CODEWITHSUNDEEP
java
Marina_Radiskovic
Marina_Radiskovic

Reputation: 43

Java: Why this program works with char and not with int?

I am learning java, and I came across one sample program in the book I am learning from. I decided to experiment a little and I don't understand why it doesn't work as expected. Program is very simple, it's a menu that should output general forms of some of Java's statements based on users input. Here's the code: 

public class SimpleMenu {
public static void main(String args[])
    throws java.io.IOException {
        int choice;
        do {
            System.out.println("Help on: ");
            System.out.println(" 1. IF");
            System.out.println(" 2. WHILE");
            System.out.println(" 3. DO WHILE");
            System.out.println(" 4. FOR");
            System.out.println(" 5. SWITCH\n");
            System.out.println("Choose one: ");
            choice = System.in.read();
        } while ((choice < 1) || (choice > 5));
        System.out.println("\n");
        switch (choice) {
            case 1:
                System.out.println("if(condition) statement1;\n else statement2;");
                break;
            case 2:
                System.out.println("while(condition){\n //body of loop \n }");
                break;
            case 3:
                System.out.println("do{ \n //body of loop \n } while(condition);");
                break;
            case 4:
                System.out.println("for(initialization; termination; increment/decrement){ \n statement; \n }");
                break;
            case 5:
                System.out.println("switch(value){\n case 1: \n statement; \n break; \n default:\n statement;\n}");
                break;
        }
    }
}

No matter what number I type in, it goes through do-while loop two times and expects input again. This is the original code: 

public class MenuFromBook {
public static void main(String args[])
        throws java.io.IOException {
    char choice;
    do {
        System.out.println("Help on: ");
        System.out.println(" 1. if");
        System.out.println(" 2. switch");
        System.out.println(" 3. while");
        System.out.println(" 4. do-while");
        System.out.println(" 5. for\n");
        System.out.println("Choose one:");
        choice = (char) System.in.read();
    } while( choice < '1' || choice > '5');
    System.out.println("\n");
    switch(choice) {
        case '1':
            System.out.println("The if:\n");
            System.out.println("if(condition) statement;");
            System.out.println("else statement;");
            break;
        case '2':
            System.out.println("The switch:\n");
            System.out.println("switch(expression) {");
            System.out.println(" case constant:");
            System.out.println(" statement sequence");
            System.out.println(" break;");
            System.out.println(" //...");
            System.out.println("}");
            break;
        case '3':
            System.out.println("The while:\n");
            System.out.println("while(condition) statement;");
            break;
        case '4':
            System.out.println("The do-while:\n");
            System.out.println("do {");
            System.out.println(" statement;");
            System.out.println("} while (condition);");
            break;
        case '5':
            System.out.println("The for:\n");
            System.out.print("for(init; condition; iteration)");
            System.out.println(" statement;");
            break;
    }
}

But I don't understand why it has to be done like that, why can't choice be int and work like that? I hope I was clear...

Upvotes: 0

Views: 80

Answers (2)

Emil Kantis
Emil Kantis

Reputation: 1114

System.in is an InputStream. The read()-method reads one byte of data. In this case, you input ASCII 1 (byte = 49). Try printing choice to see its value, which should be 49 in case you enter 1.

https://docs.oracle.com/javase/7/docs/api/java/io/InputStream.html#read%28%29 http://www.asciitable.com/

Upvotes: 1

eis
eis

Reputation: 53462

System.in.read() reads input from your keyboard as ascii characters, and if you look at ascii chart, you'll see that numbers 0-9 are actually in the range of 48-57. Thus, if you would compare the result to the actual ascii codes, your code would work correctly. Using '1' you would do just that: '1' is int 49, so comparison works.

Upvotes: 0

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