在尝试对空对象引用调用虚拟方法'android.view.View android.view.View.findViewById（int）'时发生致命异常'

问题描述 投票：0回答：2

我一直在遵循使用此处显示的视图分页器添加类似卡片的滑动视图的教程：

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

我试图使其与片段一起使用，但是我遇到了此错误：

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编程还是比较陌生，非常感谢您的帮助，谢谢您的时间，

罗马

java android android-fragments findviewbyid
2个回答
0
投票

在CardsFragment#onCreateView中，您正在尝试viewPager = fragment_cards.findViewById(R.id.viewPager);，因为在该时间点未启动fragment_cards，因此引发了此异常。

您可以尝试在CardsFragment#onCreateView中进行以下操作以避免出现此问题：

    @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; 
    }
0
投票

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

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