Question

我想读取一个输入流，我的问题是这个方法总是返回null，虽然没有Exception输出和String输出（见下文）是正确的.. 有没有人知道如何解决这个问题？

private ArrayList<String> getPlayer(String name){

    try{
        byte[] b = new byte[1024];
        int numBytes;
        InputStream in = openFileInput(name);
        ArrayList<String> content = new ArrayList<String>();

        while((numBytes = in.read(b)) != 0){
            content.add(new String(b, 0, numBytes, "UTF-8"));
            String string = new String(b, 0, numBytes, "UTF-8");
            System.out.println(string);     //StringOutput 
        }
        return content;


    }catch(Exception e){
        System.out.print(e.getMessage());   //ExceptionOutput 
        return null;
    }
}

Exception，抛出并使应用程序停止：

  E/AndroidRuntime: FATAL EXCEPTION: main
                                                               Process: auldo.trainersoccer, PID: 7733
                                                               java.lang.NullPointerException: Attempt to invoke virtual method 'int java.util.ArrayList.size()' on a null object reference
                                                                   at auldo.trainersoccer.ManageSpielerActivity$1.onItemClick(ManageSpielerActivity.java:60)
                                                                   at android.widget.AdapterView.performItemClick(AdapterView.java:310)
                                                                   at android.widget.AbsListView.performItemClick(AbsListView.java:1145)
                                                                   at android.widget.AbsListView$PerformClick.run(AbsListView.java:3066)
                                                                   at android.widget.AbsListView$3.run(AbsListView.java:3903)
                                                                   at android.os.Handler.handleCallback(Handler.java:739)
                                                                   at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                   at android.os.Looper.loop(Looper.java:148)
                                                                   at android.app.ActivityThread.main(ActivityThread.java:5417)
                                                                   at java.lang.reflect.Method.invoke(Native Method)
                                                                   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                                                                   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

Answer 1

您的while循环条件存在问题

while((numBytes = in.read(b)) != 0){

将其更改为：

while((numBytes = in.read(b)) > 0){

当您使用!=条件时会发生StringIndexOutOfBoundsException，因为当您到达文件末尾时，in.read方法会返回 -1

Answer 2

由于 StringIndexOutOfRangeException 而发生错误。您没有检查是否已到达流的末尾。正如Java doc所述，流的结尾由返回值 -1 表示。所以你应该做((numBytes = in.read(b) != -1)。

Java方法返回null

2 个答案: