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我是android新手。我試圖構建一個簡單的應用程序，但當我嘗試調用我的onClickListener時遇到了NullPointerException。被困幾個小時才能調試，但未能發現錯誤的位置。我需要幫助來解決它。這是我的代碼。java.lang.NullPointerException：試圖在OnClickListener中調用虛擬方法android
異常
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>
在此先感謝。
「PlayerActivity.java」的第63行是什麼？ – Emmanuel
保存按鈕上出現異常？或檢索按鈕？ –
線63是saveBtn.setOnClickListener（新View.OnClickListener（）{。拋出異常的保存按鈕，但如果我onclicklistner除去savebtn。retreive按鈕也拋出的異常被點擊按鈕時 – gtuner