import java.io.*; 
public class Ex6_2 { 
   
        public static void main(String arg[])throws IOException{ 
        File f = new File("D:" + File.separator + "Hello.txt"); 
        f.createNewFile(); 
        s1 += "要添加的内容";
        BufferedWriter output = new BufferedWriter(new FileWriter(f)); 
        output.write(s1); 
        output.close(); 
           }

------解决方案--------------------
write

------解决方案--------------------
S1应该是String类型的变量，1楼可能忘了在S1前加String修饰

------解决方案--------------------
String str=new String("自己要输入的内容");
File file = new File("D:\Hello.txt");  
FileWriter writer=new FileWriter(file);
writer.write(str);
writer.close() ;

------解决方案--------------------
File file=new File("c:/mulu.txt");
FileWriter filewriter=new FileWriter(file);
String s=new String();
filewrite.write(s);
filewrite.close();


------解决方案--------------------
File对象不能写文件的，必须用流才可以写入，如FileWriter或FileOutputStream