java中HashMap的7种遍历方式与性能分析

时间：2022-10-29 11:24:56|栏目：JAVA代码|点击：次

1、遍历方式

1.1 迭代器 EntrySet

/**
 * 1. 迭代器 EntrySet
 */
@Test
public void test1() {
    Map<Integer, String> map = new HashMap<>();
    map.put(1, "Java");
    map.put(2, "JavaSE");
    map.put(3, "JavaEE");
    map.put(4, "Spring");
    map.put(5, "SpringMVC");
    map.put(6, "MyBatis");

    Iterator<Map.Entry<Integer, String>> iterator = map.entrySet().iterator();
    while (iterator.hasNext()) {
        Map.Entry<Integer, String> entry = iterator.next();
        System.out.println(entry.getKey() + "：" + entry.getValue());
    }
}

1.2 迭代器 KeySet

/**
 * 2. 迭代器 KeySet
 */
@Test
public void test2() {
    Map<Integer, String> map = new HashMap<>();
    map.put(1, "Java");
    map.put(2, "JavaSE");
    map.put(3, "JavaEE");
    map.put(4, "Spring");
    map.put(5, "SpringMVC");
    map.put(6, "MyBatis");

    Iterator<Integer> iterator = map.keySet().iterator();
    while (iterator.hasNext()) {
        Integer key = iterator.next();
        System.out.println(key + "：" + map.get(key));
    }
}

1.3 ForEach EntrySet

/**
 * 3. ForEach EntrySet
 */
@Test
public void test3() {
    Map<Integer, String> map = new HashMap<>();
    map.put(1, "Java");
    map.put(2, "JavaSE");
    map.put(3, "JavaEE");
    map.put(4, "Spring");
    map.put(5, "SpringMVC");
    map.put(6, "MyBatis");

    for (Map.Entry<Integer, String> entry : map.entrySet()) {
        System.out.println(entry.getKey() + "：" + entry.getValue());
    }
}

1.4 ForEach KeySet

/**
 * 4. ForEach KeySet
 */
@Test
public void test4() {
    Map<Integer, String> map = new HashMap<>();
    map.put(1, "Java");
    map.put(2, "JavaSE");
    map.put(3, "JavaEE");
    map.put(4, "Spring");
    map.put(5, "SpringMVC");
    map.put(6, "MyBatis");

    for (Integer key : map.keySet()) {
        System.out.println(key + "：" + map.get(key));
    }
}

1.5 Lambda 表达式

/**
 * 5. Lambda 表达式
 */
@Test
public void test5() {
    Map<Integer, String> map = new HashMap<>();
    map.put(1, "Java");
    map.put(2, "JavaSE");
    map.put(3, "JavaEE");
    map.put(4, "Spring");
    map.put(5, "SpringMVC");
    map.put(6, "MyBatis");

    map.forEach((key, value) -> {
        System.out.println(key + "：" + value);
    });
}

1.6 Stream API 单线程

/**
 * 6. Stream API 单线程
 */
@Test
public void test6() {
    Map<Integer, String> map = new HashMap<>();
    map.put(1, "Java");
    map.put(2, "JavaSE");
    map.put(3, "JavaEE");
    map.put(4, "Spring");
    map.put(5, "SpringMVC");
    map.put(6, "MyBatis");

    map.entrySet().stream().forEach((entry) -> {
        System.out.println(entry.getKey() + "：" + entry.getValue());
    });
}

1.7 Stream API 多线程

/**
 * 7. Stream API 多线程
 */
@Test
public void test7() {
    Map<Integer, String> map = new HashMap<>();
    map.put(1, "Java");
    map.put(2, "JavaSE");
    map.put(3, "JavaEE");
    map.put(4, "Spring");
    map.put(5, "SpringMVC");
    map.put(6, "MyBatis");

    map.entrySet().parallelStream().forEach((entry) -> {
        System.out.println(entry.getKey() + "：" + entry.getValue());
    });
}

1.8 代码汇总

/**
 * HashMap 的 7 种遍历方式
 * @ClassName HashMapTraverse
 * @Author YH
 * @Date 2021/11/12
 * @Version 1.0
 */
public class HashMapTraverseTest {

    /**
     * 1. 迭代器 EntrySet
     */
    @Test
    public void test1() {
        Map<Integer, String> map = new HashMap<>();
        map.put(1, "Java");
        map.put(2, "JavaSE");
        map.put(3, "JavaEE");
        map.put(4, "Spring");
        map.put(5, "SpringMVC");
        map.put(6, "MyBatis");

        Iterator<Map.Entry<Integer, String>> iterator = map.entrySet().iterator();
        while (iterator.hasNext()) {
            Map.Entry<Integer, String> entry = iterator.next();
            System.out.println(entry.getKey() + "：" + entry.getValue());
        }
    }

    /**
     * 2. 迭代器 KeySet
     */
    @Test
    public void test2() {
        Map<Integer, String> map = new HashMap<>();
        map.put(1, "Java");
        map.put(2, "JavaSE");
        map.put(3, "JavaEE");
        map.put(4, "Spring");
        map.put(5, "SpringMVC");
        map.put(6, "MyBatis");

        Iterator<Integer> iterator = map.keySet().iterator();
        while (iterator.hasNext()) {
            Integer key = iterator.next();
            System.out.println(key + "：" + map.get(key));
        }
    }

    /**
     * 3. ForEach EntrySet
     */
    @Test
    public void test3() {
        Map<Integer, String> map = new HashMap<>();
        map.put(1, "Java");
        map.put(2, "JavaSE");
        map.put(3, "JavaEE");
        map.put(4, "Spring");
        map.put(5, "SpringMVC");
        map.put(6, "MyBatis");

        for (Map.Entry<Integer, String> entry : map.entrySet()) {
            System.out.println(entry.getKey() + "：" + entry.getValue());
        }
    }

    /**
     * 4. ForEach KeySet
     */
    @Test
    public void test4() {
        Map<Integer, String> map = new HashMap<>();
        map.put(1, "Java");
        map.put(2, "JavaSE");
        map.put(3, "JavaEE");
        map.put(4, "Spring");
        map.put(5, "SpringMVC");
        map.put(6, "MyBatis");

        for (Integer key : map.keySet()) {
            System.out.println(key + "：" + map.get(key));
        }
    }

    /**
     * 5. Lambda 表达式
     */
    @Test
    public void test5() {
        Map<Integer, String> map = new HashMap<>();
        map.put(1, "Java");
        map.put(2, "JavaSE");
        map.put(3, "JavaEE");
        map.put(4, "Spring");
        map.put(5, "SpringMVC");
        map.put(6, "MyBatis");

        map.forEach((key, value) -> {
            System.out.println(key + "：" + value);
        });
    }

    /**
     * 6. Stream API 单线程
     */
    @Test
    public void test6() {
        Map<Integer, String> map = new HashMap<>();
        map.put(1, "Java");
        map.put(2, "JavaSE");
        map.put(3, "JavaEE");
        map.put(4, "Spring");
        map.put(5, "SpringMVC");
        map.put(6, "MyBatis");

        map.entrySet().stream().forEach((entry) -> {
            System.out.println(entry.getKey() + "：" + entry.getValue());
        });
    }

    /**
     * 7. Stream API 多线程
     */
    @Test
    public void test7() {
        Map<Integer, String> map = new HashMap<>();
        map.put(1, "Java");
        map.put(2, "JavaSE");
        map.put(3, "JavaEE");
        map.put(4, "Spring");
        map.put(5, "SpringMVC");
        map.put(6, "MyBatis");

        map.entrySet().parallelStream().forEach((entry) -> {
            System.out.println(entry.getKey() + "：" + entry.getValue());
        });
    }
}

2、性能分析

使用 Oracle 官方提供的性能测试工具 JMH（Java Microbenchmark Harness，JAVA 微基准测试套件）来测试一下这 7 种循环的性能。

使用 JMH 进行性能基准测试

2.1 引入依赖

<!-- https://mvnrepository.com/artifact/org.openjdk.jmh/jmh-core -->
<dependency>
    <groupId>org.openjdk.jmh</groupId>
    <artifactId>jmh-core</artifactId>
    <version>1.23</version>
</dependency>
<!-- https://mvnrepository.com/artifact/org.openjdk.jmh/jmh-generator-annprocess -->
<dependency>
    <groupId>org.openjdk.jmh</groupId>
    <artifactId>jmh-generator-annprocess</artifactId>
    <version>1.23</version>
    <scope>provided</scope>
</dependency>

2.2 编写测试类

直接复制粘贴即可！

/**
 * @ClassName HashMapCycleTest
 * @Author YH
 * @Date 2021/11/12
 * @Version 1.0
 */
@BenchmarkMode(Mode.AverageTime) // 测试完成时间
@OutputTimeUnit(TimeUnit.NANOSECONDS)
@Warmup(iterations = 2, time = 1, timeUnit = TimeUnit.SECONDS) // 预热 2 轮，每次 1s
@Measurement(iterations = 5, time = 1, timeUnit = TimeUnit.SECONDS) // 测试 5 轮，每次 1s
@Fork(1) // fork 1 个线程
@State(Scope.Thread) // 每个测试线程一个实例
public class HashMapCycleTest {
    /**
     * 类加载时赋值
     */
    static Map<Integer, String> map = new HashMap() {{
        // 添加数据
        for (int i = 0; i < 100; i++) {
            put(i, "val:" + i);
        }
    }};

    public static void main(String[] args) throws RunnerException {
        // 启动基准测试
        Options opt = new OptionsBuilder()
                // 要导入的测试类
                .include(HashMapCycleTest.class.getSimpleName())
                // 输出测试结果的文件
                .output("D:/JAVA/面试/workplace/interview/jmh-hashMap.log")
                .build();
        // 执行测试
        new Runner(opt).run();
    }

    /**
     * 迭代器 EntrySet
     */
    @Benchmark
    public void entrySet() {
        // 遍历
        Iterator<Map.Entry<Integer, String>> iterator = map.entrySet().iterator();
        while (iterator.hasNext()) {
            Map.Entry<Integer, String> entry = iterator.next();
            Integer k = entry.getKey();
            String v = entry.getValue();
        }
    }

    /**
     * ForEach EntrySet
     */
    @Benchmark
    public void forEachEntrySet() {
        // 遍历
        for (Map.Entry<Integer, String> entry : map.entrySet()) {
            Integer k = entry.getKey();
            String v = entry.getValue();
        }
    }

    /**
     * 迭代器 KeySet
     */
    @Benchmark
    public void keySet() {
        // 遍历
        Iterator<Integer> iterator = map.keySet().iterator();
        while (iterator.hasNext()) {
            Integer k = iterator.next();
            String v = map.get(k);
        }
    }

    /**
     * ForEach KeySet
     */
    @Benchmark
    public void forEachKeySet() {
        // 遍历
        for (Integer key : map.keySet()) {
            Integer k = key;
            String v = map.get(k);
        }
    }

    /**
     * Lambda 表达式
     */
    @Benchmark
    public void lambda() {
        // 遍历
        map.forEach((key, value) -> {
            Integer k = key;
            String v = value;
        });
    }

    /**
     * Stream API 单线程
     */
    @Benchmark
    public void streamApi() {
        // 单线程遍历
        map.entrySet().stream().forEach((entry) -> {
            Integer k = entry.getKey();
            String v = entry.getValue();
        });
    }

    /**
     * Stream API 多线程
     * 这个不用测，可以肯定性能是最好的。
     * 如果把这个加入进测试了，理论上来说性能应该是最差的（已经测试过）
     * 为啥这么说？因为你本来就开了好几个线程来测试其他方法了，
     * 你这个方法还想再多搞几个线程来提升性能已经不可能了，线程都分配完了。
     * 线程上下文切换的时间会更长！！！所以不能一起测试！！！
     */
    public void parallelStreamApi() {
        // 多线程遍历
        map.entrySet().parallelStream().forEach((entry) -> {
            Integer k = entry.getKey();
            String v = entry.getValue();
        });
    }
}