1. Overview
In this tutorial, we’ll cover many ways of converting a String into a double in Java.
2. Double.parseDouble
We can convert a String to a double using the Double.parseDouble method:
assertEquals(1.23, Double.parseDouble("1.23"), 0.000001);
3. Double.valueOf
Similarly, we can convert a String into a boxed Double using the Double.valueOf method:
assertEquals(1.23, Double.valueOf("1.23"), 0.000001);
Note that the returned value of Double.valueOf is a boxed Double. Since Java 5, this boxed Double is converted by the compiler to a primitive double where needed.
In general, we should favor Double.parseDouble since it does not require the compiler to perform auto-unboxing.
4. DecimalFormat.parse
When a String representing a double has a more complex format, we can use a DecimalFormat.
For example, we can convert a decimal-based currency value without removing non-numeric symbols:
DecimalFormat format = new DecimalFormat("\u00A4#,##0.00");
format.setParseBigDecimal(true);
BigDecimal decimal = (BigDecimal) format.parse("-$1,000.57");
assertEquals(-1000.57, decimal.doubleValue(), 0.000001);
Similar to the Double.valueOf, the DecimalFormat.parse method returns a Number, which we can convert to a primitive double using the doubleValue method. Additionally, we use the setParseBigDecimal method to force DecimalFormat.parse to return a BigDecimal.
Usually, the DecimalFormat is more advanced than we require, thus, we should favor the Double.parseDouble or the Double.valueOf instead.
To learn more about DecimalFormat, please check a practical guide to DecimalFormat.
5. Invalid Conversions
Java provides a uniform interface for handling invalid numeric Strings.
Notably, Double.parseDouble, Double.valueOf, and DecimalFormat.parse throw a NullPointerException when we pass null.
Likewise, Double.parseDouble and Double.valueOf throw a NumberFormatException when we pass an invalid String that cannot be parsed to a double (such as &).
Lastly, DecimalFormat.parse throws a ParseException when we pass an invalid String.
6. Avoiding Deprecrated Conversions
Before Java 9, we could create a boxed Double from a String by instantiating a Double:
new Double("1.23");
As of version 9, Java officially deprecated this method.
7. Conclusion
In conclusion, Java provides us with multiple methods to convert Strings into double values.
In general, we recommend using Double.parseDouble unless a boxed Double is needed.
The source code for this article, including examples, can be found over on GitHub.
