Question

我正在尝试创建一个具有GridView的应用，并且可以从GridView中选择一个图像并将其显示在图像切换器中。有两个文件。布局文件和MainActivity。

这是MainActivity：

package com.example.grid;

import android.content.Context;
import android.support.v7.app.ActionBar;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.view.ViewGroup;
import android.view.animation.AnimationUtils;
import android.widget.AdapterView;
import android.widget.BaseAdapter;
import android.widget.Button;
import android.widget.GridView;
import android.widget.ImageSwitcher;
import android.widget.ImageView;
import android.widget.Toast;
import android.widget.ViewSwitcher;

public class MainActivity extends AppCompatActivity {
    ImageSwitcher imgSwitcher = (ImageSwitcher) findViewById(R.id.imgSwitcher);

    //---the images to display---
    Integer[] imageIDs = {
            R.mipmap.butterfly,
            R.mipmap.windows,
            R.mipmap.ic_launcher
    };
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        GridView gridView = (GridView) findViewById(R.id.gridview);
        gridView.setAdapter(new ImageAdapter(this));
        gridView.setOnItemClickListener(new AdapterView.OnItemClickListener()
        {
            public void onItemClick(AdapterView parent, View v, int position, long id) {
                imgSwitcher.setImageResource(imageIDs[position]);
                Toast.makeText(getBaseContext(),
                        "pic" + (position + 1) + " selected",
                        Toast.LENGTH_SHORT).show();

            }
        });
    }

    public class ImageAdapter extends BaseAdapter
    {
        private Context context;
        public ImageAdapter(Context c) { context = c; }
        //---returns the number of images---
        public int getCount() { return imageIDs.length;  }
        //---returns the item---
        public Object getItem(int position) { return position; }
        //---returns the ID of an item---
        public long getItemId(int position) { return position; }
        //---returns an ImageView view---
        public View getView(int position, View convertView, ViewGroup parent) {
            ImageView imageView;
            if (convertView == null) {
                imageView = new ImageView(context);
                imageView.setLayoutParams(new GridView.LayoutParams(150, 150));
                imageView.setScaleType(ImageView.ScaleType.CENTER_CROP);
                imageView.setPadding(5, 5, 5, 5);
            } else {
                imageView = (ImageView) convertView;
            }
            imageView.setImageResource(imageIDs[position]);
            return imageView;
        }
    }
}

这是我的布局文件：

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.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:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.grid.MainActivity">
    <GridView
        android:id="@+id/gridview"
        android:layout_width="384dp"
        android:layout_height="511dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="1.0" />

    <ImageSwitcher
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentStart="true"
        android:layout_below="@+id/gridview"
        android:id="@+id/imgSwitcher"
    />

</android.support.constraint.ConstraintLayout>

我遇到了错误：

由于：java.lang.NullPointerException：尝试调用虚拟方法'android.view.Window $ Callback 空对象引用上的android.view.Window.getCallback（）'

当我未声明ImageSwitcher imgSwitcher并将其保留为空时，从GridView中选择图像时会出现空指针异常。

由于ImageSwitcher findviewbyID，应用无法启动

0 个答案: