java8 lambda 将list对象改为Map对象

爱被打了一巴掌 2022-03-12 17:40 1478阅读 0赞

### list转map ###

java8 lambda将List转换为Map比较简单，但是 有坑：

List<Boy> sellOutList = Lists.newArrayList();
            Boy one = new Boy();
            one.setId(34343);
            sellOutList.add(one);
            Boy two = new Boy();
            two.setId(34343);
            sellOutList.add(two);
            Map<Integer, Boy> res = sellOutList.stream().collect(Collectors.toMap(Boy::getId, Function.identity()));
            System.out.println(res.entrySet());
            
    		public static class Boy{
            int id;
    
            int age;
    
            public int getId() {
                return id;
            }
    
            public void setId(int id) {
                this.id = id;
            }
    
            public int getAge() {
                return age;
            }
    
            public void setAge(int age) {
                this.age = age;
            }
        }

注意这里使用的是：Function.identity()；  
这里运行会报错，报错信息：

java.lang.IllegalStateException: Duplicate key com.serviceTest.ServiceTest$Boy@6c6017b9
    
    	at java.util.stream.Collectors.lambda$throwingMerger$106(Collectors.java:133)
    	at java.util.stream.Collectors$$Lambda$4/38625764.apply(Unknown Source)
    	at java.util.HashMap.merge(HashMap.java:1245)
    	at java.util.stream.Collectors.lambda$toMap$164(Collectors.java:1320)
    	at java.util.stream.Collectors$$Lambda$6/940448009.accept(Unknown Source)
    	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:512)
    	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:502)
    	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 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:483)
    	at org.testng.internal.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:80)
    	at org.testng.internal.MethodInvocationHelper$1.runTestMethod(MethodInvocationHelper.java:182)
    	at org.springframework.test.context.testng.AbstractTestNGSpringContextTests.run(AbstractTestNGSpringContextTests.java:171)
    	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:483)
    	at org.testng.internal.MethodInvocationHelper.invokeHookable(MethodInvocationHelper.java:194)
    	at org.testng.internal.Invoker.invokeMethod(Invoker.java:707)
    	at org.testng.internal.Invoker.invokeTestMethod(Invoker.java:901)
    	at org.testng.internal.Invoker.invokeTestMethods(Invoker.java:1231)
    	at org.testng.internal.TestMethodWorker.invokeTestMethods(TestMethodWorker.java:127)
    	at org.testng.internal.TestMethodWorker.run(TestMethodWorker.java:111)
    	at org.testng.TestRunner.privateRun(TestRunner.java:767)
    	at org.testng.TestRunner.run(TestRunner.java:617)
    	at org.testng.SuiteRunner.runTest(SuiteRunner.java:334)
    	at org.testng.SuiteRunner.runSequentially(SuiteRunner.java:329)
    	at org.testng.SuiteRunner.privateRun(SuiteRunner.java:291)
    	at org.testng.SuiteRunner.run(SuiteRunner.java:240)
    	at org.testng.SuiteRunnerWorker.runSuite(SuiteRunnerWorker.java:52)
    	at org.testng.SuiteRunnerWorker.run(SuiteRunnerWorker.java:86)
    	at org.testng.TestNG.runSuitesSequentially(TestNG.java:1198)
    	at org.testng.TestNG.runSuitesLocally(TestNG.java:1123)
    	at org.testng.TestNG.run(TestNG.java:1031)
    	at org.testng.IDEARemoteTestNG.run(IDEARemoteTestNG.java:72)
    	at org.testng.RemoteTestNGStarter.main(RemoteTestNGStarter.java:123)

`Function.identity() 换成 c->c,(k1, k2)->k2)`

正确的写法：

这样就不会报错了。

### 参考博客 ###

[常用lambda（Map相关）][lambda_Map]

[lambda_Map]: https://spldeolin.com/posts/lambda-map/