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Collectors.toMap介绍

在真实的业务场景中有很多集合转map的操作，例如

@Data
public class House {

    private Integer id;        //id
    private Integer ownerid;   //家主编号
    private String housename;  //家庭名称
    private String address;    //家庭地址
    
}

模拟数据

/**
 * @description: List 转 Map 操作
 */
public class ListToMap {

    public static void main(String[] args) {
        House house = new House(1,1,"aa","北京海淀");
        House house1 = new House(2,2,"bb","湖北武汉");
        House house2 = new House(3,3,"cc","浙江杭州");
        ArrayList<House> houses = new ArrayList<>();
        houses.add(house);
        houses.add(house1);
        houses.add(house2);
        //在实际项目中我们经常会用到 List 转 Map 操作 ->过去是for循环的操作，现在可以学习如下的方法Collectors.toMap
        /**
         * 我们收集一下集合中每个对象的两个单独的属性
         */
        Map<String, String> mapHouse = houses.stream().collect(Collectors.toMap(House::getHousename, House::getAddress));
        System.out.println(mapHouse);
        //{aa=湖北武汉, bb=浙江杭州, cc=北京海淀}
        
        /**
         * 前后的属性的数据类型要对应  一般时间业务中收集带有唯一表示的业务数据
         */
        Map<Integer, String> map = houses.stream().collect(Collectors.toMap(House::getOwnerid, House::getHousename));
        System.out.println(map);
        //{1=aa, 2=bb, 3=cc}

        /**
         * 收集一下属性和对象本身
         */
        Map<Integer, House> houseMap = houses.stream().collect(Collectors.toMap(House::getOwnerid, o -> o));
        Map<Integer, House> houseMap1 = houses.stream().collect(Collectors.toMap(House::getOwnerid,  Function.identity()));
        System.out.println(houseMap);
       /**
         * {1=House{id=1, ownerid=1, housename='aa', address='北京海淀'}, 
         * 2=House{id=2, ownerid=2, housename='bb', address='湖北武汉'}, 
         * 3=House{id=3, ownerid=3, housename='cc', address='浙江杭州'}}
         */
         
        //业务场景：一般会根据具体的键值 取具体的对象
        System.out.println(houseMap.get(1));
        //House{id=1, ownerid=1, housename='aa', address='北京海淀'}
        
        //此处的效果同houseMap
        System.out.println(houseMap1);
       /**
         * {1=House{id=1, ownerid=1, housename='aa', address='北京海淀'}, 
         * 2=House{id=2, ownerid=2, housename='bb', address='湖北武汉'}, 
         * 3=House{id=3, ownerid=3, housename='cc', address='浙江杭州'}}
         */

    }

}

常见操作大家实践即可

houses.stream().collect(Collectors.toMap(House::getOwnerid, House::getHousename));

深入Collectors.toMap

Collectors.toMap 有三个重载方法：

toMap(Function<? super T, ? extends K> keyMapper, Function<? super T, ? extends U> valueMapper);
toMap(Function<? super T, ? extends K> keyMapper, Function<? super T, ? extends U> valueMapper,
        BinaryOperator<U> mergeFunction);
toMap(Function<? super T, ? extends K> keyMapper, Function<? super T, ? extends U> valueMapper,
        BinaryOperator<U> mergeFunction, Supplier<M> mapSupplier);

参数含义分别是：

keyMapper：Key 的映射函数

valueMapper：Value 的映射函数

mergeFunction：当 Key 冲突时，调用的合并方法

mapSupplier：Map 构造器，在需要返回特定的 Map 时使用

业务场景最多的还是map的键为一个唯一的标识符，值为对象本身！

如果希望得到 Map 的 value 为对象本身时，可以这样写

       /**
         * 收集一下属性和对象本身
         */
Map<Integer, House> houseMap = houses.stream().collect(Collectors.toMap(House::getOwnerid, o -> o));
Map<Integer, House> houseMap1 = houses.stream().collect(Collectors.toMap(House::getOwnerid,  Function.identity()));

常见的java.lang.IllegalStateException: Duplicate key 问题处理

我们把做的测试数据 House house = new House(1,1,“aa”,“北京海淀”);

House house1 = new House(2,2,“bb”,“湖北武汉”);

House house2 = new House(3,3,“cc”,“浙江杭州”);

house2修改为： House house2 = new House(3,1,“cc”,“浙江杭州”);

这样Ownerid会重复

当我们执行如下代码时：houses.stream().collect(Collectors.toMap(House::getOwnerid, House::getHousename);

会出错，java.lang.IllegalStateException: Duplicate key

线上业务代码出现Duplicate Key的异常，影响了业务逻辑，查看抛出异常部分的代码

Exception in thread "main" java.lang.IllegalStateException: Duplicate key aa
	at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
	at java.util.HashMap.merge(HashMap.java:1254)
	at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
	at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
	at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
	at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
	at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)
	at com.example.cwgl.ListToMap.main(ListToMap.java:28)

解决办法：出现重复时，取前面value的值，或者取后面放入的value值，则覆盖先前的value值

houses.stream().collect(Collectors.toMap(House::getOwnerid, House::getHousename,(v1,v2)->v2));
houses.stream().collect(Collectors.toMap(House::getOwnerid, House::getHousename,(v1,v2)->v1));

对于结果的操作还有很多处理方法，如拼接等

houses.stream().collect(Collectors.toMap(House::getOwnerid, House::getHousename,(v1,v2)->v1+v2));

具体的实践根据具体的业务来操作数据即可