【Java集合类】 LinkedHashMap(有序的map)获取第一个元素和最后一个元素

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);
    }
}