解决使用stream将list转map时,key重复导致报错的问题

要将List对象集合转为map集合，可以通过stream流的形式快速实现转换：

//三个Users对象组成一个List集合
List<Users> list = new ArrayList<>();
list.add(Users.builder().userName("11").userId(1).build());
list.add(Users.builder().userName("11").userId(2).build());
list.add(Users.builder().userName("33").userId(3).build());
//将list转map
Map<String, Users> usersMap = list.stream()
    .collect(Collectors.toMap(Users::getUserName, user -> user));
System.out.println(usersMap.get("11"));

但是上述代码运行后报了异常：

意思为map中出现了重复的key，也就是说通过上述方法转map时，出现重复key并不会出现覆盖的情况，而是再次在map中添加一个重复的key，导致报错。

所以通过stream实现list转map时，要实现重复的key会被覆盖，可以使用Function.identity()方法：

//三个Users对象组成一个List集合
List<Users> list = new ArrayList<>();
list.add(Users.builder().userName("11").userId(1).build());
list.add(Users.builder().userName("11").userId(2).build());
list.add(Users.builder().userName("33").userId(3).build());
//将list转map，这里是出现重复key时，覆盖前一个
Map<String, Users> usersMap = list.stream()
    .collect(Collectors.toMap(Users::getUserName, Function.identity(), (user1, user2) -> user2));
System.out.println(usersMap.get("11"));
//输出结果：
edu.nf.ch08.entity.Users@41aaedaa

JDK 8 Stream List转换为Map的duplicate Key异常

Stream List to Map

Stream提供了List转换为Map提供了非常易用的方法：

Collectors.java:

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);
    }

其在转换过程中，会抛出异常：

@Test(expected = IllegalStateException.class)
   public void testStreamMap_duplicateKey() {
       Employee employee = Employee.builder().id(1).age(20).firstName("zhang").build();
       Employee employee1 = Employee.builder().id(2).age(21).firstName("Li").build();
       Employee employee2 = Employee.builder().id(3).age(22).firstName("Li").build();
       Employee employee3 = Employee.builder().id(4).age(23).firstName("Chen").build();
       List<Employee> employees = Lists.newArrayList();
       employees.add(employee);
       employees.add(employee1);
       employees.add(employee2);
       employees.add(employee3);
       Map<String, Integer> dataMap = employees.stream().collect(Collectors.toMap(e -> e.getFirstName(), e -> e.getAge()));
       //Duplicate Key
       Map<Integer, Employee> employeeMap = employees.stream().collect(Collectors.toMap(e->e.getAge(), e->e));
   }

抛出异常信息：

java.lang.IllegalStateException: Duplicate key 21
 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: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 org.cjf.java.learn.jdk8.StreamTest.testStreamMap_duplicateKey(StreamTest.java:90)
 at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
 at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
 at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
 at java.lang.reflect.Method.invoke(Method.java:498)
 at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
 at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
 at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
 at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
 at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
 at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
 at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
 at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
 at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
 at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
 at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
 at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
 at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
 at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
 at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
 at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
 at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:230)
 at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:58)
Process finished with exit code 255

如何解决

增加重复key情况下的冲突处理策略：

Map<String, Integer> dataMap = employees.stream().collect(Collectors.toMap(e -> e.getFirstName(), e -> e.getAge(), (k1, k2)-> k1));
       Assert.assertThat(dataMap, hasKey("zhang"));
       Assert.assertThat(dataMap, hasKey("Li"));
       Assert.assertThat(dataMap, hasKey("Chen"));
       Assert.assertThat(dataMap.keySet(), hasSize(3));
       Map<Integer, Employee> employeeMap = employees.stream().collect(Collectors.toMap(e->e.getAge(), e->e, (k1, k2) -> k1));
       Assert.assertThat(dataMap.keySet(), hasSize(3));