Java陷阱：Collections.unmodifiableCollection_unable to make public int java.util.collections$un-优快云博客

本文链接：https://blog.youkuaiyun.com/virgoboy2004/article/details/6131680

本文通过示例展示了如何在Java中创建不可变集合，并探讨了其背后的实现原理。特别是使用了Collections.unmodifiableCollection方法，该方法返回一个代理对象，使得原始集合在被包装后无法直接修改。文章还演示了当尝试修改不可变集合时会引发UnsupportedOperationException异常。

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import java.util.ArrayList; import java.util.Collection; import java.util.Collections; public class Test { public static void main(String[] args) { Collection<User> originalCollection = new ArrayList<User>(); originalCollection.add(new User("a")); originalCollection.add(new User("b")); originalCollection.add(new User("c")); Collection<User> unmodifiableCollection = Collections.unmodifiableCollection(originalCollection); for (User user : originalCollection) { user.name = "ssss"; } System.out.println("Original Size="+unmodifiableCollection.size()); originalCollection.add(new User("d")); System.out.println("New Size="+unmodifiableCollection.size()); for (User user : unmodifiableCollection) { System.out.println(user.name); } } } class User { public String name; public User(String name) { this.name = name; } }

Collections.unmodifiableCollection 实际是使用的代理模式，返回的Collection内部仍然使用原来Collection。

这段代码的输出为：

Original Size=3
New Size=4
ssss
ssss
ssss
d

import java.util.ArrayList; import java.util.Collection; import java.util.Collections; public class Test { public static void main(String[] args) { Collection<User> originalCollection = new ArrayList<User>(); originalCollection.add(new User("a")); originalCollection.add(new User("b")); originalCollection.add(new User("c")); Collection<User> unmodifiableCollection = Collections.unmodifiableCollection(originalCollection); for (User user : originalCollection) { user.name = "ssss"; } System.out.println("Original Size="+unmodifiableCollection.size()); unmodifiableCollection.add(new User("d")); System.out.println("New Size="+unmodifiableCollection.size()); for (User user : unmodifiableCollection) { System.out.println(user.name); } } } class User { public String name; public User(String name) { this.name = name; } }

我们不能直接操作unmodifiableCollection，这段代码运行时将抛异常

Original Size=3
Exception in thread "main" java.lang.UnsupportedOperationException
at java.util.Collections$UnmodifiableCollection.add(Collections.java:1018)
at Test.main(Test.java:22)