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Button Click Event on Android
<Button
android:id="@+id/btnOK"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Display Message"
android:onClick="onBtnClicked" />

package com.example.helloandroid;

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

public class HelloAndroid extends Activity
{
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
    }

    public void onBtnClicked(View v)
    {
        if(v.getId() == R.id.btnOK)
        {
            MessageBox("Hello World");
        }
    }

    public void MessageBox(String message)
    {
       Toast.makeText(this, message, Toast.LENGTH_SHORT);
    }
}

Toast.makeText(this, message, Toast.LENGTH_SHORT);

Toast.makeText(this, message, Toast.LENGTH_SHORT).show();

Button b = (Button) findViewById(R.id.btnOK);
b.setOnClickListener(this);

public void onClick(View v) {
    switch(v.getId()){
    case R.id.btnOK:
        /*  the instruccions of the button */
        break;
    }
}