Android入门之进度条(ProgressBar)

package com.jkxqj.helloandroid;

import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ProgressBar;

public class MainActivity extends Activity {

	private ProgressBar progressBar;
	private Button firstButton;
	private Button secondButton;
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);		
		progressBar = (ProgressBar)findViewById(R.id.firstProgressBar);
		firstButton = (Button)findViewById(R.id.firstButton);
		secondButton = (Button)findViewById(R.id.secondButton);		
		progressBar.setMax(100);		
		firstButton.setOnClickListener(new FirstListener());
		secondButton.setOnClickListener(new SecondListener());	
	}	
	class FirstListener implements OnClickListener{

		@Override
		public void onClick(View v) {
			progressBar.incrementProgressBy(10);
		}		
	}	
	class SecondListener implements OnClickListener{
		@Override
		public void onClick(View v) {
			progressBar.incrementSecondaryProgressBy(20);
		}
	}
	@Override
	public boolean onCreateOptionsMenu(Menu menu) {
		// Inflate the menu; this adds items to the action bar if it is present.
		getMenuInflater().inflate(R.menu.main, menu);
		return true;
	}

}

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

    <Button
        android:id="@+id/secondButton"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/firstProgressBar"
        android:layout_below="@+id/firstButton"
        android:layout_marginTop="26dp"
        android:text="增加第二进度" />

    <Button
        android:id="@+id/firstButton"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/secondButton"
        android:layout_below="@+id/firstProgressBar"
        android:layout_marginTop="17dp"
        android:text="增加第一进度" />

    <ProgressBar
        android:id="@+id/firstProgressBar"
        style="?android:attr/progressBarStyleHorizontal"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="30dp" />

</RelativeLayout>