文章目录
①. Collectors.toMap介绍②. Collectors.toMap有三个重载方法③. 收集一下属性和对象本身④. 常见的java.lang.IllegalStateException: Duplicate key问题处理①. Collectors.toMap介绍
①. 在项目开发过程中,我们会将List转为Map拿到特定的数据进行处理
②. 一般来说,转为Map的key尽量保持唯一
public class CollectionToMapDemo {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");List<House> houses = Lists.newArrayList();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);//{cc=浙江杭州, bb=湖北武汉, aa=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houseMapSystem.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)}*/}}@Data@AllArgsConstructorclass House {private Integer id; //idprivate Integer ownerId; //家主编号private String houseName; //家庭名称private String address; //家庭地址}
②. Collectors.toMap有三个重载方法
①. Collectors.toMap有三个重载方法： 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);
②. 第一个默认生成HashMap：如果key冲突了,会默认抛出异常
public static <T, K, U>Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,Function<? super T, ? extends U> valueMapper) {return toMap(keyMapper, valueMapper, throwingMerger(), HashMap::new);}
③. 第二个默认生成也是一个HashMap：如果key冲突了,可以按照指定的要求进行合并或者取其任意一个
public static <T, K, U>Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,Function<? super T, ? extends U> valueMapper,BinaryOperator<U> mergeFunction) {return toMap(keyMapper, valueMapper, mergeFunction, HashMap::new);}
④. 第三个可以自定义异常方法和Map类型：
public static <T, K, U, M extends Map<K, U>>Collector<T, ?, M> toMap(Function<? super T, ? extends K> keyMapper,Function<? super T, ? extends U> valueMapper,BinaryOperator<U> mergeFunction,Supplier<M> mapSupplier) {BiConsumer<M, T> accumulator= (map, element) -> map.merge(keyMapper.apply(element),valueMapper.apply(element), mergeFunction);return new CollectorImpl<>(mapSupplier, accumulator, mapMerger(mergeFunction), CH_ID);}
⑤. 如解决toMap()乱序:使用LinkedHashMap
public class CollectionToMapDemo {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");List<House> houses = Lists.newArrayList();houses.add(house);houses.add(house2);houses.add(house1);/*** 收集一下属性和对象本身*/Map<Integer, House> houseMap = houses.stream().collect(Collectors.toMap(House::getOwnerId, o -> o));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)}*/LinkedHashMap<Integer, House> collect = houses.stream().collect(Collectors.toMap(House::getOwnerId, Function.identity(), (v1, v2) -> v1, LinkedHashMap::new));//会按照添加顺序进行处理System.out.println(collect);/**{1=House(id=1, ownerId=1, houseName=aa, address=北京海淀), 3=House(id=3, ownerId=3, houseName=cc, address=浙江杭州), 2=House(id=2, ownerId=2, houseName=bb, address=湖北武汉)}*/}}@Data@AllArgsConstructorclass House {private Integer id; //idprivate Integer ownerId; //家主编号private String houseName; //家庭名称private String address; //家庭地址}
③. 收集一下属性和对象本身
houses.stream().collect(Collectors.toMap(House::getOwnerId, o -> o));// 如果key相同了,这种方式会抛出异常houses.stream().collect(Collectors.toMap(House::getOwnerId, Function.identity()));// 如果key相同了,可以按照指定规则处理houses.stream().collect(Collectors.toMap(House::getOwnerId, Function.identity(),(v1,v2)->v1));
List<User>userList=new ArrayList<>();userList.add(new User(1,"a"));userList.add(new User(2,"b"));userList.add(new User(3,"c"));userList.add(new User(4,"取前者的值"));userList.add(new User(4,"取后者的值"));userList.add(new User(5,"e"));/**如果key中有值重复,取前者(这里打印的是 4:取前者的值)1====User(id=1, name=a)2====User(id=2, name=b)3====User(id=3, name=c)4====User(id=4, name=取前者的值)5====User(id=5, name=e)*///Map<Integer, User> userMap = userList.stream().collect(Collectors.toMap(User::getId, k1 -> k1, (K1, K2) -> K1));/**如果key中有值重复,取后者(这里打印的是 4:取后者的值)1====User(id=1, name=a)2====User(id=2, name=b)3====User(id=3, name=c)4====User(id=4, name=取后者的值)5====User(id=5, name=e)*/Map<Integer, User> userMap = userList.stream().collect(Collectors.toMap(User::getId, Function.identity(), (K1, K2) -> K2));for (Integer key:userMap.keySet()){System.out.println(key+"===="+userMap.get(key));}
④. 常见的java.lang.IllegalStateException: Duplicate key问题处理
①. 修改数据,两个一样的keyHouse 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");House house3 = new House(3,3,"cc","浙江杭州");List<House> houses = Lists.newArrayList();houses.add(house);houses.add(house1);houses.add(house2);houses.add(house3);/*** 收集一下属性和对象本身*/Map<Integer, House> houseMap = houses.stream().collect(Collectors.toMap(House::getOwnerId, o -> o));System.out.println(houseMap);
②. 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 House(id=3, ownerId=3, houseName=cc, address=浙江杭州)at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)at java.util.HashMap.merge(HashMap.java:1253)at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1374)at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)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.xiaozhi.collection.CollectionToMapDemo.main(CollectionToMapDemo.java:29)Process finished with exit code 1
③. 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));
④. 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
