java的File类的lastModified()方法可以返回文件的最后修改时间：

String s1 = "file1.dat";
String s2 = "file2.dat";

File f1 = new File(s1);
File f2 = new File(s2);

if (f1.lastModified() < f2.lastModified())
System.out.println((s1 + " IS OLDER THAN " + S2);