空指针异常:尝试调用虚拟方法"void" [英] Null Pointer Exception : Attempt to invoke virtual method 'void

问题描述

我的代码有问题.我正在学习如何使用Android创建Simple Firebase CRUD应用程序.这是我的MainActivity代码:

I have problem with my code. I'm learning how to create Simple Firebase CRUD application with Android . Here is my MainActivity code :

package com.ogigpermana.crudfirebaseapp;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.text.TextUtils;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.Spinner;
import android.widget.Toast;

import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;
import com.ogigpermana.crudfirebaseapp.adapter.ArtistList;
import com.ogigpermana.crudfirebaseapp.model.Artist;

import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {
    EditText editTextName;
    Button buttonAdd;
    Spinner spinnerGenres;

    DatabaseReference databaseArtists;
    ListView listViewArtists;
    List<Artist> artistList;

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

        databaseArtists = FirebaseDatabase.getInstance().getReference("artists");

        editTextName = (EditText) findViewById(R.id.editTextName);
        buttonAdd = (Button) findViewById(R.id.buttonAddArtist);
        spinnerGenres = (Spinner) findViewById(R.id.spinnerGenres);

        listViewArtists = (ListView) findViewById(R.id.listViewArtists);

        artistList = new ArrayList<>();

        buttonAdd.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                addArtist();
            }
        });
    }

    @Override
    protected void onStart() {
        super.onStart();
        databaseArtists.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                artistList.clear();

                for (DataSnapshot artistSnapshot: dataSnapshot.getChildren()){
                    Artist artist = artistSnapshot.getValue(Artist.class);

                    artistList.add(artist);
                }

                ArtistList adapter = new ArtistList(MainActivity.this, artistList);

                listViewArtists.setAdapter(adapter);
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });
    }

    private void addArtist(){
        String name = editTextName.getText().toString().trim();
        String genre = spinnerGenres.getSelectedItem().toString();

        if (!TextUtils.isEmpty(name)){

            String id  = databaseArtists.push().getKey();

            Artist artist = new Artist(id, name, genre);

            databaseArtists.child(id).setValue(artist);

            Toast.makeText(this, "Artis berhasil ditambah", Toast.LENGTH_LONG).show();

        }else{
            Toast.makeText(this, "Nama tidak boleh kosong", Toast.LENGTH_LONG).show();
        }
    }
}

这是我运行时显示的错误代码:

And here is the errors code that displayed when i run :

07-03 00:30:45.727 30907-30907/com.ogigpermana.crudfirebaseapp E/AndroidRuntime:致命例外:主要

流程:com.ogigpermana.crudfirebaseapp，PID:30907

java.lang.NullPointerException:尝试调用虚拟方法'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' 空对象引用 在 com.ogigpermana.crudfirebaseapp.MainActivity $ 2.onDataChange(MainActivity.java:72) 在com.google.android.gms.internal.zzbmz.zza(未知来源) 在com.google.android.gms.internal.zzbnz.zzYj(未知来源) com.google.android.gms.internal.zzboc $ 1.run(未知来源) 在android.os.Handler.handleCallback(Handler.java:739) 在android.os.Handler.dispatchMessage(Handler.java:95) 在android.os.Looper.loop(Looper.java:148) 在android.app.ActivityThread.main(ActivityThread.java:5441) 在java.lang.reflect.Method.invoke(本机方法) 在 com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run(ZygoteInit.java:738) 在com.android.internal.os.ZygoteInit.main(ZygoteInit.java:628)

07-03 00:30:45.727 30907-30907/com.ogigpermana.crudfirebaseapp E/AndroidRuntime: FATAL EXCEPTION: main

Process: com.ogigpermana.crudfirebaseapp, PID: 30907

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference at com.ogigpermana.crudfirebaseapp.MainActivity$2.onDataChange(MainActivity.java:72) at com.google.android.gms.internal.zzbmz.zza(Unknown Source) at com.google.android.gms.internal.zzbnz.zzYj(Unknown Source) at com.google.android.gms.internal.zzboc$1.run(Unknown Source) at android.os.Handler.handleCallback(Handler.java:739) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:148) at android.app.ActivityThread.main(ActivityThread.java:5441) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:738) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:628)

推荐答案

尝试一下-

交叉检查activity_main.xml

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