List与String[]相互转化

一、List 转 String[]

 @Test
    void arraysTostr() {
        List list = new ArrayList<>();
        list.add("mm");
        list.add("nn");
        list.add("hh");
        String[] strings = list.toArray(new String[list.size()]);
        System.out.println(strings.length);
    }

 输出: 3

 二、 String[] 转List

 @Test
    void strToArrays() {
        String[] strs = new String[]{"mm","nn","hh"};
        List list = Arrays.asList(strs);
        System.out.println(list.toString());
    }

输出: [mm, nn, hh]

三、Error:java.lang.UnsupportedOperationException 

 @Test
    void strToArrays() {
        String[] strs = new String[]{"mm","nn","hh"};
        List list = Arrays.asList(strs);
        list.add("vv");
        System.out.println(list.toString());
    }

执行了上边的测试数据后会发现将String[]转化为List之后,再执行list.add操作会抛出以下异常

java.lang.UnsupportedOperationException
	at java.util.AbstractList.add(AbstractList.java:148)
	at java.util.AbstractList.add(AbstractList.java:108)
	at com.yd.redisassistant.RedisAssistantApplicationTests.strToArrays(RedisAssistantApplicationTests.java:32)
	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:497)
	at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:532)
	at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:115)
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$invokeTestMethod$6(TestMethodTestDescriptor.java:171)
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:72)
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.invokeTestMethod(TestMethodTestDescriptor.java:167)
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:114)
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:59)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$4(NodeTestTask.java:108)
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:72)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:98)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:74)
	at java.util.ArrayList.forEach(ArrayList.java:1249)

原因如下:

AbstractList中add方法是默认throw UnsupportedOperationException而且不作任何操作。
Arrays的内部类ArrayList没有重新实现,所以会抛出异常
java.util.ArrayList重写了方法
解决方法如下:

@Test
    void strToArrays() {
        String[] strs = new String[]{"mm","nn","hh"};
        List list = new ArrayList<>(Arrays.asList(strs));
        list.add("vv");
        System.out.println(list.toString());
    }

 输出: [mm, nn, hh, vv]

下边是Arrays.asList的内部类: 

List与String[]相互转化_第1张图片List与String[]相互转化_第2张图片 

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