import java.io.FileInputStream;
import java.io.FileNotFoundException;

public class FileInputStreamDemo {
    public static void main(String[] args) throws Exception {
        String s = "G://username.txt";
        //String s = "shit";
        //StringBuffer strbuf = null;这里不能是null，可以初始化为一个内容是空的StringBuffer，楼主为啥不用StringBuilder？
        StringBuffer strbuf = new StringBuffer();
        try {
            FileInputStream fis = new FileInputStream(s);
            // BufferedInputStream bis = new BufferedInputStream(fis);
            int ch = fis.read();// 从输入流中读入下一个字节，字节被装换成0~255之间的一个无符号整数返回
            while (ch != -1) {
                char c;
                System.out.print(c = (char) ch);
                strbuf.append(c);// 这里不对，我想得到输入流的字符串形式，怎么写？//append是带参数的
                ch = fis.read();// 似乎是read（）有参数
            }
        } catch (FileNotFoundException e) {
            System.out.println("文件不存在");
        }
        // System.out.println("the root is: "+strbuf.toString());
        // catch (Exception e2) {{System.out.println("输入流异常");}
    }
}

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Java code

import java.io.FileInputStream;
import java.io.FileNotFoundException;

public class FileInputStreamDemo {
    public static void main(String[] args) throws Exception {
        String s = ……