摘要：今天寫根過濾的時候一下子有點愣眼，先是想到用原生的循環查出來，但是覺得太，后面思考了一下可以用的，寫完了，又發現有現成的方法，這里一一列出來，供參考使用。

今天寫根過濾的時候一下子有點愣眼，先是想到用 Java 原生的 map 循環查出來，但是覺得太 low， 后面思考了一下可以用 Java8 的 Lambda，寫完了，又發現 Google Guava 有現成的方法，這里一一列出來，供參考使用。

首先提示，如果照搬我的代碼的話別忘了引這些依賴

        
            junit
            junit
            4.12
            test
            
                
                    org.hamcrest
                    hamcrest-core
                
            
        

        
            org.hamcrest
            hamcrest-library
            1.3
            test
        

        
            com.google.guava
            guava
            25.1-jre
        

public class FilterMapByKeyTest {
    private Map WEEK = new HashMap<>();

    @Before
    public void setUp () {
        WEEK.put(1, "Monday");
        WEEK.put(2, "Tuesday");
        WEEK.put(3, "Wednesday");
        WEEK.put(4, "Thursday");
        WEEK.put(5, "Friday");
        WEEK.put(6, "Saturday");
        WEEK.put(7, "Sunday");
    }

    /**
     * Java 8之前的版本
     */
    @Test
    public void filterMapByKey () {
        Map map = new HashMap<>();
        for (Map.Entry entry : WEEK.entrySet()) {
            if (entry.getKey() <= 3) {
                map.put(entry.getKey(), entry.getValue());
            }
        }
        assertThat(map.keySet(), contains(1, 2, 3));
    }

    /**
     * Java 8 Lambda
     */
    @Test
    public void filterMapByKeyJava8Lambda () {
        Map map = WEEK.entrySet().stream().filter(r -> r.getKey() <= 3)
                        .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
        assertThat(map.keySet(), contains(1, 2, 3));
    }

    /**
     * Google Guava
     */
    @Test
    public void filterMapByKeyGuava () {
        Map map = Maps.filterKeys(WEEK, r -> r <= 3);
        assertThat(map.keySet(), contains(1, 2, 3));
    }

}

public class FilterMapByValueTest {
    private Map WEEK = new HashMap<>();

    @Before
    public void setUp () {
        WEEK.put(1, "Monday");
        WEEK.put(2, "Tuesday");
        WEEK.put(3, "Wednesday");
        WEEK.put(4, "Thursday");
        WEEK.put(5, "Friday");
        WEEK.put(6, "Saturday");
        WEEK.put(7, "Sunday");
    }

    /**
     * Java 8之前的版本
     */
    @Test
    public void filterMapByValue () {
        Map map = new HashMap<>();
        for (Map.Entry entry : WEEK.entrySet()) {
            if (entry.getValue().startsWith("S")) {
                map.put(entry.getKey(), entry.getValue());
            }
        }
        assertThat(map.values(), contains("Saturday","Sunday"));
    }

    /**
     * Java 8 Lambda
     */
    @Test
    public void filterMapByValueJava8Lambda () {
        Map map = WEEK.entrySet().stream().filter(r -> r.getValue().startsWith("S"))
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
        assertThat(map.values(), contains("Saturday","Sunday"));
    }

    /**
     * Google Guava
     */
    @Test
    public void filterMapByValueGuava () {
        Map map = Maps.filterValues(WEEK, r -> r.startsWith("S"));
        assertThat(map.values(), contains("Saturday","Sunday"));
    }
}

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