java - 从另一个类调用我的主要 Activity 方法之一返回错误

Question

主要 Activity

public class MainActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Intent intent=new Intent(getApplicationContext(),SecondActivity.class);
    startActivity(intent);
}


public void function(String data) {
    EditText editText=(EditText)findViewById(R.id.EditText);
    editText.setText(data);

}

}

这是我的第二项 Activity

public class SecondActivity extends Activity {
MainActivity mainActivity=new MainActivity();
@Override
public void onCreate(Bundle save){
    super.onCreate(save);
    setContentView(R.layout.layout2);
    Button button=(Button)findViewById(R.id.button);
    button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            String data="SUCCESS!";
            mainActivity.function(data);
        }
    });

}

}

这是错误

10-07 20:56:47.678 11920-11920/wifiexplorer.braveheart.com.classtest E/AndroidRuntime: FATAL EXCEPTION: main
Process: wifiexplorer.braveheart.com.classtest, PID: 11920
java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
   at android.support.v7.app.AppCompatDelegateImplBase.<init>(AppCompatDelegateImplBase.java:68)
   at android.support.v7.app.AppCompatDelegateImplV7.<init>(AppCompatDelegateImplV7.java:145)
   at android.support.v7.app.AppCompatDelegateImplV11.<init>(AppCompatDelegateImplV11.java:28)
   at android.support.v7.app.AppCompatDelegateImplV14.<init>(AppCompatDelegateImplV14.java:41)
   at android.support.v7.app.AppCompatDelegateImplV23.<init>(AppCompatDelegateImplV23.java:29)
   at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:186)
   at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:170)
   at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:502)
   at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:174)
   at wifiexplorer.braveheart.com.classtest.MainActivity.function(MainActivity.java:20)
   at wifiexplorer.braveheart.com.classtest.SecondActivity$1.onClick(SecondActivity.java:22)
   at android.view.View.performClick(View.java:5277)
   at android.view.View$PerformClick.run(View.java:21711)
   at android.os.Handler.handleCallback(Handler.java:815)
   at android.os.Handler.dispatchMessage(Handler.java:104)
   at android.os.Looper.loop(Looper.java:207)
   at android.app.ActivityThread.main(ActivityThread.java:5896)
   at java.lang.reflect.Method.invoke(Native Method)
   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:679)

我是 android 编程新手，这里我只是使用 Intent 来调用我的第二个 Activity。

Answer 1

使用此文档 "Getting a Result from an Activity"

您想要更新您的 ui 元素。您的情况的良好做法是使用 startActivityForResult。您的案例中的示例代码:

 public class MainActivity extends AppCompatActivity {

    public static final String STRING_RESULT_DATA_KEY = "result_data";

    private static final int REQUEST_CODE_SECOND_ACTIVITY = 101;

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

        Intent intent = new Intent(this, SecondActivity.class);
        startActivityForResult(intent, REQUEST_CODE_SECOND_ACTIVITY);
    }

    public void function(String data) {
        EditText editText=(EditText)findViewById(R.id.EditText);
        editText.setText(data);

    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        // Check which request we're responding to
        if (requestCode == REQUEST_CODE_SECOND_ACTIVITY) {
            // Make sure the request was successful
            if (resultCode == RESULT_OK) {
                String dataString = data.getStringExtra(STRING_RESULT_DATA_KEY);
                function(dataString);
            }
        }
    }
}

第二个 Activity :

public class SecondActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.second_activity);
        Button button = (Button) findViewById(R.id.button);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String data = "SUCCESS!";
                Intent intent = new Intent();
                intent.putExtra(MainActivity.STRING_RESULT_DATA_KEY, data);
                setResult(RESULT_OK, intent);
                finish();
            }
        });
    }
}