Java-集合类对应数据类型简单解析

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文章将简单的结合JDK源码来展示Java 常用集合类型的数据结构,所使用的JDK为1.8版本。

1.List和Set相比,List是有序可重复的,Set是无序不可重复;

2.HashMap无序线程不安全,HashTable线程安全,TreeMap有序;

3.HashMap的Key和Value都可以为null,HashTable不可以为null;

4.编译阶段HashMap/TreeMap/HashTable Key和Value都可以null;

5.运行是阶段HashMap key和value也可以为null,当get(null)是,value=null;
6.运行是阶段TreeMap value可以为null,但是key不可以为null,否则会报空指针;

7.运行是阶段HashTable key和value都不可以为null否则会报空;

HashMap
public class Test {
    static Map map = new HashMap<>();
    static Map map1 = new TreeMap();
    static Map map2 = new Hashtable();
    public static void main(String[] args){

        map.put("123",null);
        System.out.println("Key 123 value is " + map.get("123"));
        System.out.println("====================");
        map.put(null,null);
        System.out.println("Key null value is " + map.get(null));
    }
}

输出结果:

Key 123 value is null

====================

Key null value is null

TreeMap
public class Test {
    static Map map = new HashMap<>();
    static Map map1 = new TreeMap();
    static Map map2 = new Hashtable();
    public static void main(String[] args){

        map1.put("123",null);
        System.out.println("Key 123 value is " + map1.get("123"));
        System.out.println("====================");
        map1.put(null,null);
        System.out.println("Key null value is " + map1.get(null));
    }
}

输出结果:

Exception in thread "main" java.lang.NullPointerException

at java.util.TreeMap.put(TreeMap.java:563)

at com.troy.concurrent.Test.main(Test.java:15)

Key 123 value is null

====================

HashTable
public class Test {
    static Map map = new HashMap<>();
    static Map map1 = new TreeMap();
    static Map map2 = new Hashtable();
    public static void main(String[] args){

        map2.put("123",null);
        System.out.println("Key 123 value is " + map2.get("123"));
        System.out.println("====================");
        map2.put(null,null);
        System.out.println("Key null value is " + map2.get(null));
    }
}

输出结果:

Exception in thread "main" java.lang.NullPointerException

at java.util.Hashtable.put(Hashtable.java:459)

at com.troy.concurrent.Test.main(Test.java:12)

List

ArrayList

源码展示:
根据ArrayList的源码可以看到ArrayList的底层存储结构是一个Object[] elementData,然后可以看到get(int index) 方法也是从elementData里面拿value出来,也就是说ArrayList的底层数据结构是一个数组。

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    private static final long serialVersionUID = 8683452581122892189L;

    /**
     * Default initial capacity.
     */
    private static final int DEFAULT_CAPACITY = 10;
    ...
    ...
    ...
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    /**
     * 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 == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
     * will be expanded to DEFAULT_CAPACITY when the first element is added.
     */
    transient Object[] elementData; // non-private to simplify nested class access
    ...
    ...
    ...
    /**
     * Returns the element at the specified position in this list.
     *
     * @param  index index of the element to return
     * @return the element at the specified position in this list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }
    

LinkedList

源码展示:
通过源码可以看到,LinkedList里面保存的是一个Node节点,但是Linkedlist里面的Node节点数据特点是保存了前一个Node,后一个Node还有自己本身,从这里我们可以得知LinkedList里面保存的是一个链表结构,并且在get(index)的时候通过二分法判断,然后逐个遍历Node节点通过由前到后或者由后到前的方法,最终拿到该index的Node节点并返回。

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;
        }
    }
    
    /**
     * Returns the element at the specified position in this list.
     *
     * @param index index of the element to return
     * @return the element at the specified position in this list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E get(int index) {
        checkElementIndex(index);
        return node(index).item;
    }
    
    /**
     * Returns the (non-null) Node at the specified element index.
     */
    Node<E> node(int index) {
        // assert isElementIndex(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;
        }
    }
    
    

Map

HashMap

源码展示:
通过源码可以看到HashMap里面的Node保存了一个Key的hash值,当前Node的Key & Value,还有Next Node,同时可以在get方法里面发现,HashMap拿值是从一个叫做table的变量里面去拿,通过resize方法可以看到table是一个Node数组,当从HashMap里面get数据的时候是通过对比hash值来判断,据说此方法可以显著提高效率,博主才疏学浅不知道原理,就先这样记下吧,到这里我们可以看到HashMap的数据是保存在一个带有hash值的Node数组里面,并且每个Node内部也有一个指向了下一个Node的对象。

public class HashMap<K,V> extends AbstractMap<K,V>
    implements Map<K,V>, Cloneable, Serializable {
    /**
     * Basic hash bin node, used for most entries.  (See below for
     * TreeNode subclass, and in LinkedHashMap for its Entry subclass.)
     */
    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 final int hashCode() {
            return Objects.hashCode(key) ^ Objects.hashCode(value);
        }

        public final V setValue(V newValue) {
            V oldValue = value;
            value = newValue;
            return oldValue;
        }

        public final boolean equals(Object o) {
            if (o == this)
                return true;
            if (o instanceof Map.Entry) {
                Map.Entry<?,?> e = (Map.Entry<?,?>)o;
                if (Objects.equals(key, e.getKey()) &&
                    Objects.equals(value, e.getValue()))
                    return true;
            }
            return false;
        }
    }
    /**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * <p>More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
     * key.equals(k))}, then this method returns {@code v}; otherwise
     * it returns {@code null}.  (There can be at most one such mapping.)
     *
     * <p>A return value of {@code null} does not <i>necessarily</i>
     * indicate that the map contains no mapping for the key; it's also
     * possible that the map explicitly maps the key to {@code null}.
     * The {@link #containsKey containsKey} operation may be used to
     * distinguish these two cases.
     *
     * @see #put(Object, Object)
     */
    public V get(Object key) {
        Node<K,V> e;
        return (e = getNode(hash(key), key)) == null ? null : e.value;
    }
    
    static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }


    /**
     * Implements Map.get and related methods
     *
     * @param hash hash for key
     * @param key the key
     * @return the node, or null if none
     */
    final Node<K,V> getNode(int hash, Object key) {
        Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (first = tab[(n - 1) & hash]) != null) {
            if (first.hash == hash && // always check first node
                ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
            if ((e = first.next) != null) {
                if (first instanceof TreeNode)
                    return ((TreeNode<K,V>)first).getTreeNode(hash, key);
                do {
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        return e;
                } while ((e = e.next) != null);
            }
        }
        return null;
    }
    
    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; // double threshold
        }
        else if (oldThr > 0) // initial capacity was placed in threshold
            newCap = oldThr;
        else {               // zero initial threshold signifies using defaults
            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 { // preserve order
                        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;
    }

TreeMap

源码展示:
顾名思义TreeMap的数据结构就是一个树形,Java中的TreeMap里面的数据结构对应的是一个二叉树,代码里面可以看到当我们调用get方法去拿值的时候会去一个叫做root的对象里面开始查找(也就是说从根节点开始遍历),最终返回匹配的值。

public class TreeMap<K,V>
    extends AbstractMap<K,V>
    implements NavigableMap<K,V>, Cloneable, java.io.Serializable
{
    /**
     * The comparator used to maintain order in this tree map, or
     * null if it uses the natural ordering of its keys.
     *
     * @serial
     */
    private final Comparator<? super K> comparator;

    private transient Entry<K,V> root;

    /**
     * The number of entries in the tree
     */
    private transient int size = 0;

    /**
     * The number of structural modifications to the tree.
     */
    private transient int modCount = 0;

    /**
     * Constructs a new, empty tree map, using the natural ordering of its
     * keys.  All keys inserted into the map must implement the {@link
     * Comparable} interface.  Furthermore, all such keys must be
     * <em>mutually comparable</em>: {@code k1.compareTo(k2)} must not throw
     * a {@code ClassCastException} for any keys {@code k1} and
     * {@code k2} in the map.  If the user attempts to put a key into the
     * map that violates this constraint (for example, the user attempts to
     * put a string key into a map whose keys are integers), the
     * {@code put(Object key, Object value)} call will throw a
     * {@code ClassCastException}.
     */
    public TreeMap() {
        comparator = null;
    }

    /**
     * Constructs a new, empty tree map, ordered according to the given
     * comparator.  All keys inserted into the map must be <em>mutually
     * comparable</em> by the given comparator: {@code comparator.compare(k1,
     * k2)} must not throw a {@code ClassCastException} for any keys
     * {@code k1} and {@code k2} in the map.  If the user attempts to put
     * a key into the map that violates this constraint, the {@code put(Object
     * key, Object value)} call will throw a
     * {@code ClassCastException}.
     *
     * @param comparator the comparator that will be used to order this map.
     *        If {@code null}, the {@linkplain Comparable natural
     *        ordering} of the keys will be used.
     */
    public TreeMap(Comparator<? super K> comparator) {
        this.comparator = comparator;
    }

    /**
     * Constructs a new tree map containing the same mappings as the given
     * map, ordered according to the <em>natural ordering</em> of its keys.
     * All keys inserted into the new map must implement the {@link
     * Comparable} interface.  Furthermore, all such keys must be
     * <em>mutually comparable</em>: {@code k1.compareTo(k2)} must not throw
     * a {@code ClassCastException} for any keys {@code k1} and
     * {@code k2} in the map.  This method runs in n*log(n) time.
     *
     * @param  m the map whose mappings are to be placed in this map
     * @throws ClassCastException if the keys in m are not {@link Comparable},
     *         or are not mutually comparable
     * @throws NullPointerException if the specified map is null
     */
    public TreeMap(Map<? extends K, ? extends V> m) {
        comparator = null;
        putAll(m);
    }
    public V get(Object key) {
        Entry<K,V> p = getEntry(key);
        return (p==null ? null : p.value);
    }
    /**
     * Returns this map's entry for the given key, or {@code null} if the map
     * does not contain an entry for the key.
     *
     * @return this map's entry for the given key, or {@code null} if the map
     *         does not contain an entry for the key
     * @throws ClassCastException if the specified key cannot be compared
     *         with the keys currently in the map
     * @throws NullPointerException if the specified key is null
     *         and this map uses natural ordering, or its comparator
     *         does not permit null keys
     */
    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();
        @SuppressWarnings("unchecked")
            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;
    }
    static final class Entry<K,V> implements Map.Entry<K,V> {
        K key;
        V value;
        Entry<K,V> left;
        Entry<K,V> right;
        Entry<K,V> parent;
        boolean color = BLACK;

        /**
         * Make a new cell with given key, value, and parent, and with
         * {@code null} child links, and BLACK color.
         */
        Entry(K key, V value, Entry<K,V> parent) {
            this.key = key;
            this.value = value;
            this.parent = parent;
        }

        /**
         * Returns the key.
         *
         * @return the key
         */
        public K getKey() {
            return key;
        }

        /**
         * Returns the value associated with the key.
         *
         * @return the value associated with the key
         */
        public V getValue() {
            return value;
        }

        /**
         * Replaces the value currently associated with the key with the given
         * value.
         *
         * @return the value associated with the key before this method was
         *         called
         */
        public V setValue(V value) {
            V oldValue = this.value;
            this.value = value;
            return oldValue;
        }

        public boolean equals(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry<?,?> e = (Map.Entry<?,?>)o;

            return valEquals(key,e.getKey()) && valEquals(value,e.getValue());
        }

        public int hashCode() {
            int keyHash = (key==null ? 0 : key.hashCode());
            int valueHash = (value==null ? 0 : value.hashCode());
            return keyHash ^ valueHash;
        }

        public String toString() {
            return key + "=" + value;
        }
    }    

Set

HashSet

源码展示:
通过代码可以看到HashSet里面的值都保存在HashMap中,因此可以参考HashMap解析。

public class HashSet<E>
    extends AbstractSet<E>
    implements Set<E>, Cloneable, java.io.Serializable
{
    static final long serialVersionUID = -5024744406713321676L;

    private transient HashMap<E,Object> map;

    // Dummy value to associate with an Object in the backing Map
    private static final Object PRESENT = new Object();

    /**
     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
     * default initial capacity (16) and load factor (0.75).
     */
    public HashSet() {
        map = new HashMap<>();
    }

    /**
     * Constructs a new set containing the elements in the specified
     * collection.  The <tt>HashMap</tt> is created with default load factor
     * (0.75) and an initial capacity sufficient to contain the elements in
     * the specified collection.
     *
     * @param c the collection whose elements are to be placed into this set
     * @throws NullPointerException if the specified collection is null
     */
    public HashSet(Collection<? extends E> c) {
        map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
        addAll(c);
    }

    /**
     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
     * the specified initial capacity and the specified load factor.
     *
     * @param      initialCapacity   the initial capacity of the hash map
     * @param      loadFactor        the load factor of the hash map
     * @throws     IllegalArgumentException if the initial capacity is less
     *             than zero, or if the load factor is nonpositive
     */
    public HashSet(int initialCapacity, float loadFactor) {
        map = new HashMap<>(initialCapacity, loadFactor);
    }

TreeSet

源码展示:
通过代码可以看到TreeSet里面的值都保存在HashMap中,因此可以参考TreeMap解析。

public class TreeSet<E> extends AbstractSet<E>
    implements NavigableSet<E>, Cloneable, java.io.Serializable
{
    /**
     * The backing map.
     */
    private transient NavigableMap<E,Object> m;

    // Dummy value to associate with an Object in the backing Map
    private static final Object PRESENT = new Object();

    /**
     * Constructs a set backed by the specified navigable map.
     */
    TreeSet(NavigableMap<E,Object> m) {
        this.m = m;
    }

    /**
     * Constructs a new, empty tree set, sorted according to the
     * natural ordering of its elements.  All elements inserted into
     * the set must implement the {@link Comparable} interface.
     * Furthermore, all such elements must be <i>mutually
     * comparable</i>: {@code e1.compareTo(e2)} must not throw a
     * {@code ClassCastException} for any elements {@code e1} and
     * {@code e2} in the set.  If the user attempts to add an element
     * to the set that violates this constraint (for example, the user
     * attempts to add a string element to a set whose elements are
     * integers), the {@code add} call will throw a
     * {@code ClassCastException}.
     */
    public TreeSet() {
        this(new TreeMap<E,Object>());
    }

    /**
     * Constructs a new, empty tree set, sorted according to the specified
     * comparator.  All elements inserted into the set must be <i>mutually
     * comparable</i> by the specified comparator: {@code comparator.compare(e1,
     * e2)} must not throw a {@code ClassCastException} for any elements
     * {@code e1} and {@code e2} in the set.  If the user attempts to add
     * an element to the set that violates this constraint, the
     * {@code add} call will throw a {@code ClassCastException}.
     *
     * @param comparator the comparator that will be used to order this set.
     *        If {@code null}, the {@linkplain Comparable natural
     *        ordering} of the elements will be used.
     */
    public TreeSet(Comparator<? super E> comparator) {
        this(new TreeMap<>(comparator));
    }