Why should I use Integer.toString() instead of just printing the Integer?

Question

I was following a Java tutorial and it was having me do something like:

int k = 5;
System.out.println("The number is " + Integer.toString(k));

So like a good sport I followed along, but I can't help but wonder, why use that method at all? Seems like a lot of extra typing to get the same functionality as:

int k = 5;
System.out.println("The number is " + k);

Is there a good reason to use the toString() method for non-string data types? And in my specific example, are there any int values that can't be auto toString'ed?

Answer 1

It's one of many "safe programming" practices you can adopt to improve clarity and prevent subtle errors which you or a future maintainer of your code could introduce. Take the following example:

private static void printMessage(int k) {
    System.out.println("number" + '=' + k);
}

output: number=5

Now suppose you want to print the number first and the text last. All I'm doing is swapping the first and last items in the string concatenation:

private static void printMessage2(int k) {
    System.out.println(k + '=' + "number");
}

output: 66number

Oops! The int value of = was added to k instead of being concatenated. One solution is to make the = into a String. Another solution is to explicitly convert the int to a String:

private static void printMessage2Safe(int k) {
    System.out.println(Integer.toString(k) + '=' + "number");
}   

output: 5=number

"Oh, but I'd never do that, anyway," you say. "Anyone in their right mind would have just made the = part of the other String." But what if you're implementing part of a basic calculator app, and you want to output the expression and its result? If you're really trying to be careful, I suppose you'll use StringBuilder.append() to concatenate the parts of the expression. But if not, then I hope you don't write the first implementation of the display method below.

public class Calculator {

    public static void main(String[] args) {
        display(2, '+', 3, 5);
        displaySafe(2, '+', 3, 5);
    }

    public static void display(int num1, char operator, int num2, int result) {
        System.out.println(num1 + operator + num2 + '=' + result);
    }

    public static void displaySafe(int num1, char operator, int num2, int result) {
        System.out.println(Integer.toString(num1) + operator + Integer.toString(num2) + "=" + result);
    }

}

output:

114
2+3=5