安卓工作室尝试对空对象引用调用虚拟方法" [英] Android studio " Attempt to invoke virtual method on a null object reference"

问题描述

当我为列表视图的每个项目创建自定义视图时，我得到一个空指针异常，我不知道为什么，布局 ID 似乎是正确的

when i create the custom view of each items of the list view, i get a null pointer exception and i dont know why, the layout id seems correct

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.TextView;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;


public class ActivityMainWish extends Activity {

private List<Wish> myWishs = new ArrayList<>();

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

    creerListe();
    creerListeView();
    afficherWishView();
    ajouterWishListe();
}

@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_activity_main_wish, 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) {
        //fct
        return true;
    }
    if (id ==R.id.action_add_wish){
        startActivity(new Intent(this,ActivityAddWish.class));
        return true;
    }
    return super.onOptionsItemSelected(item);
}

public void creerListe() {
    myWishs.add(new Wish("Voiture", 50000, getResources().getDrawable(R.drawable.voiture,null)));
    myWishs.add(new Wish("TV", 5000, getResources().getDrawable(R.drawable.tv,null)));
    myWishs.add(new Wish("Smartphone", 500, getResources().getDrawable(R.drawable.smarphone, null)));
}

public void creerListeView (){
    //convertire les items en views appropier
    //prend cette liste, le Layout utilise (creer), et la liste d'items a afficher
    ArrayAdapter<Wish> adapter = new MyListAdapter();
    //prend la liste
    ListView list = (ListView) findViewById(R.id.ListViewWishList);
    //configure la liste
    list.setAdapter(adapter);
}

private class MyListAdapter extends ArrayAdapter<Wish>{
    public MyListAdapter(){
        super(ActivityMainWish.this, R.layout.item_view, myWishs);
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        View itemView = convertView;
        if (itemView==null)
            itemView = getLayoutInflater().inflate(R.layout.item_view, parent, false);

        Wish currentWish = myWishs.get(position);

        TextView textView = (TextView) itemView.findViewById(R.id.TextViewWishListNom);
        textView.setText((currentWish.getNom()));

        TextView prixView = (TextView) itemView.findViewById(R.id.TextViewWishListPrix);
        prixView.setText(("" + currentWish.getPrix()));

        ImageView imageView = (ImageView) itemView.findViewById(R.id.ImageViewWishListImage);
        imageView.setImageDrawable(currentWish.getImage());

        return itemView;
    }
}
}

主要活动布局:

<RelativeLayout
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="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin"
tools:context=".ActivityMainWish">

<ListView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/ListViewWishList"
    android:layout_alignParentTop="true"
    android:layout_centerHorizontal="true" />

项目视图布局:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/wishList">

<ImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/ImageViewWishListImage"
    android:src="@drawable/voiture"
    android:minWidth="80dp"
    android:minHeight="80dp"
    android:layout_alignParentLeft="true"
    android:layout_alignParentTop="true"
    android:adjustViewBounds="true"
    android:maxHeight="80dp"
    android:maxWidth="80dp" />

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:textAppearance="?android:attr/textAppearanceLarge"
    android:text="Voiture"
    android:id="@+id/TextViewWishListNom"
    android:layout_alignParentTop="true"
    android:layout_alignParentEnd="true" />

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:textAppearance="?android:attr/textAppearanceSmall"
    android:text="50000"
    android:id="@+id/TextViewWishListPrix"
    android:layout_alignBottom="@+id/ImageViewWishListImage"
    android:layout_alignParentEnd="true" />

这里是错误

java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference
        at com.gokhan.mywishlist.ActivityMainWish$MyListAdapter.getView(ActivityMainWish.java:90)
        at android.widget.AbsListView.obtainView(AbsListView.java:2347)
        at android.widget.ListView.measureHeightOfChildren(ListView.java:1270)
        at android.widget.ListView.onMeasure(ListView.java:1182)
        at android.view.View.measure(View.java:17547)
        at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:727)
        at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:463)
        at android.view.View.measure(View.java:17547)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:436)
        at android.view.View.measure(View.java:17547)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
        at com.android.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:447)
        at android.view.View.measure(View.java:17547)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:436)
        at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2615)
        at android.view.View.measure(View.java:17547)
        at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2015)
        at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1173)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1379)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1061)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5885)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:767)
        at android.view.Choreographer.doCallbacks(Choreographer.java:580)
        at android.view.Choreographer.doFrame(Choreographer.java:550)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:753)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5254)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

欢迎任何帮助和解释

推荐答案

在你的代码中

itemView = getLayoutInflater().inflate(R.layout.item_view, parent, false);

getLayoutInflater() 方法在内部适配器类中不可用.因此，您应该尝试创建一个变量来拥有父类的 Layout inflater 对象并因此访问它，或者您可以在适配器类的构造函数中在调用类的上下文中发送.

the getLayoutInflater() method is not usable in The inner adapter class. Hence you should try creating an Variable to have the Layout inflater object of the parent class and hence access it ,or you can send in the context of the calling class in the constructor of the adapter class.

因此你可以这样使用:

public class ActivityMainWish extends Activity {

LayoutInflate inflater;
private List<Wish> myWishs = new ArrayList<>();

@Override
protected void onCreate(Bundle savedInstanceState) {
inflater=getLayoutInflater()
...
}

并在这里使用这个 inflater 变量:itemView = inflater.inflate(R.layout.item_view, parent, false);

and use this inflater variable over here: itemView = inflater.inflate(R.layout.item_view, parent, false);

或者像这样:

private class MyListAdapter extends ArrayAdapter<Wish>{
private Context mContext;
public MyListAdapter(Context ctx){
    super(ActivityMainWish.this, R.layout.item_view, myWishs);
    mContext=ctx;
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    View itemView = convertView;
    if (itemView==null)
        itemView = LayoutInflater.from(mContext).inflate(R.layout.item_view, parent, false);

并发送 Context 对象，如:

and send the Context object like:

ArrayAdapter<Wish> adapter = new MyListAdapter(getApplicationContext());

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