JDK 8 Stream List轉換爲Map的duplicate Key異常

Stream List to Map

Stream提供了List轉換爲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);
    }

其在轉換過程中，會拋出異常：

 @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));
    }

拋出異常信息：

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

如何破解

增加重複key情況下的衝突處理策略：

 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));

這裏的處理策略是：

(k1, k2) -> k1

總結

在Collectors.toMap()轉換過程中，需要注意一下duplicate key的處理邏輯，需要增加mergeFunction()處理方法。