如何用java编写 从c盘的test.txt读取文件内容,然后拷贝到c盘得test2.txt文件中,并且覆盖原有的内容

这个很简单，
使用commons-io包的工具类：
FileUtils.copyFile(new File("c:\\test.txt"), new File("c:\\test2.txt"));

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;

public class test {
public static void main(String[] args ) throws Exception {
File source = new File("c:/test.txt");
BufferedReader in = new BufferedReader(new InputStreamReader(new FileInputStream(source)));

File desc = new File("c:/test2.txt");
BufferedWriter out = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(desc)));

String str = null;
while ((str = in.readLine()) != null) {
out.write(str + "\r\n");
}
if(desc.exists())
{
System.out.print("ok！");
}
out.close();
in.close();
}
}