"尝试在空对象引用上调用虚拟方法'android.view.View android.view.View.findViewById(int)'"的致命异常

Question

我一直在按照教程使用此处显示的视图寻呼机添加类似卡片的滑动视图：

https://www.youtube.com/watch?v=UsXv6VRqZKs

我试图让它与片段一起使用，但是我遇到了这个错误：

2020-02-22 16:04:41.218 9582-9582/? E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.musicassistant, PID: 9582
java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference
at com.example.musicassistant.CardsFragment.onCreateView(CardsFragment.java:38)
at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2600)
at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:881)
at androidx.fragment.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManagerImpl.java:1238)
at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:1303)
at androidx.fragment.app.BackStackRecord.executeOps(BackStackRecord.java:439)
at androidx.fragment.app.FragmentManagerImpl.executeOps(FragmentManagerImpl.java:2079)
at androidx.fragment.app.FragmentManagerImpl.executeOpsTogether(FragmentManagerImpl.java:1869)
at androidx.fragment.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManagerImpl.java:1824)
at androidx.fragment.app.FragmentManagerImpl.execPendingActions(FragmentManagerImpl.java:1727)
at androidx.fragment.app.FragmentManagerImpl$2.run(FragmentManagerImpl.java:150)
at android.os.Handler.handleCallback(Handler.java:883)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7356)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)

这是我的代码：

fragment_cards.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@android:color/background_light"
>
<androidx.viewpager.widget.ViewPager
android:id="@+id/viewPager"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:clipToPadding="false"
android:foregroundGravity="center"
android:overScrollMode="never"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.0"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.0"></androidx.viewpager.widget.ViewPager>
<Button
android:id="@+id/button_shuffle"
android:text="SHUFFLE"
android:textColor="#fff"
android:background="@drawable/round"
android:paddingLeft="30dp"
android:paddingRight="30dp"
android:layout_marginBottom="60dp"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerHorizontal="true"
app:layout_constraintBottom_toBottomOf="@+id/viewPager"
app:layout_constraintEnd_toEndOf="@+id/viewPager"
app:layout_constraintStart_toStartOf="@+id/viewPager">
</Button>

</androidx.constraintlayout.widget.ConstraintLayout>

CardsFragment.java

package com.example.musicassistant;
import android.animation.ArgbEvaluator;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.viewpager.widget.ViewPager;
import java.util.ArrayList;
import java.util.List;
import java.util.Objects;
public class CardsFragment extends Fragment {
ViewPager viewPager;
Adapter adapter;
List<Model> models;
Integer[] colors = null;
ArgbEvaluator argbEvaluator = new ArgbEvaluator();
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
models = new ArrayList<>();
models.add(new Model(R.drawable.brochure, "Brochure", "oooooo"));
models.add(new Model(R.drawable.sticker, "Sticker", "oooooo"));
models.add(new Model(R.drawable.poster, "Poster", "oooooo"));
models.add(new Model(R.drawable.namecard, "Namecard", "oooooo"));
adapter = new Adapter(models, getActivity());
viewPager = fragment_cards.findViewById(R.id.viewPager);
viewPager.setAdapter(adapter);
viewPager.setPadding(130,0,130,0);
Integer[] colors_temp = {
getResources().getColor(R.color.color1),
getResources().getColor(R.color.color2),
getResources().getColor(R.color.color3),
getResources().getColor(R.color.color4)
};
colors = colors_temp;
viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
@Override
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
if (position < (adapter.getCount() -1) && position < (colors.length -1)) {
viewPager.setBackgroundColor(
(Integer) argbEvaluator.evaluate(
positionOffset,
colors[position],
colors[position + 1]
)
);
} else {
viewPager.setBackgroundColor(colors[colors.length - 1]);
}
}
@Override
public void onPageSelected(int position) {
}
@Override
public void onPageScrollStateChanged(int state) {
}
});
return inflater.inflate(R.layout.fragment_cards, container, false);
}
}

Adapter.java

package com.example.musicassistant;

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;
import android.widget.TextView;
import androidx.annotation.NonNull;
import androidx.fragment.app.FragmentActivity;
import androidx.viewpager.widget.PagerAdapter;
import java.util.List;
public class Adapter extends PagerAdapter {
private List<Model> models;
private LayoutInflater layoutInflater;
private Context context;
public Adapter(List<Model> models, FragmentActivity context) {
this.models = models;
this.context = context;
}
@Override
public int getCount() {
return models.size();
}
@Override
public boolean isViewFromObject(@NonNull View view, @NonNull Object object) {
return view.equals(object);
}
@NonNull
@Override
public Object instantiateItem(@NonNull ViewGroup container, int position) {
layoutInflater = LayoutInflater.from(context);
View view = layoutInflater.inflate(R.layout.item, container, false);
ImageView imageView;
TextView title, desc;
imageView = view.findViewById(R.id.image);
title = view.findViewById(R.id.title);
desc = view.findViewById(R.id.desc);
imageView.setImageResource(models.get(position).getImage());
title.setText(models.get(position).getTitle());
desc.setText(models.get(position).getDesc());
container.addView(view, 0);
return view;
}
@Override
public void destroyItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
container.removeView((View)object);
}
}

Model.java

package com.example.musicassistant;
public class Model {
private int image;
private String title;
private  String desc;
public Model(int image, String title, String desc) {
this.image = image;
this.title = title;
this.desc = desc;
}
public int getImage() {
return image;
}
public void setImage(int image) {
this.image = image;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
public String getDesc() {
return desc;
}
public void setDesc(String desc) {
this.desc = desc;
}
}

我对Android Studio编程相对较新，任何帮助将不胜感激，感谢您的时间，

罗马

Answer 1

在CardsFragment#onCreateView中，您正在尝试抛出此异常viewPager = fragment_cards.findViewById(R.id.viewPager);因为fragment_cards在该时间点未启动。

您可以尝试按照CardsFragment#onCreateView执行以下操作以避免此问题：

@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
...
fragment_cards = inflater.inflate(R.layout.fragment_cards, container, false);
viewPager = fragment_cards.findViewById(R.id.viewPager);
...
return fragment_cards; 
}

在上面的代码中，我们正在膨胀布局文件并将根视图的引用存储在fragment_cards允许您使用findViewById查找布局文件中存在的所有视图

Answer 2

View rootView = inflater.inflate(R.layout.fragment_top_rated， container， false); Viewpager=rootview.findviewbyid(R.id.viewpager) 返回根视图;