如何从Java文件更改XML按钮属性？

Question

I am attempting to change the text displayed on a button from my java file, and am not having much luck. After searching for awhile on various forums, and doing google searches, I have learned to create a new button and then use button.setText. However, after my application would suddenly end, displaying a message about being forced to quit, I decided to do some debugging. I found the problem to be in this Java class.

package com.KenanDeHart.thebasics;

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



public class Showrole extends Activity {
    public static int players;
    public static int idx;
    public static int[] playArray;
    public static Button nextplayerButton =null;

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

            Bundle extras = getIntent().getExtras();
            Showrole.players = extras.getInt("PLAYERS");


            for (idx = 0; idx < players ; ++idx){

                display(idx);
                ++idx;
            }


        }








     public void display(int idx){
        Bundle extras = getIntent().getExtras();
        playArray = extras.getIntArray("PLAYARRAY");
        nextplayerButton = (Button) findViewById(R.id.nextplayer);
        nextplayerButton.setOnClickListener(new OnClickListener() {

                    @Override
                    public void onClick(View v) {
        nextplayerButton.setText(playArray[Showrole.idx]);
        }
        });
    }

    @Override
    protected void onPause() {
        super.onPause();
    }




    public void next(){

        Bundle extras = getIntent().getExtras();
        Button  button = (Button)findViewById(R.id.nextplayer);
        button.setText("Next Player");
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }



    }
}

After using breakpoints, I think I have located the problematic source code.

 public void display(int idx){
    Bundle extras = getIntent().getExtras();
    playArray = extras.getIntArray("PLAYARRAY");
    nextplayerButton = (Button) findViewById(R.id.nextplayer);
    nextplayerButton.setOnClickListener(new OnClickListener() {

                @Override
                public void onClick(View v) {
    nextplayerButton.setText(playArray[Showrole.idx]);
    }
    });
}

More specifically, this line

nextplayerButton.setText(playArray[Showrole.idx]);

When I press continue, a string of things happen.

1. A file opens called

   ZygoteInit$MethodAndArgsCaller.run()

2. My phone vibrates once
3. About 3 seconds pass
4. My phone vibrates three times quickly
5. The program closes

Here is my logcat log

11-30 12:47:28.770: E/AndroidRuntime(9770):     at com.KenanDeHart.thebasics.Showrole$1.onClick(Showrole.java:51)

Pay attention to this error

 11-30 12:47:28.770: E/AndroidRuntime(9770): at com.KenanDeHart.thebasics.Showrole$1.onClick(Showrole.java:51) 

I really am unsure what is happening here. This is my first time using Java, and I have only been using it for two days. I hope this question is explanatory enough, and I assure you I have done my research but to no avail. I really appreciate any help someone can give me. Thank you.

Answer 1

If its just changing the text displayed on button.

add this as instance variable (outside all the methods)

Button nextplayerButton =null;

get reference of the button in onCreate() method.

nextplayerButton = (Button) findViewById(R.id.nextplayer);
nextplayerButton.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
nextplayerButton.setText(""+i);
}
});

Answer 2

According to the LogCat, you have a NullpointerException in your display() method at line 42.

Caused by: java.lang.NullPointerException 11-30 06:44:37.147: E/AndroidRuntime(6601): at com.KenanDeHart.thebasics.Showrole.display(Showrole.java:42)

Just check if there's something that's not initialized there.

Answer 3

Normally, this should work:

setContentView(R.layout.activity_mail_sender);
        int i = 2;
        btn = (Button) findViewById(R.id.autoSendBtn);
        btn.setText(""+i);

You don't have to create button in java code if its already there in xml file. There must be some other issue. Please post your logcat error

Answer 4

Alright, I figured out what was happening.

The array "playArray" was not correctly being passed from my other intent. I looked into the variable tree and found it was actually equal to null. I solved this by sending the size of the area (which was a variable determined in the previous class) with the intent, and creating the array after arrival.

Thank you everyone for your help. I am impressed with the support and helpfulness found on Stack Overflow.