布局中的Android视图

Question

Ok so I'm running into problems creating an app (force closing) and I think it has to do with the way I implemented the layout. So a few questions: First, I have a relative layout that includes a text input with a button next to it, a list view (still within the relative layout) and another button below that. This is my main xml file:

 <?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:id="@+id/RelativeLayout1"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="horizontal" >

<EditText
    android:id="@+id/edit_choice"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignBaseline="@+id/Button1"
    android:layout_alignParentLeft="true"
    android:layout_alignParentTop="true"
    android:layout_toLeftOf="@+id/Button1"
    android:hint="@string/edit_choice" />

<Button
    android:id="@+id/Button1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentRight="true"
    android:layout_alignParentTop="true"
    android:onClick="addString"
    android:text="@string/button_add" />

<Button
    android:id="@+id/Button2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    android:layout_centerHorizontal="true"
    android:layout_marginBottom="22dp"
    android:text="@string/button_random" />


<ListView
    android:id="@+id/listView1"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_above="@+id/Button2"
    android:layout_alignParentLeft="true"
    android:layout_below="@+id/Button1"
    tools:listitem="@android:layout/simple_list_item_1" >
</ListView>

</RelativeLayout>

Before I even changed anything in the .java files when I tried running this, only the text input and the 2 buttons appeared and the theme changed from Holo to Holo light. So I'm wondering if this works, I've only seen examples where the list view matches the parent layout completely.

My second question is how do I handle using the input to add values to the list view, can I do that in the main activity class or can I have another class to handle the list view and still reference the main layout.

This is my Main class:

public class MainActivity extends Activity 
{

public ArrayList <String> choices = new ArrayList <String>();
public ListView listView = (ListView) findViewById(R.id.listView1);
public String [] choicesArray;


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

    ListView listView = (ListView) findViewById(R.id.listView1);
    choicesArray = new String [] {"You have not entered a choice yet"};
    ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
            android.R.layout.simple_list_item_1, android.R.id.text1, choicesArray);
    listView.setAdapter(adapter);

}

@Override
public boolean onCreateOptionsMenu(Menu menu) 
{
    getMenuInflater().inflate(R.menu.activity_main, menu);
    return true;
}

//adds the string to the list
public void addString(View view) 
{

    choicesArray = (String[]) choices.toArray();
    EditText editText = (EditText) findViewById(R.id.edit_choice);
    String message = editText.getText().toString();
    choices.add(message);

}


}

Hopefully this makes sense and thank you for any help.

Answer 1

1. Define your ArrayAdapter as member for your class (not as variable in onCreate()).
2. Set the adapter to your listView in onCreate().
3. Set the onClickListener for your button.
4. Add the text from EditText to your adapter when you click on the button.
5. Profit!

Answer 2

Problem is you are calling this line

public ListView listView = (ListView) findViewById(R.id.listView1);

before setting content view by setContentView() method in onCreate() so you are getting NullPointerException. Do not forget that before calling findViewById you have to set content view. So delete above line because you are creating function scope listView in onCreate method and your NullPointerException problem will be solved. And also change first line of addString method like this

choicesArray = choices.toArray(choicesArray);