alertDialog.getButton() method gives null pointer exception android

Iam planing to give create 3 buttons with layout_weight=1, not interested in custom dialog.So I have written below code.It is not working.Always yes button gives me null. Whats wrong in this code?

  AlertDialog dialog= new AlertDialog.Builder(this).create();
    		dialog.setIcon(R.drawable.alert_icon);
    		dialog.setTitle("title");
    		dialog.setMessage("Message");
    		dialog.setButton(AlertDialog.BUTTON_POSITIVE,"Yes", new DialogInterface.OnClickListener() {
    
    			@Override
    			public void onClick(DialogInterface arg0, int arg1) {
    											}
    		});
    		Button yesButton = dialog.getButton(AlertDialog.BUTTON_POSITIVE);
    		Log.w("Button",""+yesButton);//here getting null
    		LinearLayout.LayoutParams layoutParams = new LinearLayout.LayoutParams(LayoutParams.FILL_PARENT, LayoutParams.WRAP_CONTENT, 1f);
    		yesButton.setLayoutParams(layoutParams);
    		dialog.show();

Regards, Android developer.

Take a look here for the answer : http://code.google.com/p/android/issues/detail?id=6360

As it says in comment #4 you must call show() on your dialog before you can access the buttons, they are not available beforehand. For an automatic solution on how to modify the buttons as soon as they are ready see Mickeys answer

This works for me :

AlertDialog alertDialog = new AlertDialog.Builder(this)
				.setMessage(message)
				.setCancelable(true)
				.setPositiveButton("Yes",
						new DialogInterface.OnClickListener() {
							public void onClick(DialogInterface dialog, int id) {
							//do smthng
						})
				.setNegativeButton("No", new DialogInterface.OnClickListener() {
					public void onClick(DialogInterface dialog, int id) {
						//do snthn
					}
				}).create();
		
		alertDialog.setOnShowListener(new OnShowListener() {
			@Override
			public void onShow(DialogInterface dialog) {					//
				Button positiveButton = ((AlertDialog) dialog)
						.getButton(AlertDialog.BUTTON_POSITIVE);
				positiveButton.setBackgroundDrawable(getResources()
						.getDrawable(R.drawable.btn_default_holo_dark));
				
				Button negativeButton = ((AlertDialog) dialog)
						.getButton(AlertDialog.BUTTON_NEGATIVE);
				positiveButton.setBackgroundDrawable(getResources()
						.getDrawable(R.drawable.btn_default_holo_dark));
			}
		});
		
		alertDialog.show(); 

only in this order, call alertDialog.setOnShowListener after create()

Thanks wieux. But for new developers understanding purpose I am re-writing code below

AlertDialog dialog= new AlertDialog.Builder(this).create();             
dialog.setIcon(R.drawable.alert_icon);             
dialog.setTitle("title");            
dialog.setMessage("Message");             
dialog.setButton(AlertDialog.BUTTON_POSITIVE,"Yes", new DialogInterface.OnClickListener() {                 
    @Override                 
    public void onClick(DialogInterface arg0, int arg1) {                                                
    }             
}); 
dialog.show(); 
Button yesButton = dialog.getButton(AlertDialog.BUTTON_POSITIVE);             
Log.w("Button",""+yesButton); //here getting null             
LinearLayout.LayoutParams layoutParams = new LinearLayout.LayoutParams(LayoutParams.FILL_PARENT, LayoutParams.WRAP_CONTENT, 1f);             
yesButton.setLayoutParams(layoutParams);        

Here is useful Kotlin extensions functions to achieve what you want with an automatic solution :

import android.content.DialogInterface.BUTTON_NEGATIVE
import android.content.DialogInterface.BUTTON_POSITIVE
import androidx.appcompat.app.AlertDialog

fun AlertDialog.onPositiveButtonClick(action: () -> Unit) {
    onButtonClick(BUTTON_POSITIVE, action)
}

fun AlertDialog.onNegativeButtonClick(action: () -> Unit) {
    onButtonClick(BUTTON_NEGATIVE, action)
}

fun AlertDialog.onButtonClick(whichButton: Int, action: () -> Unit) {
    fun onButtonClick() = getButton(whichButton).setOnClickListener { action() }

    if (isShowing) {
        onButtonClick()
    } else {
        setOnShowListener {
            onButtonClick()
        }
    }
}

How to use :

alertDialog.onPositiveButtonClick { /* DO WHAT YOU WANT HERE */ }