String newname = name.substring(0,j-1) + "_BS" + name.substring(j+1,name.length());

import java.io.File;

public class Ls {
	public static void main(String args[]) 
	{
		String[] dir = new java.io.File("test").list();
		java.util.Arrays.sort(dir); 
		File f[] = new File("test").listFiles();
		int len1 = dir.length;
		for (int i=0;i<len1; i++)
		{
			System.out.println(dir[i]);
			String name = f[i].getName();
			System.out.println(name);
			int j=name.indexOf('.');
			System.out.println(j);
			String newname = name.substring(0,j) + "_BS" + name.substring(j,name.length());	
			System.out.println(newname);
			File newFileName=new File(f[i].getParentFile(), newname);
			System.out.println("newFileName="+newFileName);
			f[i].renameTo(newFileName);   
		}
	}	
}