Java Map 通过 key 或者 value 过滤

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

junitjunit4.12testorg.hamcresthamcrest-coreorg.hamcresthamcrest-library1.3testcom.google.guavaguava25.1-jre

filter by key

publicclassFilterMapByKeyTest{privateMap WEEK =newHashMap<>();@BeforepublicvoidsetUp(){        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

    */@TestpublicvoidfilterMapByKey(){        Map map =newHashMap<>();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

    */@TestpublicvoidfilterMapByKeyJava8Lambda(){        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

    */@TestpublicvoidfilterMapByKeyGuava(){        Map map = Maps.filterKeys(WEEK, r -> r <=3);        assertThat(map.keySet(), contains(1,2,3));    }}

filter by value

publicclassFilterMapByValueTest{privateMap WEEK =newHashMap<>();@BeforepublicvoidsetUp(){        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

    */@TestpublicvoidfilterMapByValue(){        Map map =newHashMap<>();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

    */@TestpublicvoidfilterMapByValueJava8Lambda(){        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

    */@TestpublicvoidfilterMapByValueGuava(){        Map map = Maps.filterValues(WEEK, r -> r.startsWith("S"));        assertThat(map.values(), contains("Saturday","Sunday"));    }}

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