Convert Double to String in Java

class Test {
   public static void main(String[] args) {
      String s1 = String.valueOf(123.5);
      String s2 = String.valueOf(8252.1);
      String s3 = String.valueOf(12.8);
      
      System.out.println(s1);
      System.out.println(s2);
      System.out.println(s3);
   }
}

123.5
8252.1
12.8

class Test {
    public static void main(String[] args) {
        String s1 = Double.toString(123.5);
        String s2 = Double.toString(8252.1);
        String s3 = Double.toString(12.8);

        System.out.println(s1);
        System.out.println(s2);
        System.out.println(s3);
    }
}

123.5
8252.1
12.8

Formatting Double to String

In addition to converting a double to String, Java provides several options to format the String representation of a double value according to specific patterns. The following are some of the formatting options available in Java:

Decimal places

You can specify the number of decimal places to display for a double value using the %f format specifier. For example, the pattern %.2f will display a double value with two decimal places. Here’s an example:

double d = 123.456;
String s = String.format("%.2f", d);
System.out.println(s);

Output:

123.46

Scientific notation

You can use the %e format specifier to display a double value in scientific notation. Here’s an example:

double d = 1234567890;
String s = String.format("%e", d);
System.out.println(s); 

Output:

1.234568e+09

Currency symbols

You can use the NumberFormat class to format a double value as a currency amount. The NumberFormat class provides several methods to format a double value as a currency amount for different locales. Here’s an example:

double d = 123.456;
Locale locale = Locale.US;
NumberFormat currencyFormatter = NumberFormat.getCurrencyInstance(locale);
String s = currencyFormatter.format(d);
System.out.println(s); 

Output:

$123.46

Custom patterns

You can use the DecimalFormat class to specify a custom pattern for formatting a double value as a String. The DecimalFormat class allows you to specify the number of decimal places, grouping separators, and other formatting options. Here’s an example:

double d = 123456789.123456789;
DecimalFormat decimalFormatter = new DecimalFormat("#,###.##");
String s = decimalFormatter.format(d);
System.out.println(s);

Output:

123,456,789.12

By using these formatting options, you can create a String representation of a double value that is formatted according to your requirements.

Comparison of String Conversion Methods

Java provides several ways to convert a double to String, and each method has its own advantages and disadvantages. Here’s a comparison of the two most common methods:

String.valueOf() Method

The String.valueOf() method is a simple and convenient way to convert a double to String. It takes a double value as input and returns a String representation of that value.

Advantages:

• Simple and easy to use.
• Returns a String representation of the input double value.

Disadvantages:

• Less flexible than other methods for formatting the output String.
• Slightly slower than the Double.toString() method.

Double.toString() Method

The Double.toString() method is another way to convert a double to String. It is a static method of the Double class that takes a double value as input and returns a String representation of that value.

Advantages:

• Faster than the String.valueOf() method.
• More flexible than the String.valueOf() method for formatting the output String.

Disadvantages:

• Requires an additional method call compared to the String.valueOf() method.
• Returns a String representation of the input double value.

Choosing a Method

When choosing a method to convert a double to String in Java, it’s important to consider the trade-offs between convenience, flexibility, and performance. If you need a simple and quick way to convert a double to String, the String.valueOf() method may be sufficient. If you need more control over the formatting of the output String, or if performance is a concern, the Double.toString() method may be a better choice. Additionally, if you need to perform multiple double to String conversions in a loop or in a performance-critical section of code, you may want to consider using a StringBuilder or a Formatter to avoid creating multiple String objects.

Handling NaN and Infinity values

In Java, a double value can have special values that indicate a non-numeric result. These values are represented by the constants Double.NaN (Not-a-Number), Double.POSITIVE_INFINITY, and Double.NEGATIVE_INFINITY. When converting a double value that is NaN or infinite to a String, it is important to represent these values correctly in the output.

To check if a double value is NaN or infinite, you can use the static methods Double.isNaN() and Double.isInfinite(). Here’s an example:

double d1 = Double.NaN;
double d2 = Double.POSITIVE_INFINITY;
double d3 = Double.NEGATIVE_INFINITY;

System.out.println(Double.isNaN(d1)); 
System.out.println(Double.isInfinite(d2)); 
System.out.println(Double.isInfinite(d3)); 

Output:

true
true
true

To represent NaN and infinite values as a String, you can use the String class or the Double.toString() method. The output of these methods for NaN and infinite values is specified by the IEEE 754 standard, which defines the representation of floating-point numbers.

Here are some examples of converting NaN and infinite values to a String in Java:

double d1 = Double.NaN;
double d2 = Double.POSITIVE_INFINITY;
double d3 = Double.NEGATIVE_INFINITY;

String s1 = String.valueOf(d1);
String s2 = Double.toString(d2);
String s3 = Double.toString(d3);

System.out.println(s1); 
System.out.println(s2); 
System.out.println(s3); 

Output:

NaN
Infinity
-Infinity

Note that the toString() method for infinite values returns the strings “Infinity” and “-Infinity” for positive and negative infinite values, respectively.

When working with NaN and infinite values, it is important to handle them correctly in your code, as they can lead to unexpected results and errors if not handled properly.

Conclusion

In this tutorial, we’ve covered several methods for converting a double to String in Java. We’ve explored the String.valueOf() and Double.toString() methods, as well as some formatting options. We’ve also covered how to handle special values such as NaN and Infinity when converting double values to String. By following these best practices, you can ensure that your double values are represented correctly as String values in your Java programs.

If you wish to perform the opposite conversion, this tutorial Convert Java String to Double provides a guide on how to do so. By combining the knowledge from both of these tutorials, you’ll be able to convert between double and String values with ease in your Java programs.

Frequently asked questions

• What is the difference between String.valueOf() and Double.toString()?
  The main difference between String.valueOf() and Double.toString() is the way they handle null input. If the input to String.valueOf() is null, it returns the String “null”. If the input to Double.toString() is null, it throws a NullPointerException.
• Why would I need to convert a double to a String?
  There are many reasons why you might need to convert a double to String, such as when you need to display the double value in a user interface, write it to a file, or transmit it over a network.
• What is a scientific notation, and how is it used in Java?
  Scientific notation is a way of expressing very large or very small numbers in a compact form. In Java, scientific notation is represented using the letter “e” to denote the exponent. For example, the number 12345.6789 in scientific notation would be represented as 1.23456789e+04.
• Can a double value always be accurately represented as a String?
  No, in some cases a double value may not be able to be accurately represented as a String. This can occur when the value is very large or very small, or when it has a large number of decimal places. In these cases, the String representation of the double value may be an approximation rather than an exact value.
• What is the difference between the %f and %g format specifiers when converting a double to a String?
  The %f format specifier is used to format a double value with a fixed number of decimal places, while the %g format specifier is used to format a double value in a way that minimizes the number of digits displayed. The %g format specifier will automatically switch to scientific notation if the value is very large or very small.
• What is the default format used when converting a double to a String?
  When converting a double to String using String.valueOf() or Double.toString(), the default format used is the same as the %f format specifier, which formats the value with six decimal places.