開發(fā)常見錯誤ArrayIndexOutOfBoundsException(越界)
這個一般都是在取值或者是賦值的時候最為常見：

public class ExampleUnitTest {  
    private String[] data;  
    @Test  
    public void addition_isCorrect() throws Exception {  
        data = new String[]{}; //初始化  
        data[0] = "李四"; //這里就會ArrayIndexOutOfBoundsException  
    }  
}  

錯誤：

java.lang.ArrayIndexOutOfBoundsException: 0  
    at com.example.mytest.ExampleUnitTest.addition_isCorrect(ExampleUnitTest.java:29)  
    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$2.evaluate(ParentRunner.java:268)  
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)  
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)  
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)  
    at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:51)  
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)  
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)  
    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 com.intellij.rt.execution.application.AppMainV2.main(AppMainV2.java:131)  

看錯誤能知道data這個數(shù)組是沒有給分配空間導(dǎo)致下標(biāo)0報錯，那么在初始化地方修改下代碼

public class ExampleUnitTest {  
    private String[] data;  
    @Test  
    public void addition_isCorrect() throws Exception {  
        data = new String[1]; //初始化  
        data[0] = "李四"; //這里就會ArrayIndexOutOfBoundsException  
    }  
}  

這樣在運(yùn)行代碼就OK了；
取值也一樣，比方現(xiàn)在data數(shù)組有下標(biāo)0了，那么取下標(biāo)1的數(shù)據(jù)data[1]可以嗎，我們試試。

public class ExampleUnitTest {  
    private String[] data;  
    @Test  
    public void addition_isCorrect() throws Exception {  
        data = new String[1]; //初始化  
        data[0] = "李四"; //這里就會ArrayIndexOutOfBoundsException  
        System.out.print(data[1]);  
    }  
}  

結(jié)果：

java.lang.ArrayIndexOutOfBoundsException: 1  
    at com.example.mytest.ExampleUnitTest.addition_isCorrect(ExampleUnitTest.java:30)  
    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$2.evaluate(ParentRunner.java:268)  
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)  
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)  
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)  
    at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:51)  
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)  
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)  
    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 com.intellij.rt.execution.application.AppMainV2.main(AppMainV2.java:131)  

果斷的報錯了，那是因為數(shù)組一個就只有一位長度，現(xiàn)在在去值第二位當(dāng)然就報錯了；(下標(biāo)都是已0開始的)，我們把

System.out.print(data[1]);  

改為

System.out.print(data[0]);  

運(yùn)行就可以得李四的值，也就是data數(shù)組保存的值。
結(jié)束語：
但你在取某個集合或數(shù)組的長度大的下標(biāo)數(shù)據(jù)時就會報錯，但保存沒有定義數(shù)組長度的時候也會報錯，這是我的一個理解。