与Java中的缓冲读取器混淆

Question

1. when i run the code only with //1 - start, the output is ascii value of input character
2. when i run the code only with //2 - start, the output is the entered string terminated by newline character
3. when i run the code with both (as shown in below code), only //1 - start executes, readLine() is being considered in a strange way.
4. when i run the code with both (with //2 - start placed above //1 - start), both the codes executes fine,

Please explain why this strange behavior happens in case 3 but not in case 4.

public class InputBufferedReader {

  public static void main(String[] args) {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    try {
        //1-start
        //read one character
        System.out.println("enter input: ");
        int b= br.read();
        System.out.println(b);
        //end

        //2-start
        //read a string
        System.out.println("enter input string: ");
        String a = br.readLine();
        System.out.println(a);
        //end
    } catch (IOException e) {
        e.printStackTrace();
    }
  }
}

Answer 1

Your BufferedReader uses the InputStreamReader with System.in . The BufferedReader uses the read() method from InputStreamReader to read the data from the standard input stream System.in . Now lets look into the API for this read() method.

[...] This method blocks until input data is available, the end of the stream is detected, or an exception is thrown.

Blocking means in this context waits for user to input data throu the console confirming with the Enter Key.
With that in mind lets examine your cases.
1. int b= br.read(); Nothing is already typed, so this method blocks until user typed something and then prints the ascci value of first character.
2. String a = br.readLine(); Nothing is already typed, so this method blocks until user typed something and then prints the whole line.
3.

int b= br.read();

Lets image user typed a confirming with Enter Key that means the input is a\\n . Now read() reads the first character which is a .

String a = br.readLine();

This read() call will not block and ask for user input because there is unconsumed input left \\n . So readLine() will read \\n .
4.

String a = br.readLine();

User is asked for input which is confirmed with Enter Key. And the whole line will be read.

int b= br.read();

There are no unconsumed data left, because readLine() already has read the whole line including \\n character. So this read() call blocks and user is asked for input.

Answer 2

int read() method attempts to Read Next Character from the Console(or File) and Return its Unicode Value ,

As this Method Returns Unicode Value Compulsory at the Time of Printing we- should perform type- Casting.

If there is No Next Character the we will get -1 .

String readLine() Method attempts to Read Next Line from Console(or File) and Returns it, if it is available.

If the Next line is Not available, then return null.