Question

我一直在开展Android活动，其中包含一个ViewPager，上面有10张图片，下面是一个Image View，点击后会显示ViewPager的图片。

当我通过登录屏幕进入后尝试运行时，交易崩溃了。

我不确定LogCat警告的错误在哪里，所以如果你能帮助我，我会非常感激。

以下是LogCat消息：

FATAL EXCEPTION: main
java.lang.NullPointerException
at com.example.logger.SecondActivity$MyPagerAdapter.setPrimaryItem (SecondActivity.java:80)
at android.support.v4.view.ViewPager.populate (ViewPager.java:1066)
at android.support.v4.view.ViewPager.populate (ViewPager.java:914)
at android.support.v4.view.ViewPager.onMeasure (ViewPager.java:1436)
at android.view.View.measure (View.java:15848)
at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:728)
at android.widget.RelativeLayout.onMeasure (RelativeLayout.java:477)
at android.view.View.measure (View.java:15848)
at android.view.ViewGroup.measureChildWithMargins (ViewGroup.java:5012)
at com.android.internal.widget.ActionBarOverLayout.onMeasure(ActionBarOverLayout.java:302)
at android.view.View.measure (View.java:15848)
at android.view.ViewGroup.measureChildWithMargins (ViewGroup.java:5012)
at android.widget.FrameLayout.onMeasure (FrameLayout.java:310)
at android.view.View.measure (View.java:15848)

SecondActivity.java

package com.example.logger;

import android.os.Bundle;
import android.annotation.SuppressLint;
import android.app.Activity;
import android.app.Fragment;
import android.app.FragmentManager;
import android.content.DialogInterface;
import android.content.DialogInterface.OnClickListener;
import android.support.v4.app.FragmentStatePagerAdapter;
import android.support.v4.view.PagerAdapter;
import android.support.v4.view.ViewPager;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.View;
import android.widget.ImageView;
import android.view.ViewGroup;

public class SecondActivity extends Activity {

    private static final int NUM_PAGES = 10;

    private ViewPager mpager;

    private MyPagerAdapter mPagerAdapter;
    private int [] pics = {R.drawable.android_interfaz_layout_estructura_final, R.drawable.baixa, R.drawable.baixada, R.drawable.greenprogressbar,
            R.drawable.layout_keyboard, R.drawable.linerlay, R.drawable.merge1, R.drawable.o2zds, R.drawable.regbd,
            R.drawable.s7qrs};

    @SuppressLint("NewApi")
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_second);
        mPagerAdapter = new MyPagerAdapter(null);
        mpager = (ViewPager) findViewById(R.id.viewer);
        mpager.setAdapter (mPagerAdapter);
    }

    public void showImage (int position, View view)
    {
        //View is your view that you returned from instantiateItem 
        //and position is it's position in array you can get the image resource id 
        //using this position and set it to the ImageView
    }

    private class MyPagerAdapter extends PagerAdapter
    {

        SecondActivity activity;
        public MyPagerAdapter (SecondActivity activity){
            this.activity=activity;
        }


        @Override
        public int getCount() {
            // TODO Auto-generated method stub
            return pics.length;
        }

        @Override
        public boolean isViewFromObject(View arg0, Object arg1) {
            // TODO Auto-generated method stub
            return false;
        }

        @Override
        public Object instantiateItem(View collection, int position) {
            ImageView view = new ImageView (SecondActivity.this);
            view.setImageResource (pics[position]);
            ((ViewPager) collection).addView(view, 0);
            return view;
        }

        @Override
        public void setPrimaryItem (ViewGroup container, int position, Object object)
        {
            super.setPrimaryItem (container, position, object);
            activity.showImage(position,(View)object);
        }

        }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.second, menu);
        return true;
    }


    private OnClickListener mPageClickListener = new OnClickListener()
    {
     public void onClick (View v)
     {
        // TODO Auto-generated method stub
         //aquí anirà la funció per traslladar la image de la gallery a la pantalla
         Integer picId = (Integer) v.getTag();
         mpager.setVisibility(View.VISIBLE);
         mpager.setCurrentItem(v.getId());

     }

    @Override
    public void onClick(DialogInterface dialog, int which) {
        // TODO Auto-generated method stub

    }

        };
}

结束SecondActivity.java

Activity_Second.xml

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


    <android.support.v4.view.ViewPager
        android:padding="16dp"
        android:id="@+id/viewer"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
         android:layout_marginTop="10dp"
         android:layout_weight="0.30"
        />

    <ImageView 

        android:id="@+id/big_image"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="10dp"
        android:layout_below="@+id/viewer"
         android:layout_weight="0.70"

        />

</RelativeLayout>

结束Activity_Second.xml

以防万一我也会添加MainActivity。

Main_Activity.java

package com.example.logger;

import android.os.Bundle;
import android.app.Activity;
import android.content.Intent;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;

public class MainActivity extends Activity implements OnClickListener {

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

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }


    private void attachHandlers() {
        findViewById(R.id.login).setOnClickListener(this);
    }

    @Override
    public void onClick(View arg0) {
        if(arg0.getId() == R.id.login)
        {
            Intent i = new Intent(this, SecondActivity.class);
            startActivity(i);    
        }

    }

}

结束MainActivity.java

activity_main.xml中

<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:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:orientation="vertical"
    tools:context=".MainActivity" >

        <Button
        android:id="@+id/login"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="@string/Login"
        android:layout_alignParentBottom="true"  
        android:background="@drawable/shapes"
        android:textColor="@color/text"
        />

    <EditText 
        android:id="@+id/username"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="@string/input_u"
        android:layout_marginTop="10dp"
        android:textColor="@color/text"
        />

    <EditText 
        android:id="@+id/password"
        android:layout_below="@+id/username"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="@string/input_p"
        android:layout_marginTop="10dp"
        android:textColor="@color/text"
        />

</RelativeLayout>

结束Activity_Main.xml

如果能够对如何解决这些问题有所了解，我将非常感激。

您诚挚的，

莫罗。

Answer 1

更改此

 mPagerAdapter = new MyPagerAdapter(null)

到

 mPagerAdapter = new MyPagerAdapter(SecondActivity.this)

构造函数看起来像

 public MyPagerAdapter (SecondActivity activity){
            this.activity=activity; //activity is null
        }

SecondActivity.java中的第80行很可能是activity.showImage(position,(View)object);而activity是空的。

更改Pageradapter代码

  class MyPagerAdapter extends PagerAdapter{

        MyPagerAdapter (Context context) {


         }

        @Override
        public void destroyItem(View container, int position, Object object) {
            ((ViewPager) container).removeView((View) object);
        }
        @Override
        public int getCount() {
            return pics.length;
        }

        @Override
        public Object instantiateItem(View collection, int position) {
            ImageView iv =  new ImageView(SecondActivity.this);
            ((ViewPager) collection).addView(iv, 0);
            iv.setImageResource(pics[position]);
          return iv;

        }

        @Override
        public boolean isViewFromObject(View view, Object object) {
            return view.equals(object);
        }

        }

Answer 2

现在正在运作

public class SecondActivity extends Activity {

private static final int NUM_PAGES = 10;

private ViewPager mpager;

private MyPagerAdapter mPagerAdapter;
private int[] pics = { R.drawable.flower1, R.drawable.flower10,
        R.drawable.flower11, R.drawable.flower13, R.drawable.flower2,
        R.drawable.flower6, R.drawable.flower8, R.drawable.flower7,
        R.drawable.image3, R.drawable.flower3 };

@SuppressLint("NewApi")
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_second);
    mPagerAdapter = new MyPagerAdapter(this,pics);
    mpager = (ViewPager) findViewById(R.id.viewer);
    mpager.setAdapter(mPagerAdapter);
}

public void showImage(int position, View view) {
    // View is your view that you returned from instantiateItem
    // and position is it's position in array you can get the image resource
    // id
    // using this position and set it to the ImageView
}

private class MyPagerAdapter extends PagerAdapter {

    Activity activity;
    int imageArray[];

    public MyPagerAdapter(SecondActivity act, int[] imgArra) {
        imageArray = imgArra;
        activity = act;
    }

    public int getCount() {
        return imageArray.length;
    }

    public Object instantiateItem(View collection, int position) {
         ImageView view = new ImageView(activity);
         System.out.println(position);
          view.setLayoutParams(new LayoutParams(LayoutParams.FILL_PARENT,
            LayoutParams.FILL_PARENT));
          view.setScaleType(ScaleType.FIT_XY);
          view.setBackgroundResource(imageArray[position]);
          ((ViewPager) collection).addView(view, 0);
        return view;
    }

    @Override
    public void destroyItem(View arg0, int arg1, Object arg2) {
        ((ViewPager) arg0).removeView((View) arg2);
    }

    @Override
    public boolean isViewFromObject(View arg0, Object arg1) {
        return arg0 == ((View) arg1);
    }

    @Override
    public Parcelable saveState() {
        return null;
    }

}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    // getMenuInflater().inflate(R.menu.second, menu);
    return true;
}

private OnClickListener mPageClickListener = new OnClickListener() {
    public void onClick(View v) {
        // TODO Auto-generated method stub
        // aquí anirà la funció per traslladar la image de la gallery a la
        // pantalla
        Integer picId = (Integer) v.getTag();
        mpager.setVisibility(View.VISIBLE);
        mpager.setCurrentItem(v.getId());

    }

    @Override
    public void onClick(DialogInterface dialog, int which) {
        // TODO Auto-generated method stub

    }

};

}

Answer 3

由于未在MyPagerAdapter类

中获取活动的上下文，您将收到nullpointer错误

更改适配器的SecondActivity初始化，如下所示：“

而不是这个

 mPagerAdapter = new MyPagerAdapter(null);

这样做

mPagerAdapter = new MyPagerAdapter(SecondActivity.this)

Android viewpager活动问题

3 个答案: