为什么 getFragmentManager().findFragmentById 返回 null（导致 NullPointerException）

Question

我有两个片段：输入在顶部，结果在底部。

最初，仅显示 InputFragment。当用户单击 "Calculate" 按钮时，将显示 ResultFragment。一切正常。

ResultFragment 有一个 "Clear all" 按钮，我想清除 InputFragment 的字段，并使 ResultFragment 消失（隐藏、删除或分离，我不关心哪个，只要它变成隐形）。

但是当您点击 "Clear all" 按钮时它会崩溃。具体来说，致命的 NullPointerException 发生在 clearResult() 方法中。

这里是MainActivity.java

import android.support.v7.app.ActionBarActivity;
import android.app.Fragment;
import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;

public class MainActivity extends ActionBarActivity implements InputFragment.InputFragmentListener, ResultFragment.ResultFragmentListener {

    private static FragmentManager manager;

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

        if(savedInstanceState == null){
            Fragment inputFragment  = new InputFragment();

            manager = getFragmentManager();
            FragmentTransaction transaction = manager.beginTransaction();
            transaction.add(R.id.fragment_container_top,inputFragment).commit();
        }
    }

    //This gets called by the Input Fragment when the user clicks the Calculate button
    @Override
    public void createResult(String resultPhrase) {

        ResultFragment resultFragment = (ResultFragment) manager.findFragmentById(R.id.result_fragment);

        if (resultFragment != null){
            /* if resultFragment is available, we can modify it's TextView
             * by calling the method to do the updating
             */
            //ResultFragment.theResultText.setText(resultPhrase);
            resultFragment.setResultText(resultPhrase); 
        }else{
            /*...otherwise the fragment has to be created */
            ResultFragment newResultFragment = new ResultFragment();
            Bundle args = new Bundle();
            args.putString(ResultFragment.resultTxt, resultPhrase);
            newResultFragment.setArguments(args);

            FragmentTransaction transaction = manager.beginTransaction();   
            transaction.replace(R.id.fragment_container_bottom, newResultFragment);
            transaction.addToBackStack(null);
            transaction.commit();       

            //((TextView) newResultFragment.getView().get).setText(resultPhrase);

        }


    }


    //This gets called in the ResultFragment when the user clicks the Clear All button
    @Override
    public void clearResult() {
        //InputFragment inp_frag = (InputFragment) getFragmentManager().findFragmentById(R.id.fragment_input);


        // This damned object is null again 
        //inp_frag.clearFields();
        Fragment res_frag = getFragmentManager().findFragmentByTag("resultfragtag");

        FragmentTransaction transaction = getFragmentManager().beginTransaction();  
        //res_frag.clearResult();
        transaction.hide(res_frag);
        transaction.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 true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

以下是 LogCat 条消息：

03-01 23:30:34.663: E/AndroidRuntime(3399): FATAL EXCEPTION: main
03-01 23:30:34.663: E/AndroidRuntime(3399): Process: com.example.amortizersandbox, PID: 3399
03-01 23:30:34.663: E/AndroidRuntime(3399): java.lang.NullPointerException: Attempt to write to field 'int android.app.Fragment.mNextAnim' on a null object reference
03-01 23:30:34.663: E/AndroidRuntime(3399): at android.app.BackStackRecord.run(BackStackRecord.java:797)

Answer 1

添加 InputFragment 时没有使用标签 "resultfragtag"。

尝试在创建时添加它：

 transaction.add(R.id.fragment_container_top,inputFragment,"resultfragtag").commit();

Answer 2

replacing/adding 片段使用：

getSupportFragmentManager()
                            .beginTransaction()
                            .setCustomAnimations(R.anim.slide_in_top,
                                    R.anim.slide_out_top)
                            .add(R.id.list_frame, fr, "tag").commit();

现在查找此片段使用：

Fragment fr = getSupportFragmentManager()
                .findFragmentByTag("tag");
        if (fr != null) {
            //do your stuff
        } else {
//fr is null here
        }

Answer 3

使用：

Fragment res_frag = getFragmentManager().findFragmentById(R.id.result_fragment);

替换：

Fragment res_frag = getFragmentManager().findFragmentByTag("resultfragtag");

为什么 getFragmentManager().findFragmentById 返回 null（导致 NullPointerException）

Why is getFragmentManager().findFragmentById returning null (causing NullPointerException)

android

nullpointerexception

android-fragments

fragmenttransaction