java - 未显示可扩展 ListView Activity 。我该如何修复。？

Question

我是 android studio 的初学者。我在我的方案 Activity 中添加了可扩展 ListView ，但是当单击主页上的按钮时，它不会导航方案 Activity ，我将强制关闭并移回登录页面。我该如何解决这个问题有人请帮助我吗？

我将在下面添加我的每个 Activity 和代码

1.Scheme.java

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.ExpandableListView;
import android.widget.ExpandableListView.OnChildClickListener;
import android.widget.ExpandableListView.OnGroupClickListener;
import android.widget.ExpandableListView.OnGroupCollapseListener;
import android.widget.ExpandableListView.OnGroupExpandListener;
import android.widget.Toast;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;

public class Scheme extends Activity {

    My_Adapter listAdapter;
    ExpandableListView expListView;
    List<String> listDataHeader;
    HashMap<String, List<String>> listDataChild;

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

        // get the listview
        expListView = (ExpandableListView) findViewById(R.id.expandableListView);

        // preparing list data
        prepareListData();

        listAdapter = new My_Adapter(this, listDataHeader, listDataChild);

        // setting list adapter
        expListView.setAdapter(listAdapter);

        // Listview Group click listener
        expListView.setOnGroupClickListener(new OnGroupClickListener() {

            @Override
            public boolean onGroupClick(ExpandableListView parent, View v,
                                        int groupPosition, long id) {
                // Toast.makeText(getApplicationContext(),
                // "Group Clicked " + listDataHeader.get(groupPosition),
                // Toast.LENGTH_SHORT).show();
                return false;
            }
        });

        // Listview Group expanded listener
        expListView.setOnGroupExpandListener(new OnGroupExpandListener() {

            @Override
            public void onGroupExpand(int groupPosition) {
                Toast.makeText(getApplicationContext(),
                        listDataHeader.get(groupPosition) + " Expanded",
                        Toast.LENGTH_SHORT).show();
            }
        });

        // Listview Group collasped listener
        expListView.setOnGroupCollapseListener(new OnGroupCollapseListener() {

            @Override
            public void onGroupCollapse(int groupPosition) {
                Toast.makeText(getApplicationContext(),
                        listDataHeader.get(groupPosition) + " Collapsed",
                        Toast.LENGTH_SHORT).show();

            }
        });

        // Listview on child click listener
        expListView.setOnChildClickListener(new OnChildClickListener() {

            @Override
            public boolean onChildClick(ExpandableListView parent, View v,
                                        int groupPosition, int childPosition, long id) {
                // TODO Auto-generated method stub
                Toast.makeText(
                        getApplicationContext(),
                        listDataHeader.get(groupPosition)
                                + " : "
                                + listDataChild.get(
                                listDataHeader.get(groupPosition)).get(
                                childPosition), Toast.LENGTH_SHORT)
                        .show();
                return false;
            }
        });
    }

    /*
     * Preparing the list data
     */
    private void prepareListData() {
        listDataHeader = new ArrayList<String>();
        listDataChild = new HashMap<String, List<String>>();

        // Adding child data
        listDataHeader.add("Top 250");
        listDataHeader.add("Now Showing");
        listDataHeader.add("Coming Soon..");

        // Adding child data
        List<String> top250 = new ArrayList<String>();
        top250.add("The Shawshank Redemption");
        top250.add("The Godfather");
        top250.add("The Godfather: Part II");
        top250.add("Pulp Fiction");
        top250.add("The Good, the Bad and the Ugly");
        top250.add("The Dark Knight");
        top250.add("12 Angry Men");

        List<String> nowShowing = new ArrayList<String>();
        nowShowing.add("The Conjuring");
        nowShowing.add("Despicable Me 2");
        nowShowing.add("Turbo");
        nowShowing.add("Grown Ups 2");
        nowShowing.add("Red 2");
        nowShowing.add("The Wolverine");

        List<String> comingSoon = new ArrayList<String>();
        comingSoon.add("2 Guns");
        comingSoon.add("The Smurfs 2");
        comingSoon.add("The Spectacular Now");
        comingSoon.add("The Canyons");
        comingSoon.add("Europa Report");

        listDataChild.put(listDataHeader.get(0), top250); // Header, Child data
        listDataChild.put(listDataHeader.get(1), nowShowing);
        listDataChild.put(listDataHeader.get(2), comingSoon);
    }
}

2. Activity 方案

    <?xml version="1.0" encoding="utf-8"?>
<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:background="@drawable/sce"
    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="com.agte.vivo.Scheme">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Current Schemes"
        android:id="@+id/textView"
        android:textSize="30dp"
        android:layout_below="@+id/textClock"
        android:layout_centerHorizontal="true"
        android:textColor="#040404" />

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

    <ExpandableListView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/expandableListView"
        android:indicatorLeft="?android:attr/expandableListPreferredItemIndicatorLeft"
        android:divider="#A4C739"
        android:dividerHeight="0.5dp"
        android:layout_below="@+id/textView"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="86dp" />

</RelativeLayout>

3.myadapter

    package com.agte.vivo;

import android.content.Context;
import android.graphics.Typeface;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseExpandableListAdapter;
import android.widget.TextView;

import java.util.HashMap;
import java.util.List;

public class My_Adapter extends BaseExpandableListAdapter {

    private Context _context;
    private List<String> _listDataHeader; // header titles
    // child data in format of header title, child title
    private HashMap<String, List<String>> _listDataChild;

    public My_Adapter(Context context, List<String> listDataHeader,
                                 HashMap<String, List<String>> listChildData) {
        this._context = context;
        this._listDataHeader = listDataHeader;
        this._listDataChild = listChildData;
    }

    @Override
    public Object getChild(int groupPosition, int childPosititon) {
        return this._listDataChild.get(this._listDataHeader.get(groupPosition))
                .get(childPosititon);
    }

    @Override
    public long getChildId(int groupPosition, int childPosition) {
        return childPosition;
    }

    @Override
    public View getChildView(int groupPosition, final int childPosition,
                             boolean isLastChild, View convertView, ViewGroup parent) {

        final String childText = (String) getChild(groupPosition, childPosition);

        if (convertView == null) {
            LayoutInflater infalInflater = (LayoutInflater) this._context
                    .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            convertView = infalInflater.inflate(R.layout.child_layout,parent, false);
        }

        TextView txtListChild = (TextView) convertView
                .findViewById(R.id.lblListItem);

        txtListChild.setText(childText);
        return convertView;
    }

    @Override
    public int getChildrenCount(int groupPosition) {
        return this._listDataChild.get(this._listDataHeader.get(groupPosition))
                .size();
    }

    @Override
    public Object getGroup(int groupPosition) {
        return this._listDataHeader.get(groupPosition);
    }

    @Override
    public int getGroupCount() {
        return this._listDataHeader.size();
    }

    @Override
    public long getGroupId(int groupPosition) {
        return groupPosition;
    }

    @Override
    public View getGroupView(int groupPosition, boolean isExpanded,
                             View convertView, ViewGroup parent) {
        String headerTitle = (String) getGroup(groupPosition);
        if (convertView == null) {
            LayoutInflater infalInflater = (LayoutInflater) this._context
                    .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            convertView = infalInflater.inflate(R.layout.parent_layout,parent, false);
        }

        TextView lblListHeader = (TextView) convertView
                .findViewById(R.id.lblListHeader);
        lblListHeader.setTypeface(null, Typeface.BOLD);
        lblListHeader.setText(headerTitle);

        return convertView;
    }

    @Override
    public boolean hasStableIds() {
        return false;
    }

    @Override
    public boolean isChildSelectable(int groupPosition, int childPosition) {
        return true;
    }
}

4.父布局

    <TextView
        android:id="@+id/lblListHeader"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:paddingLeft="?android:attr/expandableListPreferredItemPaddingLeft"
        android:textSize="17dp"
        android:textColor="#f9f93d" />

</LinearLayout>

5.子布局

     <TextView
        android:id="@+id/lblListItem"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:textSize="17dip"
        android:paddingTop="5dp"
        android:paddingBottom="5dp"
        android:paddingLeft="?android:attr/expandableListPreferredChildPaddingLeft" />

</LinearLayout>

6.activity iistview

    <?xml version="1.0" encoding="utf-8"?>
<!--<RelativeLayout
    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="com.agte.vivo.iistview">
</RelativeLayout>-->
    <TextView  xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
        android:text="New Text"
    android:id="@+id/textView4"
    android:layout_alignParentTop="true"
    android:layout_alignParentStart="true"
    android:padding="10dip"
    android:textSize="16dip"
    android:textStyle="bold" >

没有错误显示，但运行监视器时会显示空指针问题

  08-16 16:43:24.211 25818-25818/com.agte.vivo E/AndroidRuntime: FATAL EXCEPTION: main
                                                               Process: com.agte.vivo, PID: 25818
                                                               java.lang.RuntimeException: Unable to start activity ComponentInfo{com.agte.vivo/com.agte.vivo.Scheme}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ExpandableListView.setAdapter(android.widget.ExpandableListAdapter)' on a null object reference

在 android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2342)

在 android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2404)

在 android.app.ActivityThread.access$800(ActivityThread.java:154)

在 android.app.ActivityThread$H.handleMessage(ActivityThread.java:1315)

在 android.os.Handler.dispatchMessage(Handler.java:102)

在 android.os.Looper.loop(Looper.java:135)

在 android.app.ActivityThread.main(ActivityThread.java:5290)

在 java.lang.reflect.Method.invoke( native 方法)

在 java.lang.reflect.Method.invoke(Method.java:372)

在 com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:908)

位于 com.android.internal.os.ZygoteInit.main(ZygoteInit.java:703)

引起:java.lang.NullPointerException:尝试在空对象引用上调用虚拟方法“void android.widget.ExpandableListView.setAdapter(android.widget.ExpandableListAdapter)”

在 com.agte.vivo.Scheme.onCreate(Scheme.java:40)

在 android.app.Activity.performCreate(Activity.java:6039)

在 android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)

在 android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2295)

在 android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2404) 处

在 android.app.ActivityThread.access$800(ActivityThread.java:154) 

位于 android.app.ActivityThread$H.handleMessage(ActivityThread.java:1315)

在 android.os.Handler.dispatchMessage(Handler.java:102)

在 android.os.Looper.loop(Looper.java:135)

位于 android.app.ActivityThread.main(ActivityThread.java:5290)

在 java.lang.reflect.Method.invoke( native 方法)

在 java.lang.reflect.Method.invoke(Method.java:372)

在 com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:908)

位于 com.android.internal.os.ZygoteInit.main(ZygoteInit.java:703)

请帮助我

Answer 1

expListView 似乎有一个空引用。您确定您的 R.layout.activity_iistview Activity 布局中有此 ListView 吗？

所以我认为你应该改变:

setContentView(R.layout.activity_iistview);

至

setContentView(R.layout.activity_scheme);