java - 应用程序无法在 Activity 之间传递值

Question

我正在开发一款应用，其中用户在一个 Activity 中输入一个数字，该数字显示在下一个 Activity 中。这是我目前所拥有的。

第一个 Activity :EnterText.java

public class EnterText extends Activity {

    //This is the string that will be passed to the next activity
    public static String mynumber = "com.example.ListviewExplorer.MESSAGE";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_enter_text);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.enter_text, menu);
        return true;
    }

    //This is where onClick, the button will invoke sendMessage
    //sendMessage is supposed to start the next activity
    public void sendMessage(View view){
        Intent intent = new Intent (this, DisplayText.class);
        EditText editText = (EditText) findViewById(R.id.editText1);
        mynumber = editText.getText().toString();
        intent.putExtra("MyNumber",mynumber);
        startActivity(intent);
    }

}

第二个 Activity :DisplayText.java

public class DisplayText extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        Bundle extras=getIntent().getExtras();
        String mynumber = extras.getString("MyNumber");
        TextView textview = (TextView)findViewById(R.id.textView1);
            textview.setText(EnterText.mynumber);

        setContentView(R.layout.activity_display_text);
        // Show the Up button in the action bar.
        setupActionBar();
    }
}

问题是，当我在第一个 Activity 上按下按钮并调用 sendMessage 时，这个应用程序崩溃了；第二个 Activity 甚至没有显示。

有人知道为什么会这样吗？

LogCat 贴在下面。

开始吧!

05-29 11:57:59.385: W/dalvikvm(3046): threadid=1: thread exiting with uncaught exception (group=0x42d5d140)
05-29 11:57:59.425: E/AndroidRuntime(3046): FATAL EXCEPTION: main
05-29 11:57:59.425: E/AndroidRuntime(3046): Process: com.example.listviewexplorer, PID: 3046
05-29 11:57:59.425: E/AndroidRuntime(3046): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.listviewexplorer/com.example.listviewexplorer.DisplayText}: java.lang.NullPointerException
05-29 11:57:59.425: E/AndroidRuntime(3046):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
05-29 11:57:59.425: E/AndroidRuntime(3046):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
05-29 11:57:59.425: E/AndroidRuntime(3046):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
05-29 11:57:59.425: E/AndroidRuntime(3046):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
05-29 11:57:59.425: E/AndroidRuntime(3046):     at android.os.Handler.dispatchMessage(Handler.java:102)
05-29 11:57:59.425: E/AndroidRuntime(3046):     at android.os.Looper.loop(Looper.java:136)
05-29 11:57:59.425: E/AndroidRuntime(3046):     at android.app.ActivityThread.main(ActivityThread.java:5017)
05-29 11:57:59.425: E/AndroidRuntime(3046):     at java.lang.reflect.Method.invokeNative(Native Method)
05-29 11:57:59.425: E/AndroidRuntime(3046):     at java.lang.reflect.Method.invoke(Method.java:515)
05-29 11:57:59.425: E/AndroidRuntime(3046):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
05-29 11:57:59.425: E/AndroidRuntime(3046):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
05-29 11:57:59.425: E/AndroidRuntime(3046):     at dalvik.system.NativeStart.main(Native Method)
05-29 11:57:59.425: E/AndroidRuntime(3046): Caused by: java.lang.NullPointerException
05-29 11:57:59.425: E/AndroidRuntime(3046):     at com.example.listviewexplorer.DisplayText.onCreate(DisplayText.java:21)
05-29 11:57:59.425: E/AndroidRuntime(3046):     at android.app.Activity.performCreate(Activity.java:5231)
05-29 11:57:59.425: E/AndroidRuntime(3046):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
05-29 11:57:59.425: E/AndroidRuntime(3046):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
05-29 11:57:59.425: E/AndroidRuntime(3046):     ... 11 more

Answer 1

您需要在使用前分配布局。

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    // assign layout
    setContentView(R.layout.activity_display_text);
    // then you can use it
    TextView textview = (TextView)findViewById(R.id.textView1);
    textview.setText(EnterText.mynumber);
    ...
}

由于尚未定义布局，它处于 null 阶段，当您尝试访问其中的 View 时，您会抛出 NullPointerException。