do-while循环问题

Question

I am just a beginner in java coding i was just writing a simple program : user is given a menu he has to enter number between 1-4 if user enters correct number, required task is done if wrong number is entered,user is again asked for input. Below is my program

class menu {
  public static void main(String [] args) throws java.io.IOException {
        int choice;
        do
        {
            System.out.println("HELP MENU: ");
            System.out.println("IF STATEMENT: 1 ");
            System.out.println("WHILE: 2 ");
            System.out.println("DO WHILE: 3 ");
            System.out.println("SWITCH: 4 ");
            choice = System.in.read();
            System.out.println(choice);
        }

        while( choice < 1 || choice > 4);
        System.out.println("\n");
        System.out.println(choice);

        switch (choice)
        {
        case 1:
            System.out.println("if statement is selected");
            break;
        case 2:
            System.out.println("while statement is selected");
            break;

        case 3:
            System.out.println("do while statement is selected");
            break;

        case 4:
            System.out.println("switch statement is selected");
            break;
        }
    }
}

OUTPUT: +++++++

E:\study\javacode>java menu 
HELP MENU:
IF STATEMENT: 1
WHILE: 2
DO WHILE: 3
SWITCH: 4
4
52
HELP MENU:
IF STATEMENT: 1
WHILE: 2
DO WHILE: 3
SWITCH: 4
13
HELP MENU:
IF STATEMENT: 1
WHILE: 2
DO WHILE: 3
SWITCH: 4
10
HELP MENU:
IF STATEMENT: 1
WHILE: 2
DO WHILE: 3
SWITCH: 4

what ever user is entering through keyboard ,the code keeps on iterating through do-while loop.i identified the cause by printing the input value and what i found is that input value is taken wrong by the code.Please help to solve this

Answer 1

To read numbers or String s, I would recommend you to use a Scanner object. Create and instantiate it at the begining of the main() :

Scanner in = new Scanner(System.in);

and call the nextInt() method in the do-while :

do {
    System.out.println("HELP MENU: ");
    System.out.println("IF STATEMENT: 1 ");
    System.out.println("WHILE: 2 ");
    System.out.println("DO WHILE: 3 ");
    System.out.println("SWITCH: 4 ");
    choice = in.nextInt();
    System.out.println(choice);
} while (choice < 1 || choice > 4);

Notes:

• System.in.read() is actually returning the int value of the character you input. For example if you input 1 , the method will return 49 which is equal to (int)'1'
• As I mentioned before, you can use Scanner to read String s, with the method nextLine() .

Edit:

Just to see that you can use System.in.read() (you must read the documentation to understand what it does) to read an integer, try this (in a separate file so you don't accidentally modify your code):

int i = System.in.read();
System.out.println(Integer.parseInt(Character.toString(((char) i))));

Answer 2

The issue is that you're using InputStream.read(), which reads a byte from the stream, not a character, eg if you type '1', read() will return 0x31.

Javadoc of read():

/**
 * Reads the next byte of data from the input stream. The value byte is
 * returned as an <code>int</code> in the range <code>0</code> to
 * <code>255</code>. If no byte is available because the end of the stream
 * has been reached, the value <code>-1</code> is returned. This method
 * blocks until input data is available, the end of the stream is detected,
 * or an exception is thrown.
 *

Answer 3

System.in.read() does not quite work like you think it does. It reads a character (not an int) and returns a byte value, not a integer.

When a user enters "1", read() returns 49, which is the integer byte value for character '1'. (50 is '2', 51 is '3', etc).

@Christian's scanner suggestion is very good. I think you should do that instead.

Alternately, you can change your switch statement to use 49/50/51/etc, but that's kind of ugly.