java.lang.NullPointerException：试图在OnClickListener中调用虚拟方法android

Question

我是android新手。我试图构建一个简单的应用程序，但当我尝试调用我的onClickListener时遇到了NullPointerException。被困几个小时才能调试，但未能发现错误的位置。我需要帮助来解决它。这是我的代码。

异常

E/AndroidRuntime: FATAL EXCEPTION: main 
       Process: com.example.win8user.kidlearnmath, PID: 2593 
       java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference 
        at com.example.win8user.kidlearnmath.PlayerActivity.displayDialog(PlayerActivity.java:63) 
        at com.example.win8user.kidlearnmath.PlayerActivity.access$000(PlayerActivity.java:24) 
        at com.example.win8user.kidlearnmath.PlayerActivity$1.onClick(PlayerActivity.java:47) 
        at android.view.View.performClick(View.java:4780) 
        at android.view.View$PerformClick.run(View.java:19866) 
        at android.os.Handler.handleCallback(Handler.java:739) 
        at android.os.Handler.dispatchMessage(Handler.java:95) 
        at android.os.Looper.loop(Looper.java:135) 
        at android.app.ActivityThread.main(ActivityThread.java:5257) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at java.lang.reflect.Method.invoke(Method.java:372) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698) 

的AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?> 

<application 
    android:allowBackup="true" 
    android:icon="@mipmap/ic_launcher" 
    android:label="@string/app_name" 
    android:supportsRtl="true" 
    android:theme="@style/AppTheme"> 
    <activity android:name=".MainActivity"> 
     <intent-filter> 
      <action android:name="android.intent.action.MAIN" /> 

      <category android:name="android.intent.category.LAUNCHER" /> 
     </intent-filter> 
    </activity> 
    <activity android:name=".PlayerActivity"></activity> 
</application> 

PlayerActivity.java

package com.example.win8user.kidlearnmath; 

import android.app.Dialog; 
import android.database.Cursor; 
import android.support.design.widget.FloatingActionButton; 
import android.support.v7.app.AppCompatActivity; 
import android.os.Bundle; 
import android.support.v7.widget.Toolbar; 
import android.util.Log; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.view.View; 
import android.widget.Button; 
import android.widget.EditText; 
import android.widget.ListView; 
import android.widget.Toast; 

import com.example.win8user.kidlearnmath.mDataObject.Player; 
import com.example.win8user.kidlearnmath.mDatabase.PlayerAdapter; 
import com.example.win8user.kidlearnmath.mListView.CustomAdapter; 

import java.util.ArrayList; 

public class PlayerActivity extends AppCompatActivity { 

ListView lv; 
EditText nameEditText; 
Button saveBtn, retrieveBtn; 
ArrayList<Player> players = new ArrayList<>(); 
CustomAdapter adapter; 
final boolean forUpdate = true; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_player); 
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar); 
    setSupportActionBar(toolbar); 

    lv = (ListView) findViewById(R.id.lv); 
    adapter = new CustomAdapter(this, players); 

    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab); 
    fab.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View v) { 
      displayDialog(false); 
     } 
    }); 

    this.getPlayers(); 
} 

private void displayDialog(boolean forUpdate){ 
    final Dialog d = new Dialog(this); 

    d.setContentView(R.layout.dialog_layout); 

    nameEditText = (EditText) findViewById(R.id.nameEditTxt); 
    saveBtn = (Button) findViewById(R.id.saveBtn); 
    retrieveBtn = (Button) findViewById(R.id.cancelBtn); 

    if(!forUpdate){ 
     d.setTitle("Add New Player"); 
     saveBtn.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View v) { 
       if(nameEditText.getText().toString().length() == 0){ 
        nameEditText.setError("Name is required"); 
       } 
       else{ 
        save(nameEditText.getText().toString()); 
        d.dismiss(); 
       } 
      } 
     }); 
     retrieveBtn.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View v) { 
       getPlayers(); 
       d.dismiss(); 
      } 
     }); 

    } 
    else { 
     d.setTitle("Edit Player Name"); 
     saveBtn.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View v) { 
       if(nameEditText.getText().toString().length() == 0){ 
        nameEditText.setError("Name is required"); 
       } 
       else{ 
        update(nameEditText.getText().toString()); 
        d.dismiss(); 
       } 
      } 
     }); 
     retrieveBtn.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View v) { 
       getPlayers(); 
       d.dismiss(); 
      } 
     }); 

    } 
    d.show(); 
} 

private void save(String name){ 
    PlayerAdapter db = new PlayerAdapter(this); 
    db.openDB(); 
    boolean save; 
    save = db.add(name); 

    if(save){ 
     nameEditText.setText(""); 
     getPlayers(); 
    } 
    else{ 
     Toast.makeText(this,"Unable To Save",Toast.LENGTH_SHORT).show(); 
    } 
} 

private void getPlayers(){ 
    players.clear(); 
    PlayerAdapter db = new PlayerAdapter(this); 
    db.openDB(); 
    Cursor c = db.retrieve(); 
    Player player = null; 

    while (c.moveToNext()){ 
     String pid = c.getString(0); 
     String pname = c.getString(1); 

     player = new Player(); 
     player.setPlayerId(pid); 
     player.setPlayerName(pname); 

     players.add(player); 
    } 
    db.closeDB(); 
    lv.setAdapter(adapter); 
} 

private void update(String newname){ 
    //Get id of player 
    String pid = adapter.getSelectedItemID(); 

    //UPDATE 
    PlayerAdapter db = new PlayerAdapter(this); 
    db.openDB(); 
    boolean updated = db.update(newname,pid); 
    db.closeDB(); 

    if(updated){ 
     nameEditText.setText(newname); 
     getPlayers(); 
    } 
    else { 
     Toast.makeText(this,"Unable to Update",Toast.LENGTH_SHORT).show(); 
    } 
} 

private void delete(){ 
    String pid = adapter.getSelectedItemID(); 

    //DELETE 
    PlayerAdapter db = new PlayerAdapter(this); 
    db.openDB(); 
    boolean deleted = db.delete(pid); 
    db.closeDB(); 

    if(deleted){ 
     getPlayers(); 
    } 
    else { 
     Toast.makeText(this,"Unable to Delete",Toast.LENGTH_SHORT).show(); 
    } 
} 

@Override 
public boolean onCreateOptionsMenu(Menu menu) { 
    getMenuInflater().inflate(R.menu.main_menu,menu); 
    return true; 
} 

@Override 
public boolean onContextItemSelected(MenuItem item) { 
    CharSequence title = item.getTitle(); 

    if(title=="New"){ 
     displayDialog(!forUpdate); 
    }else if(title=="Edit"){ 
     displayDialog(forUpdate); 
    }else if(title=="Delete"){ 
     delete(); 
    } 

    return super.onContextItemSelected(item); 
} 
} 

activity_player.xml

<?xml version="1.0" encoding="utf-8"?> 
<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android" 
xmlns:app="http://schemas.android.com/apk/res-auto" 
xmlns:tools="http://schemas.android.com/tools" 
android:layout_width="match_parent" 
android:layout_height="match_parent" 
android:fitsSystemWindows="true" 
tools:context="com.example.win8user.kidlearnmath.PlayerActivity"> 

<android.support.design.widget.AppBarLayout 
    android:layout_width="match_parent" 
    android:layout_height="wrap_content" 
    android:theme="@style/AppTheme.AppBarOverlay"> 

    <android.support.v7.widget.Toolbar 
     android:id="@+id/toolbar" 
     android:layout_width="match_parent" 
     android:layout_height="?attr/actionBarSize" 
     android:background="?attr/colorPrimary" 
     app:popupTheme="@style/AppTheme.PopupOverlay" 
     /> 

</android.support.design.widget.AppBarLayout> 

<include layout="@layout/content_player" /> 

<android.support.design.widget.FloatingActionButton 
    android:id="@+id/fab" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_gravity="bottom|end" 
    android:layout_margin="@dimen/fab_margin" 
    android:src="@android:drawable/ic_input_add" /> 

dialog_layout.xml

<?xml version="1.0" encoding="utf-8"?> 
<android.support.v7.widget.CardView xmlns:android="http://schemas.android.com/apk/res/android" 
android:orientation="horizontal" android:layout_width="fill_parent" 
xmlns:card_view="http://schemas.android.com/apk/res-auto" 
android:layout_margin="1dp" 
card_view:cardCornerRadius="10dp" 
card_view:cardElevation="5dp" 
android:gravity="center" 

android:layout_height="match_parent"> 

<LinearLayout 
    android:layout_width="match_parent" 
    android:orientation="vertical" 
    android:layout_height="match_parent"> 

    <android.support.design.widget.TextInputLayout 
     android:id="@+id/nameLayout" 
     android:layout_width="match_parent" 
     android:layout_height="wrap_content"> 

     <android.support.design.widget.TextInputEditText 
      android:id="@+id/nameEditTxt" 
      android:layout_width="wrap_content" 
      android:layout_height="wrap_content" 
      android:singleLine="true" 
      android:hint="Name"/> 

    </android.support.design.widget.TextInputLayout> 

    <Button android:id="@+id/saveBtn" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_gravity="center" 
     android:text="Save" 
     android:clickable="true" 
     android:background="@color/colorAccent" 
     android:layout_marginTop="40dp" 
     android:textColor="@android:color/white"/> 

    <Button android:id="@+id/cancelBtn" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_gravity="center" 
     android:text="Cancel" 
     android:clickable="true" 
     android:background="@color/colorAccent" 
     android:layout_marginTop="40dp" 
     android:textColor="@android:color/white"/> 


</LinearLayout> 

在此先感谢。

Answer 1

要调用findViewById()这是对Activity的方法。 Dialog不是与Activity相同的Window的一部分。您想在Dialog实例上拨打findViewById()。

d.findViewById(R.id.saveBtn)