Collections.unmodifiableMap用法产生一个只读的Map

刘星雨855

已于 2022-12-16 10:37:42 修改

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文章标签： java junit spring

于 2022-12-16 10:28:43 首次发布

本文链接：https://blog.youkuaiyun.com/lzy99534/article/details/128339136

版权

本文介绍了Java中如何使用Collections.unmodifiableMap创建只读Map，当尝试修改时会抛出异常。详细阐述了操作过程及测试结果，包括对原Map值的修改和深拷贝的概念。

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1、用法：产生一个只读的Map，当你调用此map的put方法时会抛错。

1.1、使用Collections.unmodifiableMap产生只读map

@Test
    public void test7(){
        HashMap<String, StringBuilder> stringStringHashMap = new HashMap<String, StringBuilder>();
        stringStringHashMap.put("1",new StringBuilder("1"));
        stringStringHashMap.put("2",new StringBuilder("2"));
        stringStringHashMap.put("3",new StringBuilder("3"));
        stringStringHashMap.put("3",new StringBuilder("4"));

        Map<String, StringBuilder> stringStringMap = Collections.unmodifiableMap(stringStringHashMap);
        System.out.println(stringStringMap);

    }

1.2、测试结果

{1=1, 2=2, 3=4}

1.3、对该只读map操作put

@Test
    public void test7(){
        HashMap<String, StringBuilder> stringStringHashMap = new HashMap<String, StringBuilder>();
        stringStringHashMap.put("1",new StringBuilder("1"));
        stringStringHashMap.put("2",new StringBuilder("2"));
        stringStringHashMap.put("3",new StringBuilder("3"));
        stringStringHashMap.put("3",new StringBuilder("4"));

        Map<String, StringBuilder> stringStringMap = Collections.unmodifiableMap(stringStringHashMap);
        stringStringMap.put("4",new StringBuilder("5"));
        //stringStringMap.get("3").append("test5");
        System.out.println(stringStringMap);

    }

1.4、测试结果

java.lang.UnsupportedOperationException
	at java.util.Collections$UnmodifiableMap.put(Collections.java:1457)
	at com.task.test.DataStructureTest.test7(DataStructureTest.java:144)
	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:498)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:69)
	at com.intellij.rt.junit.IdeaTestRunner$Repeater$1.execute(IdeaTestRunner.java:38)
	at com.intellij.rt.execution.junit.TestsRepeater.repeat(TestsRepeater.java:11)
	at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:35)
	at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:235)
	at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:54)

1.5、可以对原map值进行修改，这是对map对象的深拷贝

 @Test
    public void test7(){
        HashMap<String, StringBuilder> stringStringHashMap = new HashMap<String, StringBuilder>();
        stringStringHashMap.put("1",new StringBuilder("1"));
        stringStringHashMap.put("2",new StringBuilder("2"));
        stringStringHashMap.put("3",new StringBuilder("3"));
        stringStringHashMap.put("3",new StringBuilder("4"));

        Map<String, StringBuilder> stringStringMap = Collections.unmodifiableMap(stringStringHashMap);
        //stringStringMap.put("4",new StringBuilder("5"));
        stringStringMap.get("3").append("test5");
        System.out.println(stringStringMap);

    }

1.6、测试结果

{1=1, 2=2, 3=4test5}