Java - LinkedHashMap（有序 map）获取第一个元素和最后一个元素(一)

public <K, V> Entry<K, V> getHead(LinkedHashMap<K, V> map) {
    return map.entrySet().iterator().next();
}

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

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

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<String, Integer> 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 <K, V> Entry<K, V> getHead(LinkedHashMap<K, V> map) {
        return map.entrySet().iterator().next();
    }
    public <K, V> Entry<K, V> getTail(LinkedHashMap<K, V> map) {
        Iterator<Entry<K, V>> iterator = map.entrySet().iterator();
        Entry<K, V> tail = null;
        while (iterator.hasNext()) {
            tail = iterator.next();
        }
        return tail;
    }
    @SuppressWarnings("unchecked")
    public <K, V> Entry<K, V> getTailByReflection(LinkedHashMap<K, V> map)
            throws NoSuchFieldException, IllegalAccessException {
        Field tail = map.getClass().getDeclaredField("tail");
        tail.setAccessible(true);
        return (Entry<K, V>) tail.get(map);
    }
}