Android 下载重复文件命名规则【android源码解析七】

题记：一个人不能在两个同样的坑里摔倒。
去年4月份的时候，我有一个任务，让我写个下载保存文件的方法，如果文件的名字存在，就加“-1”，如果仍然存在，就在-后面的数字加1，例如：文件名：Keep_On_It.mp3,第一次下载是Keep_On_It.mp3，第二次下载名字就保存成：Keep_On_It-1.mp3,第三次下载名字就保存成：Keep_On_It-2.mp3，第四次下载名字就保存为：Keep_On_It-3.mp3，以此类推的形式一直实现。当时我用的方法比较笨，可读性特别差，当时用了半天就实现了，就是截取字符串进行比较。最近研究源码下载的时候发现可以用for循环来实现，可读性强，容易理解，算法简练，不愧为谷歌设计出来的，简单明了，容易理解。
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现在给大家分享一下：先看效果图片：
如果内存卡的这个目录只有这些MP3：



再次保存的名字就是最后一个Keep_It_On-3.mp3了




下面把代码奉上：
在CopyFileApp的工程里：
一、在com.cn.daming.copyfile包下的CopyFileAppActivity.java类中的代码：

package com.cn.daming.copyfile;

import java.io.File;
import java.util.Random;

import android.app.Activity;
import android.app.AlertDialog;
import android.os.Bundle;
import android.os.Environment;
import android.os.SystemClock;
import android.util.Log;
import android.widget.TextView;

public class CopyFileAppActivity extends Activity {
    /** Called when the activity is first created. */
	
	private final static String SDCARD_PATH="/mnt/sdcard/download/mp3/Keep_It_On.mp3";
	private final static String FILENAME_SEQUENCE_SEPARATOR = "-";
	private static Random sRandom = new Random(SystemClock.uptimeMillis());
	private final static String LOG = "wangxianming";
	
	private TextView m_TextView;
	private TextView m_TextView_full;
	
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        
        m_TextView = (TextView)findViewById(R.id.text_view);
        m_TextView_full = (TextView)findViewById(R.id.text_view2);
        
        Log.v(LOG, " ---->"+SystemClock.uptimeMillis());
        
        // Check to see if we have an SDCard
        String status = Environment.getExternalStorageState();
        if (!status.equals(Environment.MEDIA_MOUNTED)) {
            int title;
            String msg;
            // Check to see if the SDCard is busy, same as the music app
            if (status.equals(Environment.MEDIA_SHARED)) {
                msg = getString(R.string.download_sdcard_busy_dlg_msg);
                title = R.string.download_sdcard_busy_dlg_title;
            } else {
                msg = getString(R.string.download_no_sdcard_dlg_msg, "no sdcard");
                title = R.string.download_no_sdcard_dlg_title;
            }

            new AlertDialog.Builder(this)
                .setTitle(title)
                .setIcon(android.R.drawable.ic_dialog_alert)
                .setMessage(msg)
                .setPositiveButton(R.string.ok, null)
                .show();
            return;
        }
        
        File mp3File = new File(SDCARD_PATH);
        if(mp3File != null && mp3File.exists()) {
        	String fileNamePath = mp3File.getAbsolutePath();
        	String filename = fileNamePath.substring(0, fileNamePath.lastIndexOf("."));
        	String extension = fileNamePath.substring(fileNamePath.lastIndexOf("."), fileNamePath.length());
        	m_TextView.setText(filename + extension);
        	m_TextView_full.setText(chooseUniqueFilename(filename, extension));
        }
    }
    
    //This is method is core
    private String chooseUniqueFilename(String filename, String extension) {
    	String fullFilename = filename + extension;
    	if(!new File(fullFilename).exists()) {
    		return fullFilename;
    	}
    	
    	filename = filename + FILENAME_SEQUENCE_SEPARATOR;
    	
    	/*
         * This number is used to generate partially randomized filenames to avoid
         * collisions.
         * It starts at 1.
         * The next 9 iterations increment it by 1 at a time (up to 10).
         * The next 9 iterations increment it by 1 to 10 (random) at a time.
         * The next 9 iterations increment it by 1 to 100 (random) at a time.
         * ... Up to the point where it increases by 100000000 at a time.
         * (the maximum value that can be reached is 1000000000)
         * As soon as a number is reached that generates a filename that doesn't exist,
         *     that filename is used.
         * If the filename coming in is [base].[ext], the generated filenames are
         *     [base]-[sequence].[ext].
         */
         int sequence = 1;
         for (int magnitude = 1; magnitude < 1000000000; magnitude *= 10) {
             for (int iteration = 0; iteration < 9; ++iteration) {
                 fullFilename = filename + sequence + extension;
                 if (!new File(fullFilename).exists()) {
                     return fullFilename;
                 }
                 Log.v(LOG, "file with sequence number " + sequence + " exists");
                 sequence += sRandom.nextInt(magnitude) + 1;
             }
         }
    	
    	return fullFilename;
    }
}

二、layout目录下的main.xml中的代码如下：

<?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:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dip"
        android:gravity="center"
        android:text="@string/hello" />

    <TextView
        android:id="@+id/text_view"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dip"
        android:text="" 
        />
    <TextView
        android:id="@+id/text_view2"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dip"
        android:text="" />
</LinearLayout>

三、values目录下的strings.xml中的代码：

<?xml version="1.0" encoding="utf-8"?>
<resources>

    <string name="hello">This is daming Original</string>
    <string name="app_name">CopyFileApp</string>
    <string name="download_sdcard_busy_dlg_title" product="default" msgid="6877712666046917741">"SD 卡不可用"</string>
    <string name="download_sdcard_busy_dlg_msg">"SD 卡正忙。要允许下载，请在通知中选择“关闭 USB 存储设备”。"</string>
    <string name="download_no_sdcard_dlg_msg" product="default" msgid="2616399456116301518">"需要有 SD 卡才能下载</string>
    <string name="download_no_sdcard_dlg_title" product="default" msgid="605904452159416792">"无 SD 卡"</string>
    <string name="cancel" msgid="3017274947407233702">"取消"</string>
    <string name="ok" msgid="1509280796718850364">"确定"</string>
</resources>

有什么好的建议和新颖的想法的可以留言，欢迎大家共同讨论。。。。。。。