.setText（）使我的Android应用程序崩溃

Question

I have tried many other questions on this site today but none of them have been helpful, i seem to have identical syntax that works for other people but every time i hit send (when i want my text fields to update) my app just crashes, I have tried a bunch of different syntax for the setText() method but nothing has worked.

Thanks for any help in advance

this is my main class

package com.example.myapp;

import android.app.Activity;
import java.io.IOException;
import java.util.ArrayList;

import android.app.Activity;
import android.content.DialogInterface;
import android.content.DialogInterface.OnClickListener;
import android.os.Bundle;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

import org.apache.http.HttpResponse;
import org.apache.http.HttpStatus;
import org.apache.http.client.*;
import org.apache.http.client.methods.*;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.util.EntityUtils;

public class TestprojectsActivity extends Activity {

    private int index;
    private int QsClicked = 0;
    private ArrayList<String> categories = new ArrayList<String>();
    private ArrayList<Button> buttons = new ArrayList<Button>();
    Button b1;
    Button b2;
    Button b3;
    Button b4;
    Button changer;
    private int Qselected;
    TextView txt;
    EditText edit;
    private ArrayList<String> questions = new ArrayList<String>();
    private int qnum = 0;

    // updates all the buttons so we can see more categories
    public void updateButtons(View v){
        for(int i = 0; i < 4; i++){
              if(index >= categories.size()){
                  index = 0;
              }
              buttons.get(i).setText(categories.get(index));
              index++;
          }
    }

    public void Q1(View v){
        setContentView(R.layout.sub);
        Qselected = index;
        //send(null);
    }

    public void Q2(View v){
        setContentView(R.layout.sub);
        Qselected = (index+1);
        //send(null);
    }

    public void Q3(View v){
        setContentView(R.layout.sub);
        Qselected = (index+2);
        //send(null);
    }

    public void Q4(View v){
        setContentView(R.layout.sub);
        Qselected = (index+3);
        //send(null);
    }

    public void send(View v){
        edit.setText("WHY ISNT THIS WORKING?");
        if(qnum < questions.size()){
            txt.setText("i work!");
            qnum++;
        }
        else{
            qnum = 0;
            System.exit(0);
        }
    }


/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    index = 0;
    b1 = (Button) findViewById(R.id.B1);
    buttons.add(b1);
    b2 = (Button) findViewById(R.id.B2);
    buttons.add(b2);
    b3 = (Button) findViewById(R.id.B3);
    buttons.add(b3);
    b4 = (Button) findViewById(R.id.B4);
    buttons.add(b4);
    txt = (TextView) findViewById(R.id.textView1);
    edit = (EditText) findViewById(R.id.editText2);
    categories.add("dumb questions");
    categories.add("smart questions");
    questions.add("hi there, what is your name?");
    questions.add("what did you have for breakfast today?");
    questions.add("how old are you?");
    questions.add("what color is your hair?");
    updateButtons(null);
}
}

this is my main xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/layoutrowtop"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >
<Button
    android:id="@+id/B4"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="B4"
    android:onClick="Q4" />
<Button
    android:id="@+id/B3"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="B3"
    android:onClick="Q3" />
<Button
    android:id="@+id/B2"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="B2"
    android:onClick="Q2" />
<Button
    android:id="@+id/B1"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="B1"
    android:onClick="Q1" />
<Button
    android:id="@+id/Button05"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:text="More Categories"
    android:onClick="updateButtons" />

</LinearLayout>

this is my sub xml (i use it so i can have another layout)

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




    <TextView
        android:id="@+id/textView1"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_weight="0.22"
        android:text="Question"
         />


    <EditText
        android:id="@+id/editText2"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_weight="0.51"
        android:ems="10"
        android:inputType="text" />


    <Button
        android:id="@+id/send"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_weight="0.08"
        android:text="Send" 
        android:onClick="send" />

    </LinearLayout>

Answer 1

In the activity's onCreate method you set the content view to R.layout.main and you search for the TextView that you want to set the text. Because the TextView is not in that layout(is in the R.layout.sub and not in the R.layout.main ) it will be null and you'll get a NullPointerException when you set the text, although you set the content view as the new layout that contains that TextView (you already search for it in the onCreate method so the reference is initialized with null).

The solution is to search for the TextView after you set the new content view( R.layout.sub ), for example:

public void send(View v){
        txt = (TextView) findViewById(R.id.textView1);
        edit = (EditText) findViewById(R.id.editText2);
        edit.setText("WHY ISNT THIS WORKING?");
        if(qnum < questions.size()){
            txt.setText("i work!");
            qnum++;
        }
        else{
            qnum = 0;
            System.exit(0);
        }
    }

Ans the same thing goes for that Edittext .

Answer 2

Your activity is using main.xml as its layout, and that does not have a TextView with id editText2 . Just putting such a view in sub.xml doesn't create the view; the layout file needs to be inflated and set as the content view for the activity. Call findViewById after you change layouts and all should be well.

Answer 3

AS in when Oncreate get call main.xml was you layout xml and it has not R.id.editText2 so at that time edit would be null..........

Put this line

edit = (EditText) findViewById(R.id.editText2);

in

 public void Q1(View v){
        setContentView(R.layout.sub);
        Qselected = index;
 edit = (EditText) findViewById(R.id.editText2); <-----------------here in Q1 Q2 Q3 Q4
        //send(null);
    }