java - 使用startActivityForResult

Question

我制作了 2 个类 YehActivity.java 和 h.java。运行应用程序时，我收到错误，应用程序意外停止。这是代码

public class YehActivity extends Activity {
public static final int r=1;
Button b;
TextView tv;



@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        b.setOnClickListener(new OnClickListener() {
            public void onClick(View v) {
                // TODO Auto-generated method stub
                Intent i=new Intent(YehActivity.this,he.class);
                //startActivity(i);
                startActivityForResult(i, r);
                }
        });
       }  
    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        // TODO Auto-generated method stub
        if(requestCode==r && resultCode==RESULT_OK){
                       String h=data.getStringExtra("a");
                       tv.setText(h);
            }
        }
     }      

在哪里检查 null。

这是第二个文件

public class he extends Activity{
Button b;
EditText et;
Intent i=getIntent();
@Override
protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);
    setContentView(R.layout.h);

    b=(Button) findViewById(R.id.button12);
    et=(EditText) findViewById(R.id.editText1);

        b.setOnClickListener(new OnClickListener() {

            public void onClick(View v) {
                // TODO Auto-generated method stub
                String value=et.getText().toString().trim();

                i.putExtra("a", value);
                he.this.setResult(RESULT_OK, i);
                finish();
                }
        });

    }

}

日志文件是

02-11 23:31:46.408: I/Process(302): Sending signal. PID: 302 SIG: 9
02-11 23:45:04.778: D/AndroidRuntime(357): Shutting down VM
02-11 23:45:04.778: W/dalvikvm(357): threadid=1: thread exiting with uncaught exception (group=0x40015560)
02-11 23:45:04.798: E/AndroidRuntime(357): FATAL EXCEPTION: main
02-11 23:45:04.798: E/AndroidRuntime(357): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.ye/com.ye.YehActivity}: java.lang.NullPointerException
02-11 23:45:04.798: E/AndroidRuntime(357):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
02-11 23:45:04.798: E/AndroidRuntime(357):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
02-11 23:45:04.798: E/AndroidRuntime(357):  at android.app.ActivityThread.access$1500(ActivityThread.java:117)
02-11 23:45:04.798: E/AndroidRuntime(357):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
02-11 23:45:04.798: E/AndroidRuntime(357):  at android.os.Handler.dispatchMessage(Handler.java:99)
02-11 23:45:04.798: E/AndroidRuntime(357):  at android.os.Looper.loop(Looper.java:123)
02-11 23:45:04.798: E/AndroidRuntime(357):  at android.app.ActivityThread.main(ActivityThread.java:3683)
02-11 23:45:04.798: E/AndroidRuntime(357):  at java.lang.reflect.Method.invokeNative(Native Method)
02-11 23:45:04.798: E/AndroidRuntime(357):  at java.lang.reflect.Method.invoke(Method.java:507)
02-11 23:45:04.798: E/AndroidRuntime(357):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
02-11 23:45:04.798: E/AndroidRuntime(357):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
02-11 23:45:04.798: E/AndroidRuntime(357):  at dalvik.system.NativeStart.main(Native Method)
02-11 23:45:04.798: E/AndroidRuntime(357): Caused by: java.lang.NullPointerException
02-11 23:45:04.798: E/AndroidRuntime(357):  at com.ye.YehActivity.onCreate(YehActivity.java:23)
02-11 23:45:04.798: E/AndroidRuntime(357):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
02-11 23:45:04.798: E/AndroidRuntime(357):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
02-11 23:45:04.798: E/AndroidRuntime(357):  ... 11 more
02-11 23:45:11.158: I/Process(357): Sending signal. PID: 357 SIG: 9
02-11 23:45:22.708: D/AndroidRuntime(374): Shutting down VM
02-11 23:45:22.708: W/dalvikvm(374): threadid=1: thread exiting with uncaught exception (group=0x40015560)
02-11 23:45:22.728: E/AndroidRuntime(374): FATAL EXCEPTION: main
02-11 23:45:22.728: E/AndroidRuntime(374): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.ye/com.ye.YehActivity}: java.lang.NullPointerException
02-11 23:45:22.728: E/AndroidRuntime(374):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
02-11 23:45:22.728: E/AndroidRuntime(374):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
02-11 23:45:22.728: E/AndroidRuntime(374):  at android.app.ActivityThread.access$1500(ActivityThread.java:117)
02-11 23:45:22.728: E/AndroidRuntime(374):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
02-11 23:45:22.728: E/AndroidRuntime(374):  at android.os.Handler.dispatchMessage(Handler.java:99)
02-11 23:45:22.728: E/AndroidRuntime(374):  at android.os.Looper.loop(Looper.java:123)
02-11 23:45:22.728: E/AndroidRuntime(374):  at android.app.ActivityThread.main(ActivityThread.java:3683)
02-11 23:45:22.728: E/AndroidRuntime(374):  at java.lang.reflect.Method.invokeNative(Native Method)
02-11 23:45:22.728: E/AndroidRuntime(374):  at java.lang.reflect.Method.invoke(Method.java:507)
02-11 23:45:22.728: E/AndroidRuntime(374):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
02-11 23:45:22.728: E/AndroidRuntime(374):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
02-11 23:45:22.728: E/AndroidRuntime(374):  at dalvik.system.NativeStart.main(Native Method)
02-11 23:45:22.728: E/AndroidRuntime(374): Caused by: java.lang.NullPointerException
02-11 23:45:22.728: E/AndroidRuntime(374):  at com.ye.YehActivity.onCreate(YehActivity.java:23)
02-11 23:45:22.728: E/AndroidRuntime(374):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
02-11 23:45:22.728: E/AndroidRuntime(374):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
02-11 23:45:22.728: E/AndroidRuntime(374):  ... 11 more
02-11 23:45:25.497: I/Process(374): Sending signal. PID: 374 SIG: 9

Answer 1

在你的

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    b.setOnClickListener(new OnClickListener() {
        public void onClick(View v) {
            // TODO Auto-generated method stub
            Intent i=new Intent(YehActivity.this,he.class);
            //startActivity(i);
            startActivityForResult(i, r);
            }
    });
}

onCreate() 方法，您尝试使用 b，但您从未初始化它(我假设它声明为全局变量)。这意味着当您尝试调用 setOnClickListener() 时，您将遇到 NullPointerException。