Question

我已经尝试了一些提交到堆栈溢出的解决方案，但到目前为止没有人帮助过我。在另一个调用父活动的片段中触发按钮侦听器时会发生此问题。父活动调用此片段updateValue()方法（参见下文）。在我的Fragment的onCreateView()中，我有以下代码：

Result.class扩展了Fragment

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    view = inflater.inflate(R.layout.result, container, false);         
    resultTV = (TextView)view.findViewById(R.id.resultTV);
    resultTV.setText(resultTV.getText().toString() + "test!");
    return view;
}

这非常有效。但是，当我从父活动调用片段中的方法时，textview引用为null（空指针异常）。调用resultTV.setText()时会发生异常。

public void updateValue(View v) {
    String text = ((Button)v).getText().toString();

    if(text.toLowerCase() == "c") resultTV.setText(DEFAULT_RESULT_TEXT);
    else resultTV.setText(resultTV.getText().toString() + text);
}

例外

10-09 16:32:59.091: E/AndroidRuntime(2324): FATAL EXCEPTION: main
10-09 16:32:59.091: E/AndroidRuntime(2324): java.lang.NullPointerException
10-09 16:32:59.091: E/AndroidRuntime(2324):     at com.test.Result.updateValue(Result.java:53)
10-09 16:32:59.091: E/AndroidRuntime(2324):     at com.test.Main.onBasicButtonClicked(Main.java:65)
10-09 16:32:59.091: E/AndroidRuntime(2324):     at com.test.Basic$7.onClick(Basic.java:120)
10-09 16:32:59.091: E/AndroidRuntime(2324):     at android.view.View.performClick(View.java:4240)
10-09 16:32:59.091: E/AndroidRuntime(2324):     at android.view.View$PerformClick.run(View.java:17721)
10-09 16:32:59.091: E/AndroidRuntime(2324):     at android.os.Handler.handleCallback(Handler.java:730)
10-09 16:32:59.091: E/AndroidRuntime(2324):     at android.os.Handler.dispatchMessage(Handler.java:92)
10-09 16:32:59.091: E/AndroidRuntime(2324):     at android.os.Looper.loop(Looper.java:137)
10-09 16:32:59.091: E/AndroidRuntime(2324):     at android.app.ActivityThread.main(ActivityThread.java:5103)
10-09 16:32:59.091: E/AndroidRuntime(2324):     at java.lang.reflect.Method.invokeNative(Native Method)
10-09 16:32:59.091: E/AndroidRuntime(2324):     at java.lang.reflect.Method.invoke(Method.java:525)
10-09 16:32:59.091: E/AndroidRuntime(2324):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
10-09 16:32:59.091: E/AndroidRuntime(2324):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
10-09 16:32:59.091: E/AndroidRuntime(2324):     at dalvik.system.NativeStart.main(Native Method)

修改

Main有一些片段：basic和scientific片段只是按钮。 matrix片段尚未使用，因此我们可以忽略该片段。 result片段是找到问题TextView的地方。

Main.class（主要活动类）

public class Main extends Activity implements 
OnBasicButtonClickedListener, 
OnScientificButtonClickedListener, 
OnMatrixButtonClickedListener {

private FragmentManager     fm = getFragmentManager();
private FragmentTransaction ft = fm.beginTransaction();
private Basic               basic = new Basic();
private Scientific          scientific = new Scientific();
private Result              result = new Result();
private MatrixFragment      matrixFrag = new MatrixFragment();
private static final String LOG = "Main.class";

/* this list view will hold all submitted formulas */
private ListView formulaLV;

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

    if(findViewById(R.id.result) == null)
        ft.add(R.id.result, result);

    if(findViewById(R.id.basic) == null)
        ft.add(R.id.basic, basic);

    if(findViewById(R.id.scientific) == null)
        ft.add(R.id.scientific, scientific);

    if(findViewById(R.id.matrix) == null)
        ft.add(R.id.matrix, matrixFrag);
    ft.commit();        
}

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

/*
 *  This method acts as a listener for fragment to fragment communication 
 *  from Basic fragment.
 */
public void onBasicButtonClicked(View v) {
    Log.i(LOG, ((Button)v).getText() + " button pressed!");
    result.updateValue(v);
}

/*
 *  This method acts as a listener for fragment to fragment communication 
 *  from Scientific fragment.
 */
public void onScientificButtonClicked(View v) {
    Log.i(LOG, ((Button)v).getText() + " button pressed!");
    result.updateValue(v);
}

@Override
public void matrixButtonClicked(View view) {
    // matrix button clicked 

}

}

Answer 1

空指针可能是因为resultTV没有向updateValue()方法显示。你是如何在片段类中声明resultTV的？如果您要实例化该类的多个实例并希望它随时间保持不变，您可能想尝试将其声明为静态

您也可以尝试

TextView resultTV = (TextView)view.findViewById(R.id.resultTV);

updateValue方法

中的

在onCreateView：nullpointerexception之后TextView为null

1 个答案: