public abstract class AbstractMap<K,V>
extends implements <K,V> 作用:
1,提供 Map 接口的实现，最大限度地减少实现此接口所需的工作.package java.util;import java.util.Map.Entry;public abstract class AbstractMap<K,V> implements Map<K,V> { protected AbstractMap() {} //返回Map中映射关系(entry)的数量 public int size() { return entrySet().size(); } //判断Map中是否包含映射关系(entry) public boolean isEmpty() { return size() == 0; } //判断Map中是否包含值为value的映射关系. 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; } //判断Map中是否包含键为key的映射关系. 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; } //返回指定key所对应的entry的value,若不存在key对应的value,返回null. 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; } //不支持映射关系的添加，应由实现类实现。 public V put(K key, V value) { throw new UnsupportedOperationException(); } //用于移除一个key对应的映射关系entry，返回该entry的value,若不存在该entry,返回null. 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;//停止循环的开关 } } 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) { //判断是否包含key对应的entry oldValue = correctEntry.getValue(); i.remove(); } return oldValue; } // Bulk Operations 批量操作 //将m包含的映射关系添加到当前Map之中. public void putAll(Map<? extends K, ? extends V> m) { for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) put(e.getKey(), e.getValue()); } //移除Map内所有映射关系 public void clear() { entrySet().clear(); } // Views //Map中包含的key的Set(不会重复) transient volatile Set<K> keySet = null; //Map中所有映射关系的value的Collection(可重复) transient volatile Collection<V> values = null; public Set<K> keySet() { if (keySet == null) { keySet = 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 contains(Object k) { return AbstractMap.this.containsKey(k); } }; } return keySet; } public Collection<V> values() { if (values == null) { values = 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 contains(Object v) { return AbstractMap.this.containsValue(v); } }; } return values; } //返回Map中包含映射关系entry的Set. public abstract Set<Entry<K,V>> entrySet(); public boolean equals(Object o) { if (o == this) return true; if (!(o instanceof Map)) return false; Map<K,V> m = (Map<K,V>) o; if (m.size() != size()) return false; 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; } return true; } public int hashCode() { int h = 0; Iterator<Entry<K,V>> i = entrySet().iterator(); while (i.hasNext()) h += i.next().hashCode(); return h; } 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(", "); } } protected Object clone() throws CloneNotSupportedException { AbstractMap<K,V> result = (AbstractMap<K,V>)super.clone(); result.keySet = null; result.values = null; return result; } /** * Utility method for SimpleEntry and SimpleImmutableEntry. * Test for equality, checking for nulls. */ //判断2个对象的相等性. 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. /** * An Entry maintaining a key and a value. The value may be * changed using the <tt>setValue</tt> method. This class * facilitates the process of building custom map * implementations. For example, it may be convenient to return * arrays of <tt>SimpleEntry</tt> instances in method * <tt>Map.entrySet().toArray</tt>. * * @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;//键不可变 private V value; /** * Creates an entry representing a mapping from the specified * key to the specified value. * */ public SimpleEntry(K key, V value) { this.key = key; this.value = value; } /** * Creates an entry representing the same mapping as the * specified entry. */ 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())) * && * (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}. */ 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. */ public String toString() { return key + "=" + value; } } /** * An Entry maintaining an immutable key and value. This class * does not support method <tt>setValue</tt>. This class may be * convenient in methods that return thread-safe snapshots of * key-value mappings. * * @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;//键不可变 private final V value;//值不可变 /** * Creates an entry representing a mapping from the specified * key to the specified value. */ public SimpleImmutableEntry(K key, V value) { this.key = key; this.value = value; } /** * Creates an entry representing the same mapping as the * specified entry. */ public SimpleImmutableEntry(Entry<? extends K, ? extends V> entry) { this.key = entry.getKey(); this.value = entry.getValue(); } /** * Returns the key corresponding to this entry. */ public K getKey() { return key; } /** * Returns 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. */ 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())) * && * (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. * */ 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}. */ 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. */ public String toString() { return key + "=" + value; } }}