Question

我正在创建一个可以查看多个列表的应用，希望能够搜索列表。我是java的初学者，我确信我的代码中有一些错误，但我希望能够测试它，看看我是否可以创建应用程序。如果我发现它太难创建我将放弃该项目。我问，因为如果这是一个简单的解决方案，我很接近，那么我希望看到它。谢谢您的帮助。

主要活动

package com.rocklandrecycles.rcsw.rcswwheredoirecycle;

import android.content.Intent;
import android.media.Image;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ImageButton;
import android.widget.ListAdapter;
import android.widget.ListView;
import android.widget.Toast;

import java.util.ArrayList;


public class MainActivity extends ActionBarActivity {

private ImageButton imagebutton1, imagebutton2, imagebutton3, imagebutton4, imagebutton5, imagebutton6;
private EditText SearchBox;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    imagebutton1 = (ImageButton)findViewById(R.id.metal_button);
    imagebutton2 = (ImageButton) findViewById(R.id.imageButton2);
    imagebutton3 = (ImageButton) findViewById(R.id.imageButton3);
    imagebutton4 = (ImageButton) findViewById(R.id.imageButton4);
    imagebutton5 = (ImageButton) findViewById(R.id.imageButton5);
    imagebutton6 = (ImageButton) findViewById(R.id.imageButton6);
    SearchBox = (EditText) findViewById(R.id.search_box);
String[] MetalList= {"metal", "metal cans", "aluminum foil", "copper", "steel", "scrap metal",
                        "hangers", "aluminum can", "knife", "exercise equipment"};

    ListAdapter theAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1,
            MetalList);

    ListView theListView = (ListView) findViewById(R.id.metal_list_view);

    theListView.setAdapter(theAdapter);
    theListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
ListView theListView = (ListView) findViewById(R.id.metal_list_view);

        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {

            String MetalPicked = "Please put in Green Bin";
            Toast.makeText(MainActivity.this, MetalPicked, Toast.LENGTH_LONG).show();
        }
    });

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

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}
public void onMetalButtonClick(View view) {

    Intent getmetalscreenIntent= new Intent(this,
            MetalScreen.class);

    final int result = 1;
    getmetalscreenIntent.putExtra("callingActivity", "Main Activity");
    getmetalscreenIntent.putExtra("MetalActivity" , "MetalList");
    startActivityForResult(getmetalscreenIntent, result);


}

}

清单

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name=".MainActivity"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>

    <activity android:name=".MetalScreen"
        android:label="MetalScreen"
        android:theme="@style/AppTheme"/>


</application>

金属屏幕

package com.rocklandrecycles.rcsw.rcswwheredoirecycle;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.ListView;


public class MetalScreen extends Activity{
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

setContentView(R.layout.metal_layout);

    Intent activityThatCalled = getIntent();

    String previousActivity =      activityThatCalled.getExtras().getString("callingActivity");
    String previousActivity2 = activityThatCalled.getExtras().getString("MetalActivity");

    ListView callingActivityList = (ListView) findViewById(R.id.metal_list_view);


}

}

Activity_Main

<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="com.example.rcswwheredoirecyclethisproject.MainActivity"
android:gravity="top"
android:background="@drawable/recyclesymbol">


<LinearLayout
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="horizontal"
    android:layout_below="@+id/LinearLayout2">

    <ImageButton
    android:id="@+id/metal_button"
    android:layout_width="50dp"
    android:layout_height="50dp"
    android:contentDescription="@string/content"
    android:text="@string/ImageButton1text"
        android:onClick="onMetalButtonClick"
        android:background="@drawable/metal_texture"
        android:maxHeight="10dp"
        android:maxWidth="10dp"
        android:cropToPadding="true"
        android:clickable="true"/>


    <ImageButton
    android:id="@+id/imageButton2"
    android:layout_width="50dp"
    android:layout_height="50dp"
    android:contentDescription="@string/content"
    android:text="@string/ImageButton2text"
        android:background="@drawable/biohazard"
        android:clickable="true"/>

  <ImageButton
    android:id="@+id/imageButton3"
    android:layout_width="50dp"
    android:layout_height="50dp"
    android:contentDescription="@string/content"
    android:text="@string/ImageButton3text"
      android:clickable="true"/>

  <ImageButton
    android:id="@+id/imageButton4"
    android:layout_width="50dp"
    android:layout_height="50dp"
    android:contentDescription="@string/content"
    android:text="@string/ImageButton4text"/>

  <ImageButton
    android:id="@+id/imageButton5"
    android:layout_width="50dp"
    android:layout_height="50dp"
    android:contentDescription="@string/content"
    android:text="@string/ImageButton5text"
      android:clickable="true"/>

    <ImageButton
    android:id="@+id/imageButton6"
    android:layout_width="50dp"
    android:layout_height="50dp"
    android:contentDescription="@string/content"
    android:text="@string/ImageButton6text"
        android:clickable="true"/>
</LinearLayout>

<LinearLayout
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="horizontal"
    android:weightSum="5"
    android:id="@+id/LinearLayout2"
    android:layout_below="@+id/LinearLayout3">

    <EditText
        android:layout_width="200dp"
        android:layout_height="150dp"
        android:inputType="text"
        android:hint="@string/EditText"
        android:id="@+id/search_box"/>


</LinearLayout>

<LinearLayout
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="vertical"
android:id="@+id/LinearLayout3"
android:layout_centerHorizontal="true">

 <TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="RCSW Recycle"
     android:textSize="25sp"/>

 <TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Where Do I Recycle This?"
     android:textSize="25sp"/>
</LinearLayout>
<requestFocus />
</RelativeLayout>

金属布局

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout  xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent" android:layout_height="match_parent">

<ListView
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:layout_gravity="left"
    android:id="@+id/metal_list_view"
    android:padding="10dp">

</ListView>

</LinearLayout>

钉刺

<resources>
<string name="app_name">RCSW Where Do I Recycle</string>
<string name="content">Image</string>
<string name="menu_settings">Settings</string>
<string name="ImageButton1text">Metal</string>
<string name="ImageButton2text">Hazard</string>
<string name="ImageButton3text">Paper</string>
<string name="ImageButton4text">Glass</string>
<string name="ImageButton5text">Plastic</string>
<string name="ImageButton6text">Other</string>
<string name="EditText">Search Box</string>
<string name="action_settings">RCSWRecycle</string>
</resources>

Answer 1

您的ActivityMain xml布局文件中没有任何Listview，但是，您的onCreate方法正在调用它并设置适配器！您可能在theListView上获得了一个nullpointer？

你打电话两次：

ListView theListView = (ListView) findViewById(R.id.metal_list_view);

返回null，因为android无法在activity_main.xml上找到该视图。

编辑：启动主活动时，您只能使用activity_layout上的元素。由于ListView在金属版面上，而不在activity_layout上，因此无法对其进行引用。

您需要做的是找到关于您的金属活动的视图。像这样。您的创建MainActivity的方法：

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

    imagebutton1 = (ImageButton)findViewById(R.id.metal_button);
    imagebutton2 = (ImageButton) findViewById(R.id.imageButton2);
    imagebutton3 = (ImageButton) findViewById(R.id.imageButton3);
    imagebutton4 = (ImageButton) findViewById(R.id.imageButton4);
    imagebutton5 = (ImageButton) findViewById(R.id.imageButton5);
    imagebutton6 = (ImageButton) findViewById(R.id.imageButton6);
    SearchBox = (EditText) findViewById(R.id.search_box);



}

然后在你的Metal Activity上你应该有这样的东西:(在这个布局中，你找到listview，创建和设置适配器）

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

    setContentView(R.layout.metal_layout);

        Intent activityThatCalled = getIntent();

        String previousActivity =      activityThatCalled.getExtras().getString("callingActivity");
        String previousActivity2 = activityThatCalled.getExtras().getString("MetalActivity");

        ListAdapter theAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1,
            MetalList);

String[] MetalList= {"metal", "metal cans", "aluminum foil", "copper", "steel", "scrap metal",
                            "hangers", "aluminum can", "knife", "exercise equipment"};

    ListView theListView = (ListView) findViewById(R.id.metal_list_view);

    theListView.setAdapter(theAdapter);
    theListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {

        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {

            String MetalPicked = "Please put in Green Bin";
            Toast.makeText(MainActivity.this, MetalPicked, Toast.LENGTH_LONG).show();
        }
    });


    }

Answer 2

Rui Santos是对的。您只能访问活动setContentView()方法中定义的文件中的xml元素。在这种情况下，在您的主要活动中，您声明setContentView(R.layout.activity_main)。

稍后您致电findViewById(R.id.metal_list_view)，但身份metal_list_view位于metal_layout。 findViewById次搜索activity_main，找不到metal_list_view，然后返回null。当您尝试将该null转换为ListView时，它将抛出NullPointerException。

如果你收到NPE以外的其他错误，你应该告诉我们，我们会帮助解决这个问题，但这个问题需要解决。

继续开展两项活动：

你应该可以通过移动

让它运行

String[] MetalList= {"metal", "metal cans", "aluminum foil", "copper", "steel", "scrap metal",
                    "hangers", "aluminum can", "knife", "exercise equipment"};

ListAdapter theAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1,
        MetalList);

ListView theListView = (ListView) findViewById(R.id.metal_list_view);

theListView.setAdapter(theAdapter);
theListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
    ListView theListView = (ListView) findViewById(R.id.metal_list_view);

    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {

        String MetalPicked = "Please put in Green Bin";
        Toast.makeText(MetalActivity.this, MetalPicked, Toast.LENGTH_LONG).show(); // edited to remove outdated reference to MainActivity
    }
});

从您的主要活动onCreate()到您的金属活动onCreate()

编辑：

有点不幸的是，Android中的活动和片段与其xml布局紧密耦合。每个活动或片段仅包含对单个布局的引用，但包含框架元素的布局可以包含其他布局。在这种情况下，您的金属活动会与您的布局相关联，该布局包含＆＃39; listview＆＃39;您正在尝试修改，因此需要在金属活动中进行修改。我并没有真正弄清楚你的意图是什么，但你可能想要的是一个带有两个片段的单一活动。然后每个片段都有一个布局，活动包含对两个片段的引用，并且通过使用一些回调接口，您可以在三者之间进行通信，并托管您活动中的所有逻辑。

替代地

仔细查看您正在做的事情，您可能需要在主活动布局中添加framelayout，将您的金属活动转换为片段，然后将片段附加到framelayout。这将为您提供一种活动内活动感，可以提高可重用性。如果不知道你的目的是什么，很难说。

Android应用程序不会运行，不断崩溃

2 个答案:

继续开展两项活动：

编辑：

替代地