Java集合类源码分析（六）：AbstractMap类

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Java集合类源码分析（一）：Collcetion接口
Java集合类源码分析（二）：List接口
Java集合类源码分析（三）：AbstractList类
Java集合类源码分析（四）：ArrayList&LinkedList
java集合类源码分析（五）：Map接口
Java集合类源码分析（六）：AbstractMap类
Java集合类源码分析（七）：HashMap&LinkedHashMap
Java集合类源码分析（八）：AbstractSet类
Java集合类源码分析（九）：HashSet&LinkedHashSet

AbstractMap源码解析

提供了对Map接口大致性的实现

如果要实现一个不可修改的map，只需要继承此类并实现entrySet方法。这个set不支持add,remove,它的迭代器也不支持remove

如果要实现一个可修改的map，还要额外重写这个类的put，迭代器也要实现remove

public boolean containsValue(Object value) {
    Iterator<Entry<K,V>> i = entrySet().iterator();
    if (value==null) {
        while (i.hasNext()) {
            Entry<K,V> e = i.next();
            if (e.getValue()==null)
                return true;
        }
    } else {
        while (i.hasNext()) {
            Entry<K,V> e = i.next();
            if (value.equals(e.getValue()))
                return true;
        }
    }
    return false;
}

使用迭代器遍历entrySet，寻找对应的值与给定参数value相同的键值对

public boolean containsKey(Object key) {
    Iterator<Map.Entry<K,V>> i = entrySet().iterator();
    if (key==null) {
        while (i.hasNext()) {
            Entry<K,V> e = i.next();
            if (e.getKey()==null)
                return true;
        }
    } else {
        while (i.hasNext()) {
            Entry<K,V> e = i.next();
            if (key.equals(e.getKey()))
                return true;
        }
    }
    return false;
}

使用迭代器遍历entrySet，寻找键与给定参数key相同的键值对

public V get(Object key) {
    Iterator<Entry<K,V>> i = entrySet().iterator();
    if (key==null) {
        while (i.hasNext()) {
            Entry<K,V> e = i.next();
            if (e.getKey()==null)
                return e.getValue();
        }
    } else {
        while (i.hasNext()) {
            Entry<K,V> e = i.next();
            if (key.equals(e.getKey()))
                return e.getValue();
        }
    }
    return null;
}

使用迭代器遍历entrySet，寻找键与给定参数key相同的键值对，返回对应的值

public V remove(Object key) {
    Iterator<Entry<K,V>> i = entrySet().iterator();
    Entry<K,V> correctEntry = null;
    if (key==null) {
        while (correctEntry==null && i.hasNext()) {
            Entry<K,V> e = i.next();
            if (e.getKey()==null)
                correctEntry = e;
        }
    } else {
        while (correctEntry==null && i.hasNext()) {
            Entry<K,V> e = i.next();
            if (key.equals(e.getKey()))
                correctEntry = e;
        }
    }

    V oldValue = null;
    if (correctEntry !=null) {
        oldValue = correctEntry.getValue();
        i.remove();
    }
    return oldValue;
}

遍历entrySet，寻找与给定参数key相同的键，如果找到，就调用迭代器的remove将该entry删除，返回对应的值，否则返回null

public V put(K key, V value) {
    throw new UnsupportedOperationException();
}

public void putAll(Map<? extends K, ? extends V> m) {
    for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
        put(e.getKey(), e.getValue());
}

迭代参数map，依次调用put方法

transient Set<K>        keySet;
transient Collection<V> values;

public Set<K> keySet() {
    Set<K> ks = keySet;
    if (ks == null) {
        ks = new AbstractSet<K>() {
            public Iterator<K> iterator() {
                return new Iterator<K>() {
                    private Iterator<Entry<K,V>> i = entrySet().iterator();

                    public boolean hasNext() {
                        return i.hasNext();
                    }

                    public K next() {
                        return i.next().getKey();
                    }

                    public void remove() {
                        i.remove();
                    }
                };
            }

            public int size() {
                return AbstractMap.this.size();
            }

            public boolean isEmpty() {
                return AbstractMap.this.isEmpty();
            }

            public void clear() {
                AbstractMap.this.clear();
            }

            public boolean contains(Object k) {
                return AbstractMap.this.containsKey(k);
            }
        };
        keySet = ks;
    }
    return ks;
}

在KeySet方法第一次被调用时，创建这个set，该方法没有锁

public Collection<V> values() {
    Collection<V> vals = values;
    if (vals == null) {
        vals = new AbstractCollection<V>() {
            public Iterator<V> iterator() {
                return new Iterator<V>() {
                    private Iterator<Entry<K,V>> i = entrySet().iterator();

                    public boolean hasNext() {
                        return i.hasNext();
                    }

                    public V next() {
                        return i.next().getValue();
                    }

                    public void remove() {
                        i.remove();
                    }
                };
            }

            public int size() {
                return AbstractMap.this.size();
            }

            public boolean isEmpty() {
                return AbstractMap.this.isEmpty();
            }

            public void clear() {
                AbstractMap.this.clear();
            }

            public boolean contains(Object v) {
                return AbstractMap.this.containsValue(v);
            }
        };
        values = vals;
    }
    return vals;
}

在Values方法第一次被调用时，创建这个集合，该方法没有锁

public abstract Set<Entry<K,V>> entrySet();    

public boolean equals(Object o) {
    if (o == this)
        return true;//判断是不是原map

    if (!(o instanceof Map))
        return false;//判断参数对象的类型
    Map<?,?> m = (Map<?,?>) o;
    if (m.size() != size())
        return false;//判断长度是否相等

    try {
        Iterator<Entry<K,V>> i = entrySet().iterator();
        while (i.hasNext()) {
            Entry<K,V> e = i.next();
            K key = e.getKey();
            V value = e.getValue();
            if (value == null) {
                if (!(m.get(key)==null && m.containsKey(key)))
                    return false;
            } else {
                if (!value.equals(m.get(key)))
                    return false;
            }
        }
    } catch (ClassCastException unused) {
        return false;
    } catch (NullPointerException unused) {
        return false;
    }//遍历，看entrySet是否相同

    return true;
}

两个map相等当且仅当m1.entrySet().equals(m2.entrySet())

public int hashCode() {
    int h = 0;
    Iterator<Entry<K,V>> i = entrySet().iterator();
    while (i.hasNext())
        h += i.next().hashCode();
    return h;
}

返回map的哈希值，是所有entry的哈希值之和，确保m1.equals(m2)能推出m1.hashCode()==m2.hashCode()

protected Object clone() throws CloneNotSupportedException {
    AbstractMap<?,?> result = (AbstractMap<?,?>)super.clone();
    result.keySet = null;
    result.values = null;
    return result;
}

浅拷贝

public static class SimpleEntry<K,V>
    implements Entry<K,V>, java.io.Serializable
{
    private static final long serialVersionUID = -8499721149061103585L;

    private final K key;
    private V value;

    /**
     * Creates an entry representing a mapping from the specified
     * key to the specified value.
     *
     * @param key the key represented by this entry
     * @param value the value represented by this entry
     */
    public SimpleEntry(K key, V value) {
        this.key   = key;
        this.value = value;
    }

    /**
     * Creates an entry representing the same mapping as the
     * specified entry.
     *
     * @param entry the entry to copy
     */
    public SimpleEntry(Entry<? extends K, ? extends V> entry) {
        this.key   = entry.getKey();
        this.value = entry.getValue();
    }

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

    /**
     * Returns the value corresponding to this entry.
     *
     * @return the value corresponding to this entry
     */
    public V getValue() {
        return value;
    }

    /**
     * Replaces the value corresponding to this entry with the specified
     * value.
     *
     * @param value new value to be stored in this entry
     * @return the old value corresponding to the entry
     */
    public V setValue(V value) {
        V oldValue = this.value;
        this.value = value;
        return oldValue;
    }

    /**
     * Compares the specified object with this entry for equality.
     * Returns {@code true} if the given object is also a map entry and
     * the two entries represent the same mapping.  More formally, two
     * entries {@code e1} and {@code e2} represent the same mapping
     * if<pre>
     *   (e1.getKey()==null ?
     *    e2.getKey()==null :
     *    e1.getKey().equals(e2.getKey()))
     *   &amp;&amp;
     *   (e1.getValue()==null ?
     *    e2.getValue()==null :
     *    e1.getValue().equals(e2.getValue()))</pre>
     * This ensures that the {@code equals} method works properly across
     * different implementations of the {@code Map.Entry} interface.
     *
     * @param o object to be compared for equality with this map entry
     * @return {@code true} if the specified object is equal to this map
     *         entry
     * @see    #hashCode
     */
    public boolean equals(Object o) {
        if (!(o instanceof Map.Entry))
            return false;
        Map.Entry<?,?> e = (Map.Entry<?,?>)o;
        return eq(key, e.getKey()) && eq(value, e.getValue());
    }

    /**
     * Returns the hash code value for this map entry.  The hash code
     * of a map entry {@code e} is defined to be: <pre>
     *   (e.getKey()==null   ? 0 : e.getKey().hashCode()) ^
     *   (e.getValue()==null ? 0 : e.getValue().hashCode())</pre>
     * This ensures that {@code e1.equals(e2)} implies that
     * {@code e1.hashCode()==e2.hashCode()} for any two Entries
     * {@code e1} and {@code e2}, as required by the general
     * contract of {@link Object#hashCode}.
     *
     * @return the hash code value for this map entry
     * @see    #equals
     */
    public int hashCode() {
        return (key   == null ? 0 :   key.hashCode()) ^
               (value == null ? 0 : value.hashCode());
    }

    /**
     * Returns a String representation of this map entry.  This
     * implementation returns the string representation of this
     * entry's key followed by the equals character ("<tt>=</tt>")
     * followed by the string representation of this entry's value.
     *
     * @return a String representation of this map entry
     */
    public String toString() {
        return key + "=" + value;
    }

}

public static class SimpleImmutableEntry<K,V>
    implements Entry<K,V>, java.io.Serializable
{
    private static final long serialVersionUID = 7138329143949025153L;

    private final K key;
    private final V value;

    /**
     * Creates an entry representing a mapping from the specified
     * key to the specified value.
     *
     * @param key the key represented by this entry
     * @param value the value represented by this entry
     */
    public SimpleImmutableEntry(K key, V value) {
        this.key   = key;
        this.value = value;
    }

    /**
     * Creates an entry representing the same mapping as the
     * specified entry.
     *
     * @param entry the entry to copy
     */
    public SimpleImmutableEntry(Entry<? extends K, ? extends V> entry) {
        this.key   = entry.getKey();
        this.value = entry.getValue();
    }

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

    /**
     * Returns the value corresponding to this entry.
     *
     * @return the value corresponding to this entry
     */
    public V getValue() {
        return value;
    }

    /**
     * Replaces the value corresponding to this entry with the specified
     * value (optional operation).  This implementation simply throws
     * <tt>UnsupportedOperationException</tt>, as this class implements
     * an <i>immutable</i> map entry.
     *
     * @param value new value to be stored in this entry
     * @return (Does not return)
     * @throws UnsupportedOperationException always
     */
    public V setValue(V value) {
        throw new UnsupportedOperationException();
    }

    /**
     * Compares the specified object with this entry for equality.
     * Returns {@code true} if the given object is also a map entry and
     * the two entries represent the same mapping.  More formally, two
     * entries {@code e1} and {@code e2} represent the same mapping
     * if<pre>
     *   (e1.getKey()==null ?
     *    e2.getKey()==null :
     *    e1.getKey().equals(e2.getKey()))
     *   &amp;&amp;
     *   (e1.getValue()==null ?
     *    e2.getValue()==null :
     *    e1.getValue().equals(e2.getValue()))</pre>
     * This ensures that the {@code equals} method works properly across
     * different implementations of the {@code Map.Entry} interface.
     *
     * @param o object to be compared for equality with this map entry
     * @return {@code true} if the specified object is equal to this map
     *         entry
     * @see    #hashCode
     */
    public boolean equals(Object o) {
        if (!(o instanceof Map.Entry))
            return false;
        Map.Entry<?,?> e = (Map.Entry<?,?>)o;
        return eq(key, e.getKey()) && eq(value, e.getValue());
    }

    /**
     * Returns the hash code value for this map entry.  The hash code
     * of a map entry {@code e} is defined to be: <pre>
     *   (e.getKey()==null   ? 0 : e.getKey().hashCode()) ^
     *   (e.getValue()==null ? 0 : e.getValue().hashCode())</pre>
     * This ensures that {@code e1.equals(e2)} implies that
     * {@code e1.hashCode()==e2.hashCode()} for any two Entries
     * {@code e1} and {@code e2}, as required by the general
     * contract of {@link Object#hashCode}.
     *
     * @return the hash code value for this map entry
     * @see    #equals
     */
    public int hashCode() {
        return (key   == null ? 0 :   key.hashCode()) ^
               (value == null ? 0 : value.hashCode());
    }

    /**
     * Returns a String representation of this map entry.  This
     * implementation returns the string representation of this
     * entry's key followed by the equals character ("<tt>=</tt>")
     * followed by the string representation of this entry's value.
     *
     * @return a String representation of this map entry
     */
    public String toString() {
        return key + "=" + value;
    }

}

内部Entry类的实现