Collectors.toMap应用

文章目录

• • Collectors.toMap介绍
  • 深入Collectors.toMap
  • 常见的java.lang.IllegalStateException: Duplicate key 问题处理

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
    
}
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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 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 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 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 houseMap = houses.stream().collect(Collectors.toMap(House::getOwnerid, o -> o));
        Map 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'}}
         */

    }

}
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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 keyMapper, Function valueMapper);
toMap(Function keyMapper, Function valueMapper,
        BinaryOperator mergeFunction);
toMap(Function keyMapper, Function valueMapper,
        BinaryOperator mergeFunction, Supplier mapSupplier);
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       /**
         * 收集一下属性和对象本身
         */
Map houseMap = houses.stream().collect(Collectors.toMap(House::getOwnerid, o -> o));
Map houseMap1 = houses.stream().collect(Collectors.toMap(House::getOwnerid,  Function.identity()));
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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)
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houses.stream().collect(Collectors.toMap(House::getOwnerid, House::getHousename,(v1,v2)->v2));
houses.stream().collect(Collectors.toMap(House::getOwnerid, House::getHousename,(v1,v2)->v1));
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