一、简介

Guava - BiMap是Guava中新出的集合类型。官网对其描述如下：

The traditional way to map values back to keys is to maintain two separate maps and keep them both in sync, but this is bug-prone and can get extremely confusing when a value is already present in the map

大致意思是：
将值映射回键的传统方法是维护两个独立的映射并使它们保持同步，但是这很容易出现bug，而且当一个值已经出现在映射中时，可能会非常混乱。For example:

        Map<String, Integer> nameToId = Maps.newHashMap();
        Map<Integer, String> idToName = Maps.newHashMap();

        nameToId.put("Bob", 42);
        idToName.put(42, "Bob");
        // what happens if "Bob" or 42 are already present?
        // weird bugs can arise if we forget to keep these in sync...

BiMap是一个Map,咱可以通过inverse()方法获取原Map的k - v相反的Map以此来替代上诉场景...

二、BiMap的创建

Guava针对BiMap有多种实现.每种实现针对k -v以及v - k的实现不同.
官方给出的列表如下:

Implementations

实验代码:

        //通过HashBiMap创建
        BiMap<String, String> testHashBiMap = HashBiMap.create();
        //通过ImmutableBiMap创建
        Map<String,String> normalMap = Maps.newHashMap();
        BiMap<String, String> testImmutableBiMap= ImmutableBiMap.copyOf(normalMap);
        //通过EnumBiMap创建
        BiMap<TestEnum, TestEnum> testEnumBiMap = EnumBiMap.create(TestEnum.class,TestEnum.class);
        //通过EnumHashBiMap创建
        BiMap<TestEnum, String> testEnumHashBiMap = EnumHashBiMap.create(TestEnum.class);

三、常用方法

1、inverse()方法

inverse()方法可以获取当前BiMap的键值方向映射的Map.

实验代码:

        //通过HashBiMap创建
        BiMap<String, String> testHashBiMap = HashBiMap.create();

        testHashBiMap.put("KeyA", "ValueA");
        testHashBiMap.put("KeyA", "ValueB");
        testHashBiMap.put("KeyC", "ValueC");
        System.out.println("=================测试BiMap的no - inverse()方法=============" + testHashBiMap);
        System.out.println("=================测试BiMap的inverse()方法=============" + testHashBiMap.inverse());

实验结果:

=================测试BiMap的no - inverse()方法============={KeyA=ValueB, KeyC=ValueC}
=================测试BiMap的inverse()方法============={ValueB=KeyA, ValueC=KeyC}

2、put()方法

put()方法对BiMap添加一个key - value映射
值得注意的是:如果添加不同的key但是value在之前的key中已经存在的话，会抛出IllegalArgumentException异常.

实验代码:

        BiMap<String, String> testHashBiMap = HashBiMap.create();

        testHashBiMap.put("KeyA", "ValueA");
        testHashBiMap.put("KeyA", "ValueB");
        testHashBiMap.put("KeyC", "ValueC");

        testHashBiMap.put("KeyD", "ValueB");

实验结果:

Exception in thread "main" java.lang.IllegalArgumentException: value already present: ValueB
    at com.google.common.collect.HashBiMap.put(HashBiMap.java:285)
    at com.google.common.collect.HashBiMap.put(HashBiMap.java:260)
    at com.toxic.anepoch.guava.parsing.collection.newtypes.TestBiMap.main(TestBiMap.java:30)

3、forcePut()方法

forcePut()方法是作为put()方法的补充,之前在put()方法中提到了,value需要保持唯一,但是当我们实际使用中的时候,如果想在后来增加的key中使用已经存在的value就可以forcePut()方法.使用之后它会将已经存在与new value值相同的key给移除掉.

实验代码:

        //通过HashBiMap创建
        BiMap<String, String> testHashBiMap = HashBiMap.create();

        testHashBiMap.put("KeyA", "ValueA");
        testHashBiMap.put("KeyB", "ValueB");
        testHashBiMap.put("KeyC", "ValueC");
        System.out.println("=================测试BiMap的no - inverse()方法=============" + testHashBiMap);

        testHashBiMap.forcePut("KeyD", "ValueB");
        System.out.println("=================测试BiMap的no - inverse()方法=============" + testHashBiMap);

实验结果:

=================测试BiMap的no - inverse()方法============={KeyA=ValueA, KeyB=ValueB, KeyC=ValueC}
=================测试BiMap的no - inverse()方法============={KeyA=ValueA, KeyC=ValueC, KeyD=ValueB}

从实验结果中可以发现.以前持有ValueB的Key为KeyB,使用了forcePut()方法之后其对应的持有Key变为了KeyD.

Note: BiMap utilities like synchronizedBiMap live in Maps.

......未完待续