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ConcurrentHashMap NoSuchMethodError
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info.md

当用jdk8编绎，生成target 为 jdk6/7 的代码时，如果有使用到了类似的代码，就会抛出异常

ConcurrentHashMap<String, String> map = new ConcurrentHashMap<String, String>();
map.keySet();

抛出

java.lang.NoSuchMethodError: java.util.concurrent.ConcurrentHashMap.keySet()Ljava/util/concurrent/ConcurrentHashMap$KeySetView;

原因很简单，jdk8里的ConcurrentHashMap#keySet()返回了一个新的类型，这个在旧的jdk上没有。

网上有很多文章说编绎时，用-bootclasspath指定旧版本的jdk来解决。但是太麻烦，没有通用性。

最简单实用的办法是把接口改为 ConcurrentMap。

ConcurrentMap<String, String> map = new ConcurrentHashMap<String, String>();

这个接口在jdk6上也有putIfAbsent函数。

https://docs.oracle.com/javase/6/docs/api/

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