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获取LinkedHashMap中的头部元素（最早添加的元素）：时间复杂度O(1)

public 
   
     Entry
    
      getHead(LinkedHashMap
     
       map) {    return map.entrySet().iterator().next();}
     
    
   

获取LinkedHashMap中的末尾元素（最近添加的元素）：时间复杂度O(n)

public 
   
     Entry
    
      getTail(LinkedHashMap
     
       map) {    Iterator
      
       
        > iterator = map.entrySet().iterator();    Entry
        
          tail = null; while (iterator.hasNext()) { tail = iterator.next(); } return tail;}
        
       
      
     
    
   

通过反射获取LinkedHashMap中的末尾元素：时间复杂度O(1)，访问tail属性

public 
   
     Entry
    
      getTailByReflection(LinkedHashMap
     
       map)        throws NoSuchFieldException, IllegalAccessException {    Field tail = map.getClass().getDeclaredField("tail");    tail.setAccessible(true);    return (Entry
      
       ) tail.get(map);}
      
     
    
   

测试代码

import static org.junit.Assert.assertEquals;import java.lang.reflect.Field;import java.util.Iterator;import java.util.LinkedHashMap;import java.util.Map.Entry;import org.junit.Before;import org.junit.Test;public class TestLinkedHashMap {    private LinkedHashMap
   
     map = new LinkedHashMap<>();    private String letters[] = { "a", "b", "c", "d", "e" };    @Before    public void init() {        for (int i = 0; i < letters.length; i++) {            map.put(letters[i], i + 1);        }    }    @Test    public void testGetHead() {        assertEquals(getHead(map).getKey(), "a");        assertEquals(getHead(map).getValue(), Integer.valueOf(1));    }    @Test    public void testGetTail() {        assertEquals(getTail(map).getKey(), "e");        assertEquals(getTail(map).getValue(), Integer.valueOf(5));    }    @Test    public void testGetTailByReflection() throws NoSuchFieldException, IllegalAccessException {        assertEquals(getTailByReflection(map).getKey(), "e");        assertEquals(getTailByReflection(map).getValue(), Integer.valueOf(5));    }    public 
    
      Entry
     
       getHead(LinkedHashMap
      
        map) {        return map.entrySet().iterator().next();    }    public 
       
         Entry
        
          getTail(LinkedHashMap
         
           map) { Iterator
          
           
            > iterator = map.entrySet().iterator(); Entry
            
              tail = null; while (iterator.hasNext()) { tail = iterator.next(); } return tail; } @SuppressWarnings("unchecked") public 
             
               Entry
              
                getTailByReflection(LinkedHashMap
               
                 map) throws NoSuchFieldException, IllegalAccessException { Field tail = map.getClass().getDeclaredField("tail"); tail.setAccessible(true); return (Entry
                
                 ) tail.get(map); }}
                
               
              
             
            
           
          
         
        
       
      
     
    
   

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