Converting Between String and Other Types

Everything a user types, every value in a file and every field in a web request arrives as text. Before you can add a quantity or compare a price, you have to turn that text into a number. And when you show results, you turn numbers back into text. These conversions happen in almost every program, so it's worth knowing the right method for each direction and what goes wrong when the text isn't valid.

Text to Values

public class FromText {
    public static void main(String[] args) {
        int qty = Integer.parseInt("12");
        double price = Double.parseDouble("49.50");
        boolean member = Boolean.parseBoolean("true");
        long phone = Long.parseLong("9876543210");
 
        System.out.println("Bill: " + qty * price);
        System.out.println(member + " " + phone);
 
        double fromForm = Double.parseDouble("1,234.5".replace(",", ""));
        System.out.println(fromForm);
 
        try {
            Integer.parseInt("12.5");
        } catch (NumberFormatException e) {
            System.out.println("Not a whole number: " + e.getMessage());
        }
    }
}

Bill: 594.0
true 9876543210
1234.5
Not a whole number: For input string: "12.5"

How it works

1. Each wrapper class has a parseXxx method: Integer.parseInt, Double.parseDouble, Long.parseLong, Boolean.parseBoolean.
2. A 10-digit phone number is too big for int, so it needs long.
3. Text with thousands separators must be cleaned first; parseDouble doesn't understand commas.
4. "12.5" isn't a whole number, so parseInt throws NumberFormatException. Always be ready for it with user input.

Values to Text

import java.util.Arrays;
 
public class ToText {
    public static void main(String[] args) {
        int qty = 12;
        double price = 49.5;
 
        String a = String.valueOf(qty);
        String b = Integer.toString(255, 2);
        String c = "" + price;
        String d = String.format("%.2f", price);
        System.out.println(a + " " + b + " " + c + " " + d);
 
        char[] letters = "Java".toCharArray();
        Arrays.sort(letters);
        System.out.println(new String(letters));
 
        Object nothing = null;
        System.out.println(String.valueOf(nothing));
    }
}

12 11111111 49.5 49.50
Jaav
null

How it works

1. String.valueOf(x) works for every type and is the clearest choice.
2. Integer.toString(255, 2) converts to base 2 (binary). Use 16 for hexadecimal.
3. "" + price works too, but valueOf says what you mean.
4. String.format controls the look, such as two decimal places (see "String Formatting in Java").
5. A char[] becomes a string with new String(chars) or String.valueOf(chars). Sorting the letters of "Java" puts the capital J first.
6. String.valueOf(null object) returns the text "null" instead of throwing.

Conversion Table

Common Mistakes

Parsing text with spaces or commas. Call strip() and remove separators first.

Using parseInt for big numbers. Phone numbers and account numbers need long, or should stay as text.

Calling obj.toString() on a possibly null reference. It throws; String.valueOf(obj) doesn't.

Practice

1. Easy: Read two numbers as strings from args, convert them and print their sum.
2. Harder: Write static int toIntOrDefault(String text, int fallback) that strips the text, removes commas, and returns fallback for null, blank or invalid input. Test it with " 1,200 ", "abc" and null.

Key Takeaways