关于Java8当中List转Map报错“Duplicate key”的原因以及解决办法

    其实Java8的lamdba表达式了解的不是很透彻，在工作中也只是使用最多的就是 for循环遍历、list转map、map转list、分组等功能。需要什么就从网上百度查找，然后按照样子在自己代码中修改下。其中的细节并没有留意过，就在今天我不小心就掉进了坑里。

    悲剧发生在今天早上，领导过来问我为什么业务上的数据有的时候会丢失，并让我排查下原因。我起初还怀疑是不是公司另外一端的接口有问题，于是就去问了下有没有上线之类的操作。得到的结果是：没有！

    那我就只能从自己的代码入手了，我开始本地启动连接访问线上的数据进行数据排查。运行了一遍之后发现没有什么问题，之后又连续点了几遍，结果报错了！！！

java.lang.IllegalStateException: Duplicate key com.houry.spring.bean.People@e2144e4
  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.houry.spring.bean.People.main(People.java:50)

    定位到报错的代码之后，我顿时懵逼了。这里怎么会报错呢？就是一个简单的List转Map啊！仔细看看提示信息“Duplicate key”就感受到了，那一定是重复的key引起的。代码类似下面这样的。（为了保密具体的业务代码我就不粘出来了图片）

public class People {

    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public People(String name) {
        this.name = name;
    }

    public static void main(String[] args) {
        People p1 = new People("MonkeyBrother");
        People p2 = new People("MonkeyBrother");
        People p3 = new People("MonkeyBrother");
        People p4 = new People("MonkeyBrother");
        People p5 = new People("MonkeyBrother");
        List list = new ArrayList<>();

        list.add(p1);
        list.add(p2);
        list.add(p3);
        list.add(p4);
        list.add(p5);
        
        Map collect = list.stream().collect(Collectors.toMap(People::getName, p -> p));
        System.out.println(collect.toString());
    }

}

    我原本想的就是使用List转Map，然后相同的Key能够给我自动替换掉，但是按照我上面这样的写法它一定会报错。

list.stream().collect(Collectors.toMap(People::getName, p -> p));

    这样写是存在风险的，因为我生产业务代码中也是这样写的，所以我猜大概也是因为这样的原因报错了。为了死地更加明白些，我看了一下源码：

public static  Collector> toMap(Function keyMapper,
                                    Function valueMapper) {
        return toMap(keyMapper, valueMapper, throwingMerger(), HashMap::new);
}

public static >
    Collector toMap(Function keyMapper,
                                Function valueMapper,
                                BinaryOperator mergeFunction,
                                Supplier mapSupplier) {
        BiConsumer accumulator
                = (map, element) -> map.merge(keyMapper.apply(element),
                                              valueMapper.apply(element), mergeFunction);
        return new CollectorImpl<>(mapSupplier, accumulator, mapMerger(mergeFunction), CH_ID);
    }

private static  BinaryOperator throwingMerger {
    return (u,v) -> { throw new IllegalStateException(String.format("Duplicate key %s", u)); };
}

public class People {

    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public People(String name) {
        this.name = name;
    }

    public static void main(String[] args) {
        People p1 = new People("MonkeyBrother");
        People p2 = new People("MonkeyBrother");
        People p3 = new People("MonkeyBrother");
        People p4 = new People("MonkeyBrother");
        People p5 = new People("MonkeyBrother");
        List list = new ArrayList<>();

        list.add(p1);
        list.add(p2);
        list.add(p3);
        list.add(p4);
        list.add(p5);

        Map collect = list.stream().collect(Collectors.toMap(People::getName, p -> p, (k, v) -> k));

        System.out.println(collect.toString());

    }

}

Map collect = list.stream().collect(Collectors.toMap(People::getName, Function.identity(), (k, v) -> k));