JAVA集合05_Collection.toMap()应用三个重载方法解决重复key问题
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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)
*/
@Data
@AllArgsConstructor
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
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
@AllArgsConstructor
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
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key
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");
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, 05_集合对象注入
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