java - 尝试将字符串写入 Android 应用程序上的文件，打开时崩溃

Question

这个问题已经有答案了:

getFileDir() cause NullPointer exception (1 个回答)

已关闭 6 年前。

我正在编写一个简单的日记应用程序。我目前已经可以打开它，在文本框中输入内容，然后单击按钮保存它(但尚未实际保存)。如果我按如下方式运行它(将/注释掉/部分注释掉)，那么它就可以工作，但是如果我将注释设为代码，它就会在启动时崩溃。我已将其范围缩小到第一行注释代码:File path = this.getFilesDir();导致崩溃，尽管其他地方可能还有更多我尚未发现的问题。

知道为什么会崩溃吗？或者有更好的方法让我将文本保存到文件中？

public class MainActivity extends AppCompatActivity {
/*    File path = this.getFilesDir();
File file = new File(path, "dreamWritings.txt");

FileOutputStream stream;*/
@Override
protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    final TextView tv = (TextView)findViewById(dreamText);
    final Button button = (Button) findViewById(R.id.recordDream);

    button.setOnClickListener(new View.OnClickListener(){
        public void onClick(View v){
            String dreamWords = tv.getText().toString();

/*      try {
                stream = new FileOutputStream(file, true);
            }
        catch(FileNotFoundException ex2){
                System.out.println(ex2);
            }
        try {
            stream.write(dreamWords.getBytes());
            stream.close();
            tv.setText("Hey I think we wrote that to a file!");
        }
        catch(IOException ex) {
            System.out.println(ex);
        }*/
        tv.setText("Your dream has been recorded");
    }
});

}


}

这是崩溃日志:

  08-11 22:23:22.772 1443-1443/? E/AndroidRuntime: FATAL EXCEPTION: main
                                                 Process: com.sixpencegames.www.dreamjournal, PID: 1443
                                                 java.lang.RuntimeException: Unable to instantiate activity 

    ComponentInfo{com.sixpencegames.www.dreamjournal/com.sixpencegames.www.dreamjournal.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.io.File android.content.Context.getFilesDir()' on a null object reference
                                                         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2568)
                                                         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2727)
                                                         at android.app.ActivityThread.-wrap12(ActivityThread.java)
                                                         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1478)
                                                         at android.os.Handler.dispatchMessage(Handler.java:102)
                                                         at android.os.Looper.loop(Looper.java:154)
                                                         at android.app.ActivityThread.main(ActivityThread.java:6121)
                                                         at java.lang.reflect.Method.invoke(Native Method)
                                                         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:889)
                                                         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:779)
                                                      Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.io.File android.content.Context.getFilesDir()' on a null object reference
                                                         at android.content.ContextWrapper.getFilesDir(ContextWrapper.java:223)
                                                         at com.sixpencegames.www.dreamjournal.MainActivity.<init>(MainActivity.java:22)
                                                         at java.lang.Class.newInstance(Native Method)
                                                         at android.app.Instrumentation.newActivity(Instrumentation.java:1078)
                                                         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2558)
                                                         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2727) 
                                                         at android.app.ActivityThread.-wrap12(ActivityThread.java) 
                                                         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1478) 
                                                         at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                         at android.os.Looper.loop(Looper.java:154) 
                                                         at android.app.ActivityThread.main(ActivityThread.java:6121) 
                                                         at java.lang.reflect.Method.invoke(Native Method) 
                                                         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:889) 
                                                         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:779) 

Answer 1

问题是您无法全局访问 this，因为不会分配上下文，因此返回 null。

尝试以下

public class MainActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    final TextView tv = (TextView)findViewById(dreamText);
    final Button button = (Button) findViewById(R.id.recordDream);
    File path = this.getFilesDir();
    File file = new File(path, "dreamWritings.txt");
    FileOutputStream stream;
    button.setOnClickListener(new View.OnClickListener(){
        public void onClick(View v){
            String dreamWords = tv.getText().toString();

      try {
                stream = new FileOutputStream(file, true);
            }
        catch(FileNotFoundException ex2){
                System.out.println(ex2);
            }
        try {
            stream.write(dreamWords.getBytes());
            tv.setText("Hey I think we wrote that to a file!");
        }
        catch(IOException ex) {
            System.out.println(ex);
        }finally{ 
           stream.close();
        }
        tv.setText("Your dream has been recorded");
    }
});

}