Java LinkedHashMap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 <K, V> Entry<K, V> getHead(LinkedHashMap<K, V> 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 <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;
}

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 <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);
}

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<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);
    }
}

 

本文链接:http://bookshadow.com/weblog/2016/10/27/java-linked-hash-map-get-first-and-get-last/
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