今天写根过滤的时候一下子有点愣眼，先是想到用 Java 原生的 map 循环查出来，但是觉得太 low， 后面思考了一下可以用 Java8 的 Lambda，写完了，又发现 Google Guava 有现成的方法，这里一一列出来，供参考使用。

首先提示，如果照搬我的代码的话别忘了引这些依赖

<dependencies>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.12</version>
<scope>test</scope>
<exclusions>
<exclusion>
<groupId>org.hamcrest</groupId>
<artifactId>hamcrest-core</artifactId>
</exclusion>
</exclusions>
</dependency>

    <dependency>        <groupId>org.hamcrest</groupId>        <artifactId>hamcrest-library</artifactId>        <version>1.3</version>        <scope>test</scope>    </dependency>    <dependency>        <groupId>com.google.guava</groupId>        <artifactId>guava</artifactId>        <version>25.1-jre</version>    </dependency>

</dependencies>
filter by key
public class FilterMapByKeyTest {
private Map<Integer, String> WEEK = new HashMap<>();

@Beforepublic 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 */@Testpublic void filterMapByKey () {    Map<Integer, String> map = new HashMap<>();    for (Map.Entry<Integer, String> entry : WEEK.entrySet()) {        if (entry.getKey() <= 3) {            map.put(entry.getKey(), entry.getValue());        }    }    assertThat(map.keySet(), contains(1, 2, 3));}/** * Java 8 Lambda */@Testpublic void filterMapByKeyJava8Lambda () {    Map<Integer, String> 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 */@Testpublic void filterMapByKeyGuava () {    Map<Integer, String> map = Maps.filterKeys(WEEK, r -> r <= 3);    assertThat(map.keySet(), contains(1, 2, 3));}

}
filter by value
public class FilterMapByValueTest {
private Map<Integer, String> WEEK = new HashMap<>();

@Beforepublic 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 */@Testpublic void filterMapByValue () {    Map<Integer, String> map = new HashMap<>();    for (Map.Entry<Integer, String> entry : WEEK.entrySet()) {        if (entry.getValue().startsWith("S")) {            map.put(entry.getKey(), entry.getValue());        }    }    assertThat(map.values(), contains("Saturday","Sunday"));}/** * Java 8 Lambda */@Testpublic void filterMapByValueJava8Lambda () {    Map<Integer, String> 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 */@Testpublic void filterMapByValueGuava () {    Map<Integer, String> map = Maps.filterValues(WEEK, r -> r.startsWith("S"));    assertThat(map.values(), contains("Saturday","Sunday"));}

}
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