AbstractMap

类关系图

功能介绍

包路径：java.util

说明：AbstractMap直接实现了Map接口，是一个抽象类，而抽象类通常作为骨架使用，实现一些简单且通用的功能，而核心功能则由子类实现。

AbstractMap是大部分Map接口实现类的父类，典型常用的几个如HashMap、TreeMap、WeakHashMap等。

源码解析

/**
 * This class provides a skeletal implementation of the <tt>Map</tt>
 * interface, to minimize the effort required to implement this interface.
 *
 * 此类提供了一个Map接口的骨架实现，以尽可能减少其他类实现Map接口的工作量
 *
 * @param <K> the type of keys maintained by this map Map映射的key类型
 * @param <V> the type of mapped values Map映射的value类型
 *
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @see Map
 * @see Collection
 * @since 1.2
 */
public abstract class AbstractMap<K,V> implements Map<K,V> {
    /**
     * Sole constructor.  (For invocation by subclass constructors, typically
     * implicit.)
     */
    protected AbstractMap() {
    }
    // Query Operations 检索操作实现
    /**
     * {@inheritDoc}
     *
     * @implSpec
     * This implementation returns <tt>entrySet().size()</tt>.
     */
    public int size() {
        return entrySet().size(); // 直接返回Map内部维护的Entry实体个数，即映射个数
    }
    /**
     * {@inheritDoc}
     *
     * @implSpec
     * This implementation returns <tt>size() == 0</tt>.
     */
    public boolean isEmpty() {
        return size() == 0;
    }
    /**
     * Map中是否存在目标value
     *
     * @implSpec
     * This implementation iterates over <tt>entrySet()</tt> searching
     * for an entry with the specified value.  If such an entry is found,
     * <tt>true</tt> is returned.  If the iteration terminates without
     * finding such an entry, <tt>false</tt> is returned.  Note that this
     * implementation requires linear time in the size of the map.
     */
    public boolean containsValue(Object value) {
        // Entry集合迭代器
        Iterator<Entry<K,V>> i = entrySet().iterator();
        if (value==null) {
            // 遍历Entry集合并比较value
            while (i.hasNext()) {
                Entry<K,V> e = i.next();
                if (e.getValue()==null)
                    return true;
            }
        } else {
            // 遍历Entry集合并比较value
            while (i.hasNext()) {
                Entry<K,V> e = i.next();
                if (value.equals(e.getValue()))
                    return true;
            }
        }
        return false;
    }
    /**
     * Map中是否存在目标key对应映射
     *
     * @implSpec
     * This implementation iterates over <tt>entrySet()</tt> searching
     * for an entry with the specified key.  If such an entry is found,
     * <tt>true</tt> is returned.  If the iteration terminates without
     * finding such an entry, <tt>false</tt> is returned.  Note that this
     * implementation requires linear time in the size of the map; many
     * implementations will override this method.
     */
    public boolean containsKey(Object key) {
        // Entry集合迭代器
        Iterator<Map.Entry<K,V>> i = entrySet().iterator();
        if (key==null) {
            // 遍历Entry集合并比较key
            while (i.hasNext()) {
                Entry<K,V> e = i.next();
                if (e.getKey()==null)
                    return true;
            }
        } else {
            // 遍历Entry集合并比较key
            while (i.hasNext()) {
                Entry<K,V> e = i.next();
                if (key.equals(e.getKey()))
                    return true;
            }
        }
        return false;
    }
    /**
     * 据传入的key获取当前映射中对应的value，如果存在对应映射关系，则返回对应值，否则返回null
     *
     * @implSpec
     * This implementation iterates over <tt>entrySet()</tt> searching
     * for an entry with the specified key.  If such an entry is found,
     * the entry's value is returned.  If the iteration terminates without
     * finding such an entry, <tt>null</tt> is returned.  Note that this
     * implementation requires linear time in the size of the map; many
     * implementations will override this method.
     */
    public V get(Object key) {
        // Entry集合迭代器
        Iterator<Entry<K,V>> i = entrySet().iterator();
        if (key==null) {
            // 遍历Entry集合并比较key
            while (i.hasNext()) {
                Entry<K,V> e = i.next();
                if (e.getKey()==null)
                    return e.getValue();
            }
        } else {
            // 遍历Entry集合并比较key
            while (i.hasNext()) {
                Entry<K,V> e = i.next();
                if (key.equals(e.getKey()))
                    return e.getValue();
            }
        }
        // 默认返回null
        return null;
    }
    // Modification Operations 变更操作
    /**
     * 向当前Map映射中存放键值对,由子类实现
     *
     * @implSpec
     * This implementation always throws an
     * <tt>UnsupportedOperationException</tt>.
     *
     */
    public V put(K key, V value) {
        throw new UnsupportedOperationException();
    }
    /**
     * 从当前Map映射中移除以目标key为键的映射关系，并返回key对应的value，如果没有映射关系则返回null
     *
     * @implSpec
     * This implementation iterates over <tt>entrySet()</tt> searching for an
     * entry with the specified key.  If such an entry is found, its value is
     * obtained with its <tt>getValue</tt> operation, the entry is removed
     * from the collection (and the backing map) with the iterator's
     * <tt>remove</tt> operation, and the saved value is returned.  If the
     * iteration terminates without finding such an entry, <tt>null</tt> is
     * returned.  Note that this implementation requires linear time in the
     * size of the map; many implementations will override this method.
     */
    public V remove(Object key) {
        // Entry集合迭代器
        Iterator<Entry<K,V>> i = entrySet().iterator();
        // 定义目标Entry，即目标映射对象
        Entry<K,V> correctEntry = null;
        // 如果key为null
        if (key==null) {
            // 遍历Entry集合，当前entry的key为null时，赋值给correctEntry
            while (correctEntry==null && i.hasNext()) {
                Entry<K,V> e = i.next();
                if (e.getKey()==null)
                    correctEntry = e;
            }
        } else {
            // 遍历Entry集合，当前entry的key与目标key相等时，赋值给correctEntry
            while (correctEntry==null && i.hasNext()) {
                Entry<K,V> e = i.next();
                if (key.equals(e.getKey()))
                    correctEntry = e;
            }
        }
        // 获取目标映射的值并删除映射
        V oldValue = null;
        if (correctEntry !=null) {
            oldValue = correctEntry.getValue();
            i.remove();
        }
        // 目标映射的值
        return oldValue;
    }
    // Bulk Operations 扩展操作
    /**
     * 从指定的map中拷贝所有的映射到该map中
     *
     * @implSpec
     * This implementation iterates over the specified map's
     * <tt>entrySet()</tt> collection, and calls this map's <tt>put</tt>
     * operation once for each entry returned by the iteration.
     */
    public void putAll(Map<? extends K, ? extends V> m) {
        // 遍历m中的Entry集合
        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
            // 拷贝映射
            put(e.getKey(), e.getValue());
    }
    /**
     * 清空当前Map所有映射
     *
     * @implSpec
     * This implementation calls <tt>entrySet().clear()</tt>.
     *
     * <p>Note that this implementation throws an
     * <tt>UnsupportedOperationException</tt> if the <tt>entrySet</tt>
     * does not support the <tt>clear</tt> operation.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     */
    public void clear() {
        entrySet().clear(); // Set集合的Clear方法
    }
    // Views 视图
    /**
     * Each of these fields are initialized to contain an instance of the
     * appropriate view the first time this view is requested.  The views are
     * stateless, so there's no reason to create more than one of each.
     * 
     * transient关键字防止被序列化
     */
    transient Set<K>        keySet; // 存储所有key的set集合
    transient Collection<V> values; // 存储所有value的集合
    /**
     * 返回key的set集合
     *
     * @implSpec
     * This implementation returns a set that subclasses {@link AbstractSet}.
     * The subclass's iterator method returns a "wrapper object" over this
     * map's <tt>entrySet()</tt> iterator.  The <tt>size</tt> method
     * delegates to this map's <tt>size</tt> method and the
     * <tt>contains</tt> method delegates to this map's
     * <tt>containsKey</tt> method.
     */
    public Set<K> keySet() {
        Set<K> ks = keySet;
        // 如果当前keySet为null
        if (ks == null) {
            // 构建一个AbstractSet，以Map的key类型为泛型参数，即key的set集合
            ks = new AbstractSet<K>() {
                // 定义AbstractSet的迭代器
                public Iterator<K> iterator() {
                    return new Iterator<K>() {
                        // Entry的set集合迭代器
                        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();
                        }
                    };
                }
                // AbstractMap.this即ks
                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;
        }
        // 返回keySet
        return ks;
    }
    /**
     * 获取value集合
     *
     * @implSpec
     * This implementation returns a collection that subclasses {@link
     * AbstractCollection}.  The subclass's iterator method returns a
     * "wrapper object" over this map's <tt>entrySet()</tt> iterator.
     * The <tt>size</tt> method delegates to this map's <tt>size</tt>
     * method and the <tt>contains</tt> method delegates to this map's
     * <tt>containsValue</tt> method.
     *
     */
    public Collection<V> values() {
        Collection<V> vals = values;
        if (vals == null) {
            // 构建匿名内部AbstractCollection，以Map的value类型为泛型参数，即value的列表
            vals = new AbstractCollection<V>() {
                // 迭代器方法
                public Iterator<V> iterator() {
                    // 构建内部迭代器
                    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;
    }
    // 返回该map中所有映射的Set视图。由子类实现
    public abstract Set<Entry<K,V>> entrySet();
    // Comparison and hashing
    /**
     * 比较两个Map，如果两个Map映射完全一样【ker和value完全一致】，返回true
     *
     * @param o object to be compared for equality with this map
     * @return <tt>true</tt> if the specified object is equal to this map
     */
    public boolean equals(Object o) {
        // 如果就是本对象
        if (o == this)
            return true;
        // 如果传入对象不是Map类型，直接返回false
        if (!(o instanceof Map))
            return false;
        // 强转
        Map<?,?> m = (Map<?,?>) o;
        // 先比较记录数
        if (m.size() != size())
            return false;
        try {
            // Entry集合迭代器
            Iterator<Entry<K,V>> i = entrySet().iterator();
            while (i.hasNext()) {
                // 比对当前Entry与传入Map对应key的映射是否一致
                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;
        }
        return true;
    }
    /**
     *
     * @return the hash code value for this map
     * @see Map.Entry#hashCode()
     * @see Object#equals(Object)
     * @see Set#equals(Object)
     */
    public int hashCode() {
        int h = 0;
        Iterator<Entry<K,V>> i = entrySet().iterator();
        while (i.hasNext())
            h += i.next().hashCode();
        return h;
    }
    /**
     *
     * @return a string representation of this map
     */
    public String toString() {
        Iterator<Entry<K,V>> i = entrySet().iterator();
        if (! i.hasNext())
            return "{}";
        StringBuilder sb = new StringBuilder();
        sb.append('{');
        for (;;) {
            Entry<K,V> e = i.next();
            K key = e.getKey();
            V value = e.getValue();
            sb.append(key   == this ? "(this Map)" : key);
            sb.append('=');
            sb.append(value == this ? "(this Map)" : value);
            if (! i.hasNext())
                return sb.append('}').toString();
            sb.append(',').append(' ');
        }
    }
    /**
     * Returns a shallow copy of this <tt>AbstractMap</tt> instance: the keys
     * and values themselves are not cloned.
     *
     * @return a shallow copy of this map
     */
    protected Object clone() throws CloneNotSupportedException {
        AbstractMap<?,?> result = (AbstractMap<?,?>)super.clone();
        result.keySet = null;
        result.values = null;
        return result;
    }
    /**
     * Utility method for SimpleEntry and SimpleImmutableEntry.
     * Test for equality, checking for nulls.
     *
     * NB: Do not replace with Object.equals until JDK-8015417 is resolved.
     */
    private static boolean eq(Object o1, Object o2) {
        return o1 == null ? o2 == null : o1.equals(o2);
    }
    // Implementation Note: SimpleEntry and SimpleImmutableEntry
    // are distinct unrelated classes, even though they share
    // some code. Since you can't add or subtract final-ness
    // of a field in a subclass, they can't share representations,
    // and the amount of duplicated code is too small to warrant
    // exposing a common abstract class.
    /**
     * Map中内部接口Entry的简单实现
     *
     * @since 1.6
     */
    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;
        }
        /**
         *
         * @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());
        }
        /**
         *
         * @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;
        }
    }
    /**
     * Entry接口的实现类，但entry对象是不可改变的，即key和value确定后，不能更改
     *
     * @since 1.6
     */
    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;
        }
        /**
         * value仅能通过初始创建即设置，不能进行修改，抛出异常
         */
        public V setValue(V value) {
            throw new UnsupportedOperationException();
        }
        /**
         * 
         * @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());
        }
        /**
         *
         * @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());
        }
        /**
         *
         * @return a String representation of this map entry
         */
        public String toString() {
            return key + "=" + value;
        }
    }
}

总结

1、AbstractMap实现了Map接口，是一个抽象类，而抽象类通常作为骨架使用，实现一些简单且通用的功能，而核心功能则由子类实现。

2、AbstractMap是大部分Map接口实现类的父类，典型常用的几个如HashMap、TreeMap、WeakHashMap等

思考

transient关键字

在java中，对象一旦实现了Serializable接口，这个对象就可以被序列化，同理，其方法和属性都会被自动序列化。而有时候我们不希望其中的某些属性被序列化【如密码，卡号等】，这时候只要在该属性前面加上transient关键字，就可以标识该属性不需要序列化，当对象序列化时不会序列化该属性。

transient不能被用在方法上。且如果标识的属性是一个对象，该对象必须也实现了Serializable接口

测试效果：

public class JavaTest {

    /**
     * 测试transient关键字【指定某个属性不被序列化】
     */
   @Test
   public void testTransient(){
       User user = new User();
       user.setAge(20);
       user.setName("test");
       user.setPassword("123456");

       System.out.println("序列化之前："+user);

       // 使用阿里的fastJson序列化对象
       String userJsonString = JSON.toJSONString(user);
       System.out.println("序列化结果："+userJsonString);

       // 使用阿里的fastJson反序列化对象
       Object userByJson = JSON.parseObject(userJsonString);
       System.out.println("反序列化结果："+userByJson);
   }

   /**
    *  定义User类实现Serializable接口
    */
   // @Data注解来自lombok
   @Data
   class User implements Serializable{
       private String name;
       private int age;
       private transient String password;
   }
}

执行结果：