Java.lang.NullpointerException嘗試調用虛擬方法void android.widget.ListView.setAdapter（android.widget.setAdapter）

Question

我整天都在努力了解錯誤。 我是一個學習者，第一次嘗試自定義適配器和自定義對象類。 請幫助我理解我的錯誤。 我嘗試了很多次沖浪，直到最終在這里輸入我的懷疑。 任何幫助將不勝感激。 從網站上學習后，我已經嘗試過了。 我遵循相同的步驟，但無法解決此問題。 日志貓說

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.miwok, PID: 7814
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.miwok/com.example.miwok.NumbersActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2487)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2547)
    at android.app.ActivityThread.access$1100(ActivityThread.java:151)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1398)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:157)
    at android.app.ActivityThread.main(ActivityThread.java:5603)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:774)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:652)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
    at com.example.miwok.NumbersActivity.onCreate(NumbersActivity.java:30)
    at android.app.Activity.performCreate(Activity.java:6355)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1108)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2440)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2547) 
    at android.app.ActivityThread.access$1100(ActivityThread.java:151) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1398) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:157) 
    at android.app.ActivityThread.main(ActivityThread.java:5603) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:774) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:652) package com.example.miwok;

這是NumbersActivity

import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.Adapter;
import android.widget.ArrayAdapter;
import android.widget.ListView;

import java.util.ArrayList;

public class NumbersActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.intial);
        ArrayList<Word>words1=new ArrayList<Word>();
        words1.add(new Word("One","lutti"));
        words1.add(new Word("Two","ottiko"));
        words1.add(new Word("Three","tolookosu"));
        words1.add(new Word("Four","oyyisa"));
        words1.add(new Word("Five","massokka"));
        words1.add(new Word("Six","temmokka"));
        words1.add(new Word("seven","kenukaku"));
        words1.add(new Word("Eight","kawinta"));
        words1.add(new Word("Nine","wo'e"));
        words1.add(new Word("Ten","na'aacha"));
        WordAdapter itemsAdapter=new WordAdapter(this, words1);
        ListView listview=(ListView) findViewById(R.id.word_list);
        listview.setAdapter(itemsAdapter);

    }
}

WordAdapter在這里

package com.example.miwok;

import android.app.Activity;
import android.support.annotation.NonNull;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.TextView;

import java.util.ArrayList;

public class WordAdapter extends ArrayAdapter<Word> {
    private static final String LOG_TAG = WordAdapter.class.getSimpleName();
    public WordAdapter(Activity context,ArrayList<Word> words){

        super(context,0,words);
    }


    @Override
    public View getView(int position, @NonNull View convertView, @NonNull ViewGroup parent) {
        View listItemview=convertView;
        if(listItemview==null)
        {
            listItemview=LayoutInflater.from(getContext()).inflate(R.layout.list_item,parent,false);
        }
        Word current=getItem(position);
        TextView txt=(TextView)listItemview.findViewById(R.id.text_view);
        txt.setText(current.getDefaultTranslation());
        TextView txt1=(TextView)listItemview.findViewById(R.id.miwok_translation);
        txt1.setText(current.getMiwokTranslation());
        return listItemview;
    }
    }

這是單詞java文件

package com.example.miwok;

public class Word {
    private String mMiwokTranslation;
    private String mDefaultTranslation;

    public Word(String vDefaultTranslation, String vMiwokTranslation) {
        mMiwokTranslation = vMiwokTranslation;
        mDefaultTranslation = vDefaultTranslation;
    }

    public String getDefaultTranslation() {
        return mDefaultTranslation;
    }
    public String getMiwokTranslation() {
        return mMiwokTranslation;
    }
}

請幫幫我。

Answer 1

NullPointerException表示您正在嘗試訪問不存在的事物。

原因：java.lang.NullPointerException：嘗試在com.example.miwok.NumbersActivity.onCreate（NumbersActivity.java）上的空對象引用上調用虛擬方法'void android.widget.ListView.setAdapter（android.widget.ListAdapter）' ：30）

您尚未檢索ListView。 您確定使用的是正確的xml文件嗎？ 是正確的ID嗎？