前言

昨天項(xiàng)目報(bào)錯(cuò)

java.lang.IllegalStateException: Duplicate key 1

代碼如下：

Map userId2BatchId = userLevelList.stream().collect(Collectors.toMap(RazUserLevel::getUserId, RazUserLevel::getBatchId);

原因是存在重復(fù)key導(dǎo)致

正文

查看toMap文檔

toMap

public static Collector> toMap(Function super T,? extends K> keyMapper,

Function super T,? extends U> valueMapper)

1

2

Returns a Collector that accumulates elements into a Map whose keys and values are the result of applying the provided mapping functions to the input elements.

If the mapped keys contains duplicates (according to Object.equals(Object)), an IllegalStateException is thrown when the collection operation is performed. If the mapped keys may have duplicates, use toMap(Function, Function, BinaryOperator) instead.

API Note:

It is common for either the key or the value to be the input elements. In this case, the utility method Function.identity() may be helpful. For example, the following produces a Map mapping students to their grade point average:

Map studentToGPA students.stream()

.collect(toMap(Functions.identity(), student -> computeGPA(student)));

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And the following produces a Map mapping a unique identifier to students:

Map studentIdToStudent

students.stream().collect(toMap(Student::getId, Functions.identity());

如果在最后生成map的時(shí)候，mapped到的keys中如果包含重復(fù)的鍵(通過key類型的equals方法來判斷)，則會(huì)拋出異常IllegalStateException。但是，后面也提到，如果keys中包含有相同的鍵，則可以使用toMap(Function, Function, BinaryOperator)方法來替代。

我的改進(jìn)如下：

Map userId2BatchId = userLevelList.stream().collect(Collectors.toMap(RazUserLevel::getUserId, RazUserLevel::getBatchId, (e1, e2) -> e1));

也就是有重復(fù)值的時(shí)候，我取先進(jìn)來的值，也就是e1。

總結(jié)

java8不熟，查api

總結(jié)

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