FileInputStream返回空

Question

我有两种方法，都使用FileInputStream对象。第一个返回期望值。这种方法效果很好。但是第二种方法什么也不返回。传递给第二个方法的值不是null。我需要获得传递给方法的文件的十六进制格式。为什么会这样呢？请解释一下。这是我的密码

public String binaryFile1(File file1){
        try{
        stringBuilder1=new StringBuilder();
        is1=new FileInputStream(file1);
        while(b!=-1){
            counter++;
            b=is1.read();
            String s = Integer.toHexString(b).toUpperCase();
        if (s.length() == 1) {
            stringBuilder1.append('0');
        }
            if(counter%5==0){
               stringBuilder1.append(s).append("\n");
               counter=0;
            }else
                stringBuilder1.append(s).append(' ');

        }
        is1.close();
        }catch(Exception e){
            e.printStackTrace();
        }

        return stringBuilder1.toString();
}


public String binaryFile2(File file2){
        try{
        stringBuilder2=new StringBuilder();
        is2=new FileInputStream(file2);
        while(b!=-1){
            counter++;
            b=is2.read(); //Here b does not get any content assigned.
            String s = Integer.toHexString(b).toUpperCase();
        if (s.length() == 1) {
            stringBuilder2.append('0');
        }
            if(counter%5==0){
               stringBuilder2.append(s).append("\n");
               counter=0;
            }else
                stringBuilder2.append(s).append(' ');
        }
        is2.close();
        }catch(Exception e){
            e.printStackTrace();
        }

        return stringBuilder2.toString(); //Here stringBuilder2 is null
}

Answer 1

由于b是共享的，并且您没有在binaryFile1之后重置它，所以在binaryFile2的开头仍然是-1。我建议你用，

int b;
while ((b = is2.read()) != -1) {
  // ...
}

编辑

当你完成任务时关闭你的资源是很重要的。我还建议您尽量限制变量范围。使用try-with-resources，您可以编写类似于

public String binaryFile2(File file) {
    StringBuilder sb = new StringBuilder();
    int counter = 0;
    try (InputStream is = new FileInputStream(file)) {
        int b;
        while ((b = is.read()) != -1) {
            counter++;
            String s = Integer.toHexString(b).toUpperCase();
            if (s.length() == 1) {
                sb.append('0');
            }
            sb.append(s);
            if (counter % 5 == 0) {
                sb.append(System.lineSeparator());
                counter = 0;
            } else {
                sb.append(' ');
            }
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
    return sb.toString();
}