文件列表的问题

import java.io.*;

public class FileList {
	public static void main(String[] args) {
		File f = new File("G:\\笔试         面试题");
		System.out.println(f.getName());
		tree(f, 1);
	}
	private static void tree(File f, int level) {
		String preStr = "";
		for(int i=0; i<level; i++) {
			preStr += "    ";
		}
		
		File[] childs = f.listFiles();
		for(int i=0; i<childs.length; i++) {
			System.out.println(preStr + childs[i].getName());
			if(childs[i].isDirectory()) {
				tree(childs[i], level + 1);
			}
		}
	}
}