Java setAdapter返回null
我正在尝试创建一个导航抽屉,并且完全遵循了一个在线教程,在一个空对象引用上出现了错误“尝试调用虚拟方法'void android.widget.ListView.setAdapter(android.widget.ListAdapter)” 完整错误消息:06-21 13:59:08.134 8883-8883/com.example.saarikakumar.factsappfull E/AndroidRuntime:致命异常:main 流程:com.example.saarikakumar.factsappull,PID:8883 java.lang.RuntimeException:无法启动activity ComponentInfo{com.example.saarikakumar.factsappull/com.example.saarikakumar.factsappull.MainActivity}:java.lang.NullPointerException:尝试在空对象引用上调用虚拟方法“void android.widget.ListView.setAdapter(android.widget.ListAdapter)” 在android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)上 位于android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 位于android.app.ActivityThread.-wrap11(ActivityThread.java) 在android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)上 位于android.os.Handler.dispatchMessage(Handler.java:102) 位于android.os.Looper.loop(Looper.java:148) 位于android.app.ActivityThread.main(ActivityThread.java:5417) 位于java.lang.reflect.Method.invoke(本机方法) 在com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run上(ZygoteInit.java:726) 位于com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 原因:java.lang.NullPointerException:尝试在空对象引用上调用虚拟方法“void android.widget.ListView.setAdapter(android.widget.ListAdapter)” 位于com.example.saarikakumar.factsappull.MainActivity.addDrawerItems(MainActivity.java:97) 位于com.example.saarikakumar.factsappull.MainActivity.onCreate(MainActivity.java:43) 位于android.app.Activity.performCreate(Activity.java:6237) 位于android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107) 在android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)上 位于android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 位于android.app.ActivityThread.-wrap11(ActivityThread.java) 在android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)上 位于android.os.Handler.dispatchMessage(Handler.java:102) 位于android.os.Looper.loop(Looper.java:148) 位于android.app.ActivityThread.main(ActivityThread.java:5417) 位于java.lang.reflect.Method.invoke(本机方法) 在com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run上(ZygoteInit.java:726) 位于com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 错误具体在这一行: mDrawerList.setAdapter(mAdapter); 和 addDrawerItems() 我的主要活动代码是:Java setAdapter返回null,java,android,navigation-drawer,Java,Android,Navigation Drawer,我正在尝试创建一个导航抽屉,并且完全遵循了一个在线教程,在一个空对象引用上出现了错误“尝试调用虚拟方法'void android.widget.ListView.setAdapter(android.widget.ListAdapter)” 完整错误消息:06-21 13:59:08.134 8883-8883/com.example.saarikakumar.factsappfull E/AndroidRuntime:致命异常:main 流程:com.example.saarikakumar.
import android.content.Intent;
import android.content.res.Configuration;
import android.os.Bundle;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;
import com.facebook.CallbackManager;
import com.facebook.FacebookCallback;
import com.facebook.FacebookException;
import com.facebook.FacebookSdk;
import com.facebook.login.LoginResult;
import com.facebook.login.widget.LoginButton;
public class MainActivity extends AppCompatActivity {
private TextView info;
private LoginButton loginButton;
private CallbackManager callbackManager;
private ListView mDrawerList;
private DrawerLayout mDrawerLayout;
private ArrayAdapter<String> mAdapter;
private ActionBarDrawerToggle mDrawerToggle;
private String mActivityTitle;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mDrawerList = (ListView) findViewById(R.id.navList);mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
mActivityTitle = getTitle().toString();
addDrawerItems();
setupDrawer();
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
getSupportActionBar().setHomeButtonEnabled(true);
FacebookSdk.sdkInitialize(getApplicationContext());
callbackManager = CallbackManager.Factory.create();
setContentView(R.layout.activity_main);
info = (TextView) findViewById(R.id.info);
loginButton = (LoginButton) findViewById(R.id.login_button);
loginButton.registerCallback(callbackManager, new FacebookCallback<LoginResult>() {
@Override
public void onSuccess(LoginResult loginResult) {
info.setText("Login successful");
info.setText(
"User ID: "
+ loginResult.getAccessToken().getUserId()
+ "\n" +
"Auth Token: "
+ loginResult.getAccessToken().getToken()
);
}
@Override
public void onCancel() {
info.setText("Login attempt cancelled");
}
@Override
public void onError(FacebookException error) {
info.setText("Login attempt failed");
}
});
Button switchButton = (Button) findViewById(R.id.login_button);
switchButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent intent = new Intent(MainActivity.this, MainActivity5.class);
startActivity(intent);
}
});
}
private void addDrawerItems() {
String[] osArray = {"Android", "iOS", "Windows", "OS X", "Linux"};
mAdapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, osArray);
mDrawerList.setAdapter(mAdapter);
mDrawerList.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
Toast.makeText(MainActivity.this, "Time for an upgrade!", Toast.LENGTH_SHORT).show();
}
});
}
private void setupDrawer() {
mDrawerToggle = new ActionBarDrawerToggle(this, mDrawerLayout, R.string.drawer_open, R.string.drawer_close) {
/** Called when a drawer has settled in a completely open state. */
public void onDrawerOpened(View drawerView) {
super.onDrawerOpened(drawerView);
getSupportActionBar().setTitle("Navigation!");
invalidateOptionsMenu(); // creates call to onPrepareOptionsMenu()
}
/** Called when a drawer has settled in a completely closed state. */
public void onDrawerClosed(View view) {
super.onDrawerClosed(view);
getSupportActionBar().setTitle(mActivityTitle);
invalidateOptionsMenu(); // creates call to onPrepareOptionsMenu()
}
};
mDrawerToggle.setDrawerIndicatorEnabled(true);
mDrawerLayout.setDrawerListener(mDrawerToggle);
}
@Override
protected void onPostCreate(Bundle savedInstanceState) {
super.onPostCreate(savedInstanceState);
// Sync the toggle state after onRestoreInstanceState has occurred.
mDrawerToggle.syncState();
}
@Override
public void onConfigurationChanged(Configuration newConfig) {
super.onConfigurationChanged(newConfig);
mDrawerToggle.onConfigurationChanged(newConfig);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.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;
}
// Activate the navigation drawer toggle
if (mDrawerToggle.onOptionsItemSelected(item)) {
return true;
}
return super.onOptionsItemSelected(item);
}
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
callbackManager.onActivityResult(requestCode, resultCode, data);
}
}
导入android.content.Intent;
导入android.content.res.Configuration;
导入android.os.Bundle;
导入android.support.v4.widget.DrawerLayout;
导入android.support.v7.app.ActionBarDrawerToggle;
导入android.support.v7.app.AppActivity;
导入android.view.Menu;
导入android.view.MenuItem;
导入android.view.view;
导入android.widget.AdapterView;
导入android.widget.ArrayAdapter;
导入android.widget.Button;
导入android.widget.ListView;
导入android.widget.TextView;
导入android.widget.Toast;
导入com.facebook.CallbackManager;
导入com.facebook.facebook回调;
导入com.facebook.facebook异常;
导入com.facebook.FacebookSdk;
导入com.facebook.login.LoginResult;
导入com.facebook.login.widget.LoginButton;
公共类MainActivity扩展了AppCompatActivity{
私有文本视图信息;
私人登录按钮登录按钮;
私人CallbackManager CallbackManager;
私有列表视图mDrawerList;
私人抽屉布局mDrawerLayout;
私人ArrayaAdapter mAdapter;
私有操作bardrawertoggle mDrawerToggle;
私有字符串mactivitytle;
@凌驾
创建时受保护的void(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mDrawerList=(ListView)findViewById(R.id.navList);mDrawerLayout=(DrawerLayout)findViewById(R.id.drawer\u布局);
mActivityTitle=getTitle().toString();
addDrawerItems();
setupDrawer();
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
getSupportActionBar().setHomeButtonEnabled(true);
sdkinInitialize(getApplicationContext());
callbackManager=callbackManager.Factory.create();
setContentView(R.layout.activity_main);
info=(TextView)findViewById(R.id.info);
loginButton=(loginButton)findviewbyd(R.id.login_按钮);
registerCallback(callbackManager,newfacebookcallback()){
@凌驾
成功时公共无效(LoginResult LoginResult){
info.setText(“登录成功”);
info.setText(
“用户ID:”
+loginResult.getAccessToken().getUserId()
+“\n”+
“身份验证令牌:”
+loginResult.getAccessToken().getToken()
);
}
@凌驾
公开作废{
info.setText(“登录尝试已取消”);
}
@凌驾
public void onError(facebook异常错误){
info.setText(“登录尝试失败”);
}
});
按钮开关按钮=(按钮)findViewById(R.id.login_按钮);
switchButton.setOnClickListener(新视图.OnClickListener(){
@凌驾
公共void onClick(视图v){
意向意向=新意向(MainActivity.this,MainActivity5.class);
星触觉(意向);
}
});
}
私人voi
<android.support.v4.widget.DrawerLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/drawer_layout"
android:layout_width="match_parent"
android:layout_height="match_parent">
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
android:layout_height="match_parent" android:paddingLeft="16dp"
android:paddingRight="16dp"
android:paddingTop="16dp"
android:gravity="center"
android:background="#f44336"
android:orientation="vertical"
android:id="@+id/fragment_placeholder"
android:paddingBottom="16dp" tools:context=".MainActivity"
android:weightSum="1"
android:padding="41dp">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/info"
android:layout_alignParentLeft="true"
android:layout_alignParentTop="true"
android:textSize="18sp"
/>
<com.facebook.login.widget.LoginButton
android:id="@+id/login_button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerInParent="true"/>
</LinearLayout>
<ListView
android:id="@+id/navList"
android:layout_width="200dp"
android:layout_height="match_parent"
android:layout_gravity="left|start"
android:background="#ffeeeeee"/>
</android.support.v4.widget.DrawerLayout>
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
/* add this */
setContentView(R.layout.your_layout);
mDrawerList = (ListView) findViewById(R.id.navList);mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
mActivityTitle = getTitle().toString();
setContentView(R.layout.name_of_this_activity_layout);