}

String的hashCode()方法做了一些优化, 叫闪存散列码, 详见数据结构与算法分析 : Java语言描述

• 自定义Bean

/**
 * @author jifang
 * @since 16/1/13下午7:50.
 */
public class Bean implements Serializable {

    private static final long serialVersionUID = 2975296536292876992L;

    private boolean isUsed;

    private double rate;

    private String name;

    @Override
    public int hashCode() {
        long rateHash = Double.doubleToLongBits(rate);
        int isUsedHash = isUsed ? 1 : 0;
        int nameHash = name.hashCode();

        return nameHash * 11 + (int) (rateHash ^ (rateHash >>> 32)) * 13 + isUsedHash;
    }

    // ..

}

• HashMap的主要实现逻辑:

/**
 * Associates the specified value with the specified key in this map.
 * If the map previously contained a mapping for the key, the old
 * value is replaced.
 *
 * @param key key with which the specified value is to be associated
 * @param value value to be associated with the specified key
 * @return the previous value associated with <tt>key</tt>, or
 *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
 *         (A <tt>null</tt> return can also indicate that the map
 *         previously associated <tt>null</tt> with <tt>key</tt>.)
 */
public V put(K key, V value) {
    if (table == EMPTY_TABLE) {
        inflateTable(threshold);
    }
    if (key == null)
        return putForNullKey(value);
    int hash = hash(key);
    int i = indexFor(hash, table.length);
    for (Entry<K,V> e = table[i]; e != null; e = e.next) {
        Object k;
        if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
            V oldValue = e.value;
            e.value = value;
            e.recordAccess(this);
            return oldValue;
        }
    }

    modCount++;
    addEntry(hash, key, value, i);
    return null;
}

/**
 * Adds a new entry with the specified key, value and hash code to
 * the specified bucket.  It is the responsibility of this
 * method to resize the table if appropriate.
 *
 * Subclass overrides this to alter the behavior of put method.
 */
void addEntry(int hash, K key, V value, int bucketIndex) {
    if ((size >= threshold) && (null != table[bucketIndex])) {
        resize(2 * table.length);
        hash = (null != key) ? hash(key) : 0;
        bucketIndex = indexFor(hash, table.length);
    }

    createEntry(hash, key, value, bucketIndex);
}

/**
 * Like addEntry except that this version is used when creating entries
 * as part of Map construction or "pseudo-construction" (cloning,
 * deserialization).  This version needn't worry about resizing the table.
 *
 * Subclass overrides this to alter the behavior of HashMap(Map),
 * clone, and readObject.
 */
void createEntry(int hash, K key, V value, int bucketIndex) {
    Entry<K,V> e = table[bucketIndex];
    table[bucketIndex] = new Entry<>(hash, key, value, e);
    size++;
}

注意

• 当向Map类容器(如HashMap TreeMap 或后面的HashSet TreeSet)中添加可变对象时,必须十分小心,如果修改Map中的key,有可能导致该key与集合中的其他key相等,从而导致无法准确访问该key-value.因此尽量不要使用可变对象作为Map的key,或不要修改作为key的对象(Set的value于此类同)
• Map还支持containsValue()方法来判断一个value是否存在于Map中, 但该方法会遍历所有的桶查找这个值, 因此性能较差, 不推荐使用

public boolean containsValue(Object value) {
    if (value == null)
        return containsNullValue();

    Entry[] tab = table;
    for (int i = 0; i < tab.length ; i++)
        for (Entry e = tab[i] ; e != null ; e = e.next)
            if (value.equals(e.value))
                return true;
    return false;
}

/**
 * Special-case code for containsValue with null argument
 */
private boolean containsNullValue() {
    Entry[] tab = table;
    for (int i = 0; i < tab.length ; i++)
        for (Entry e = tab[i] ; e != null ; e = e.next)
            if (e.value == null)
                return true;
    return false;
}

---

LinkedHashMap

LinkedHashMap使用双向链表来维护key-value插入顺序,因此性能略低于HashMap,但在需要顺序迭代Map的场景下会有非常好的效率.

LinkedHashMap提供的addEntry()方法与HashMap有所不同,当使用LinkedHashMap的put()时, 会从HashMap调回到LinkedHashMap的addEntry()方法,将新元素添加到链表尾:

/**
 * This override alters behavior of superclass put method. It causes newly
 * allocated entry to get inserted at the end of the linked list and
 * removes the eldest entry if appropriate.
 */
void addEntry(int hash, K key, V value, int bucketIndex) {
    super.addEntry(hash, key, value, bucketIndex);

    // Remove eldest entry if instructed
    Entry<K,V> eldest = header.after;
    if (removeEldestEntry(eldest)) {
        removeEntryForKey(eldest.key);
    }
}

/**
 * This override differs from addEntry in that it doesn't resize the
 * table or remove the eldest entry.
 */
void createEntry(int hash, K key, V value, int bucketIndex) {
    HashMap.Entry<K,V> old = table[bucketIndex];
    Entry<K,V> e = new Entry<>(hash, key, value, old);
    table[bucketIndex] = e;
    e.addBefore(header);
    size++;
}

• 使用LinkedHashMap统计word出现次数

/**
 * @author jifang
 * @since 16/1/28 上午10:33.
 */
public class MapClient {

    private Random random = new Random();

    @Test
    public void clientLinkedHashMap() {
        Map<String, Integer> map = new LinkedHashMap<>();
        System.out.print("insert key:");
        for (int i = 0; i < 20; ++i) {
            String key = String.valueOf(random.nextInt(10));
            System.out.printf(" %s", key);
            if (map.get(key) == null) {
                map.put(key, 1);
            } else {
                map.put(key, map.get(key) + 1);
            }
        }
        System.out.printf("%n iterator:");

        for (Map.Entry<String, Integer> entry : map.entrySet()) {
            System.out.printf(" <%s -> %s>", entry.getKey(), entry.getValue());
        }

    }
}

---

WeakHashMap

WeakHashMap与HashMap的区别在于:HashMap的key保留了对实际对象的强引用, 这意味着只要该HashMap不被销毁,则Map的所有key所引用的对象不会被垃圾回收;但WeakHashMap的key只保留对实际对象的弱引用, 这意味着如果该key所引用的对象没有被其他强引用变量引用,则该对象可能被垃圾回收,WeakHashMap也会自动删除这些key对应的key-value对.

@Test
public void clientWeakHashMap() {
    Map<String, String> map = new WeakHashMap<>();
    String key = "key";
    map.put(key, "value");
    map.put(new String("key1"), "value");
    map.put(new String("key2"), "value");
    System.out.printf("Before : %d%n", map.size());

    System.gc();
    System.runFinalization();
    System.out.printf("After : %d ", map.size());
}

如果使用WeakHashMap来保留对象的弱引用,则不要让该key所引用的对象具有任何强引用, 否则将失去使用WeakHashMap的意义.

---

IdentityHashMap

与HashMap不同,IdentityHashMap判断元素是否相等的标准是用==而不是equals();

public boolean containsKey(Object key) {
    Object k = maskNull(key);
    Object[] tab = table;
    int len = tab.length;
    int i = hash(k, len);
    while (true) {
        Object item = tab[i];
        if (item == k)
            return true;
        if (item == null)
            return false;
        i = nextKeyIndex(i, len);
    }
}

---

SortedMap-TreeMap

Map接口派生出SortedMap接口代表根据key排序的key-value集合, TreeMap作为SortedMap的实现类是一个红黑树结构,每个key-value作为红黑树的一个节点.TreeMap存储key-value时,根据key值进行排序.因此TreeMap可以保证所有元素都处于有序状态,因此SortedMap在Map的基础上又添加了如下方法:

方法	描述
Comparator<? super K> comparator()	Returns the comparator used to order the keys in this map, or null if this map uses the natural ordering of its keys.
K firstKey()	Returns the first (lowest) key currently in this map.
K lastKey()	Returns the last (highest) key currently in this map.
SortedMap<K,V> headMap(K toKey)	Returns a view of the portion of this map whose keys are strictly less than toKey.
SortedMap<K,V> tailMap(K fromKey)	Returns a view of the portion of this map whose keys are greater than or equal to fromKey.
SortedMap<K,V> subMap(K fromKey, K toKey)	Returns a view of the portion of this map whose keys range from fromKey, inclusive, to toKey, exclusive.

而TreeMap又在SortedMap的基础上扩展了如下方法:

方法	描述
Map.Entry<K,V> ceilingEntry(K key)	Returns a key-value mapping associated with the least key greater than or equal to the given key, or null if there is no such key.
K ceilingKey(K key)	Returns the least key greater than or equal to the given key, or null if there is no such key.
Map.Entry<K,V> floorEntry(K key)	Returns a key-value mapping associated with the greatest key less than or equal to the given key, or null if there is no such key.
K floorKey(K key)	Returns the greatest key less than or equal to the given key, or null if there is no such key.
Map.Entry<K,V> higherEntry(K key)	Returns a key-value mapping associated with the least key strictly greater than the given key, or null if there is no such key.
K higherKey(K key)	Returns the least key strictly greater than the given key, or null if there is no such key.
Map.Entry<K,V> lowerEntry(K key)	Returns a key-value mapping associated with the greatest key strictly less than the given key, or null if there is no such key.
K lowerKey(K key)	Returns the greatest key strictly less than the given key, or null if there is no such key.
Map.Entry<K,V> pollFirstEntry()	Removes and returns a key-value mapping associated with the least key in this map, or null if the map is empty.
Map.Entry<K,V> pollLastEntry()	Removes and returns a key-value mapping associated with the greatest key in this map, or null if the map is empty.
Map.Entry<K,V> firstEntry()	Returns a key-value mapping associated with the least key in this map, or null if the map is empty.
Map.Entry<K,V> lastEntry()	Returns a key-value mapping associated with the greatest key in this map, or null if the map is empty.

TreeMap有两种排序方式:

---

自然排序

TreeMap的所有key必须实现Comparable接口,TreeMap会调用key的int compareTo(T o);方法来比较元素的大小,然后将集合元素升序排列.

/**
 * Compares two keys using the correct comparison method for this TreeMap.
 */
final int compare(Object k1, Object k2) {
    return comparator==null ? ((Comparable<? super K>)k1).compareTo((K)k2)
        : comparator.compare((K)k1, (K)k2);
}

Java提供的java.lang.Comparable接口仅包含一个int compareTo(T o);方法.大部分常用类都实现了该接口(如String, Integer等).

public int compareTo(String anotherString) {
    int len1 = value.length;
    int len2 = anotherString.value.length;
    int lim = Math.min(len1, len2);
    char v1[] = value;
    char v2[] = anotherString.value;

    int k = 0;
    while (k < lim) {
        char c1 = v1[k];
        char c2 = v2[k];
        if (c1 != c2) {
            return c1 - c2;
        }
        k++;
    }
    return len1 - len2;
}

与HashMap不同,TreeMap判断两个key是否相等的唯一标准是:通过compareTo方法比较返回值是否为0.

public V get(Object key) {
    Entry<K,V> p = getEntry(key);
    return (p==null ? null : p.value);
}

final Entry<K,V> getEntry(Object key) {
    // Offload comparator-based version for sake of performance
    if (comparator != null)
        return getEntryUsingComparator(key);
    if (key == null)
        throw new NullPointerException();
    Comparable<? super K> k = (Comparable<? super K>) key;
    Entry<K,V> p = root;
    while (p != null) {
        int cmp = k.compareTo(p.key);
        if (cmp < 0)
            p = p.left;
        else if (cmp > 0)
            p = p.right;
        else
            return p;
    }
    return null;
}

• 实现Comparable

/**
 * @author jifang
 * @since 16/1/13下午7:50.
 */
public class Bean implements Comparable<Bean> {

    private boolean isUsed;

    private double rate;

    private String name;

    public Bean(boolean isUsed, double rate, String name) {
        this.isUsed = isUsed;
        this.rate = rate;
        this.name = name;
    }

    @Override
    public int compareTo(Bean anotherBean) {
        double another = (anotherBean.isUsed ? 1 : 0) +
                anotherBean.rate + anotherBean.name.length();
        double self = (isUsed ? 1 : 0) + rate + name.length();
        return (int) (self - another);
    }

    @Override
    public String toString() {
        return "Bean{" +
                "isUsed=" + isUsed +
                ", rate=" + rate +
                ", name='" + name + '\\'' +
                '}';
    }
}

@Test
public void clientSortedMap() {
    // value作为期望的order
    SortedMap<Bean, Integer> map = new TreeMap<>();
    map.put(new Bean(true, 3.14, "true"), 1);
    // 该对象与上面的bean compare会返回0
    map.put(new Bean(false, 3.14, "false"), 1);
    map.put(new Bean(true, 3.14, "false"), 2);
    map.put(new Bean(false, 3.14, "true"), 0);
    System.out.println(map);

    Bean firstKey = map.firstKey();
    System.out.printf("first: %s -> %d%n", firstKey, map.get(firstKey));
    Bean lastKey = map.lastKey();
    System.out.printf("last: %s -> %d%n", lastKey, map.get(lastKey));

    map.remove(firstKey);
    Map.Entry<Bean, Integer> firstEntry = ((TreeMap<Bean, Integer>) map).firstEntry();
    System.out.printf("A first: %s -> %d%n", firstEntry.getKey(), firstEntry.getValue());
}

当执行了remove方法后, TreeMap会对集合中的元素重新索引, 这一点可以在调试时看到.

---

自定义排序

TreeMap默认的是使用升序排序,如果需要自定义排序规则,需要为其传入一个Comparator实例, 采用定制排序时不要求key实现Comparable.

public class MapClient {

    private Comparator<Bean> comparator = new Comparator<Bean>() {
        @Override
        public int compare(Bean o1, Bean o2) {
            // 返回正数: 说明o1 > o2
            // 返回负数: 说明o1 < o2
            return -o1.compareTo(o2);
        }
    };

    @Test
    public void clientSortedMap() {
        SortedMap<Bean, Integer> map = new TreeMap<>(comparator);
        // ...
    }
}

由于TreeMap是基于红黑树实现,因此相比HashMap性能要慢一点(Hash平均O(1),Tree平均O(lgN)详细可参考[1. TreeMap源码解析, 2.红黑树的设计与实现(上)]),但其中的key-value已是有序状态,无需再有专门的排序操作.因此适用于key有序的场景.

---

EnumMap

EnumMap是一个需要与枚举类一起使用的Map,其所有key都必须是单个枚举类的枚举值.EnumMap具有以下特征:

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