Java笔记——java.util.Map.Entry

Map.Entry 是Map声明的内部接口

public interface Map<K,V> {
省略n行。。。
interface Entry<K,V> {
        /**
         * Returns the key corresponding to this entry.
         *
         * @return the key corresponding to this entry
         * @throws IllegalStateException implementations may, but are not
         *         required to, throw this exception if the entry has been
         *         removed from the backing map.
         */
        K getKey();

        /**
         * Returns the value corresponding to this entry.  If the mapping
         * has been removed from the backing map (by the iterator's
         * <tt>remove</tt> operation), the results of this call are undefined.
         *
         * @return the value corresponding to this entry
         * @throws IllegalStateException implementations may, but are not
         *         required to, throw this exception if the entry has been
         *         removed from the backing map.
         */
        V getValue();

        /**
         * Replaces the value corresponding to this entry with the specified
         * value (optional operation).  (Writes through to the map.)  The
         * behavior of this call is undefined if the mapping has already been
         * removed from the map (by the iterator's <tt>remove</tt> operation).
         *
         * @param value new value to be stored in this entry
         * @return old value corresponding to the entry
         * @throws UnsupportedOperationException if the <tt>put</tt> operation
         *         is not supported by the backing map
         * @throws ClassCastException if the class of the specified value
         *         prevents it from being stored in the backing map
         * @throws NullPointerException if the backing map does not permit
         *         null values, and the specified value is null
         * @throws IllegalArgumentException if some property of this value
         *         prevents it from being stored in the backing map
         * @throws IllegalStateException implementations may, but are not
         *         required to, throw this exception if the entry has been
         *         removed from the backing map.
         */
        V setValue(V value);

        /**
         * Compares the specified object with this entry for equality.
         * Returns <tt>true</tt> if the given object is also a map entry and
         * the two entries represent the same mapping.  More formally, two
         * entries <tt>e1</tt> and <tt>e2</tt> 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 <tt>equals</tt> method works properly across
         * different implementations of the <tt>Map.Entry</tt> interface.
         *
         * @param o object to be compared for equality with this map entry
         * @return <tt>true</tt> if the specified object is equal to this map
         *         entry
         */
        boolean equals(Object o);

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

如果不用Entry，我们获取Map总的信息的时候可以先获取key的信息，再根据Key值去找相应分value

public static void main(String[] args){
        Map<String, String> mp = new HashMap<String, String>();
        mp.put("A", "1");
        mp.put("B", "2");
        Set<String> set = mp.keySet();
        if(set != null) {
            Iterator<String> iter = set.iterator();
            while(iter.hasNext()) {
                String key = iter.next();
                String value = mp.get(key);
                System.out.println(key);
                System.out.println(value);  
            }
        }
}

这样做可能会比较费时。我们可以用Entry先取出对应的key和value键值对，然后调用Entry中的getKey()和getValue()取出对应的key和value值

public static void main(String[] args){
        Map<String, String> mp = new HashMap<String, String>();
        mp.put("A", "1");
        mp.put("B", "2");
        Set<Entry<String, String>> entrySet =  mp.entrySet();
        if(entrySet != null) {
            Iterator<Entry<String, String>> iter = entrySet.iterator();
            while(iter.hasNext()) {
                Entry<String, String> entry = iter.next();
                System.out.println(entry.getKey());
                System.out.println(entry.getValue());
            }
        }
    }

当然，也可以写的更加简洁一点

public static void main(String[] args){
        Map<String, String> mp = new HashMap<String, String>();
        mp.put("A", "1");
        mp.put("B", "2");
        for(Entry<String,String> entry : mp.entrySet()) {
            System.out.println(entry.getKey());
            System.out.println(entry.getValue());
        }