问题描述
如何修复此代码？由于这些原因，它在 logcat 中不断关闭应用程序？
每次我运行它时它都会崩溃或停止。我没有构建错误，但我检查了我的 logcat 的原因，但仍然无法解决它。 请发送帮助 错误：如果需要更清晰，请参阅图片
2021-04-26 04:31:41.006 29414-29414/com.mobdeve.santosm.androidchallenge E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.mobdeve.santosm.androidchallenge,PID: 29414
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.mobdeve.santosm.androidchallenge/com.mobdeve.santosm.androidchallenge.MainActivity}: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
at android.app.ActivityThread.performlaunchActivity(ActivityThread.java:3123)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3266)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1957)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7099)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:494)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:965)
Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
at android.widget.ArrayAdapter.getCount(ArrayAdapter.java:388)
at android.widget.ListView.setAdapter(ListView.java:614)
at com.mobdeve.santosm.androidchallenge.MainActivity.onCreate(MainActivity.java:29)
at android.app.Activity.performCreate(Activity.java:7327)
at android.app.Activity.performCreate(Activity.java:7318)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1275)
at android.app.ActivityThread.performlaunchActivity(ActivityThread.java:3103)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3266)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1957)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7099)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:494)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:965)
这是我的代码
主要活动：
import androidx.appcompat.app.AppCompatActivity;
import android.content.Intent;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.ListView;
import java.util.ArrayList;
public class MainActivity extends AppCompatActivity {
private Button newEmail;
private ListView listView;
private EmailAdapter emailAdapter;
private ArrayList<Emails> emailsArrayList;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
init();
emailAdapter = new EmailAdapter(this,emailsArrayList);
listView.setAdapter(emailAdapter);
newEmail.setonClickListener(new View.OnClickListener() {
public void onClick(View v) {
Intent intent = new Intent(MainActivity.this,SendActivity.class);
startActivity(intent);
}
});
}
@Override
protected void onStart() {
super.onStart();
Log.d("MainActivity","onStart invoked");
}
@Override
protected void onResume() {
super.onResume();
Log.d("MainActivity","onResume invoked");
}
@Override
protected void onPause() {
super.onPause();
Log.d("MainActivity","onPause invoked");
}
@Override
protected void onStop() {
super.onStop();
}
@Override
protected void onRestart() {
super.onRestart();
}
@Override
protected void onDestroy() {
super.onDestroy();
}
private void init(){
newEmail = (Button) findViewById(R.id.newBtn);
listView = (ListView) findViewById(R.id.list);
}
}
适配器：
package com.mobdeve.santosm.androidchallenge;
import android.app.Activity;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.TextView;
import java.util.ArrayList;
public class EmailAdapter extends ArrayAdapter<Emails>{
private Activity activity;
public EmailAdapter(Activity activity,ArrayList<Emails> emailsArrayList){
super(activity,R.layout.emails_row,emailsArrayList);
this.activity = activity;
}
@Override
public View getView(int position,View convertView,ViewGroup parent) {
View rowView = convertView;
if (rowView == null){
LayoutInflater inflater = activity.getLayoutInflater();
rowView = inflater.inflate(R.layout.emails_row,null);
ViewHolder emailsViewHolder = new ViewHolder();
emailsViewHolder.rEmail= (TextView) rowView.findViewById(R.id.rEmail);
emailsViewHolder.rBody= (TextView) rowView.findViewById(R.id.rBody);
emailsViewHolder.rSubject= (TextView) rowView.findViewById(R.id.rSubject);
rowView.setTag(emailsViewHolder);
}
final ViewHolder holder = (ViewHolder) rowView.getTag();
Emails info = getItem(position);
holder.rEmail.setText(info.getEmails());
holder.rBody.setText(info.getBody());
holder.rSubject.setText(info.getSubject());
return rowView;
}
static class ViewHolder{
public TextView rEmail;
public TextView rBody;
public TextView rSubject;
}
}
解决方法
只需将以下行添加到您的 init() 方法中：
emailsArrayList = new ArrayList<>();
那应该可以解决NullPointerException