Convert double to String in Java [8 Methods]

Why convert double to String in Java?

The double data type in Java is used to represent floating-point numbers with double precision. However, there are several scenarios where a double might need to be represented as a string:

1. User Display: Floating-point numbers might need to be presented in a specific format to the user, and converting them to strings allows for that customization.
2. File I/O: When saving data to a file or reading from one, numbers might be represented as text. Converting between double and String is crucial for these operations.
3. Data Transmission: While transmitting data across networks, especially in formats like JSON or XML, numerical data is often sent as a string.
4. Formatting and Rounding: Representing double values as strings allows for specific formatting and rounding operations, ensuring that data is presented with a desired level of precision.
5. Comparisons: In some scenarios, it might be easier or more efficient to compare numbers in their string representation, especially when dealing with systems that store or process data as strings.

 

The double Data Type in Java

In Java, the double data type is a primitive type that represents floating-point numbers, providing a way to store fractional numbers with a significant degree of precision. It is a part of the larger family of floating-point data types, which also includes float.

Precision:

1. Size: The double data type is 64 bits in size, or 8 bytes.
2. Range: It can represent values ranging approximately between $4.9e-324$ and $1.8e+308$, both positive and negative.
3. Decimal Places: The precision of a double can be up to about 15 decimal places. However, it's crucial to note that not all decimal numbers can be represented exactly due to the binary nature of computer arithmetic.
4. Binary Representation: Java uses the IEEE 754 standard for floating-point arithmetic, which divides the bits into three parts: sign, exponent, and mantissa (or fraction). This representation allows for a wide range of values but can lead to some rounding errors or approximations.

 

Methods to Convert double to String in Java

In java, double is a primitive data type whereas Double is a wrapper class. As we know that java supports auto-boxing, so we can use double and Double interchangeably in most of the cases.

In several applications, we may need to convert a number to a string because we need to operate on the value in its string form. In many GUI based application also, if we have to display double value in text field we may need to convert. There are several ways to convert double type data to a string.

The list below shows eight different ways in which we can convert double to string.

• Using + operator
• Using String.valueOf()
• Using Double.toString()
• Using DecimalFormat class
• Using String.format()
• Using StringBuilder class
• Using StringBuffer class
• Using Double constructor

 

1. Using + operator

In Java, the + operator can be used to concatenate strings. When a double value is combined with a string using the + operator, Java automatically converts the double to its string representation and then concatenates it.

Example : In this example, we concatenate blank string "" with double variable d.

double value = 567.890;
String stringValue = "Value: " + value;
System.out.println(stringValue);

Output

Value: 567.89

The + operator for string concatenation is straightforward and readable, making it a popular choice for simple conversions and displays. However, it's worth noting that repeated use of the + operator for string concatenation (especially inside loops) can be inefficient due to the immutability of strings in Java. For such scenarios, using a StringBuilder or other methods might be more efficient.

 

2. Using String.valueOf(double d) method:

The String.valueOf() method is a static method that provides overloads for various data types, enabling them to be converted to their string representations. When you use this method with a double value, it returns the string representation of the specified double value.

double value = 987.654;
String stringValue = String.valueOf(value);
System.out.println(stringValue);

Output

987.654

Advantages:

• Explicit Conversion: Using String.valueOf() makes the data type conversion explicit in the code, making it clearer for readers that a conversion is happening.
• Null Safety: When used with object types (like Double), String.valueOf() safely converts null references to the string "null", whereas direct concatenation with the + operator would result in a NullPointerException.

 

3. Using Double.toString(double d) method

The Double.toString(double d) method is a static method of the Double wrapper class. It provides a straightforward way to obtain a string representation of a double value.

double value = 1234.5678;
String stringValue = Double.toString(value);
System.out.println(stringValue);

Output

1234.5678

Advantages:

• Directness: This method offers a direct and clear approach for converting a double primitive to a string representation. Its intention is evident from the method name and the class it belongs to.
• Precision: The Double.toString() method will present the double value in its full precision, making it suitable for applications where the exact representation of the number is required.

 

4. Using DecimalFormat class

The DecimalFormat class is part of the java.text package and provides capabilities to format decimal numbers. It's especially useful when you want to format double values according to specific patterns, like rounding to a fixed number of decimal places, adding commas for thousands separators, or using other locale-specific conventions.

double value = 1234567.8910;
DecimalFormat df = new DecimalFormat("#,###.##");
String stringValue = df.format(value);
System.out.println(stringValue);

Output

1,234,567.89

Advantages:

• Custom Formatting: The primary advantage of DecimalFormat is its ability to format numbers according to custom patterns. You can control how many decimal places to show, whether to use grouping separators, and more.
• Rounding: DecimalFormat provides automatic rounding of numbers based on the pattern provided. In the example above, the number was rounded to two decimal places.
• Locale-Specific Formatting: DecimalFormat can be used with DecimalFormatSymbols to format numbers according to specific locales, accommodating regional preferences for decimal points, grouping separators, and other number-related notations.
• Consistency: For applications where consistent number formatting is crucial, like financial or data reporting software, DecimalFormat offers a reliable solution.

 

5. Using String.format() method

The String.format() method offers a way to format strings using specific patterns and templates. It's very versatile, allowing you to embed multiple values of different data types into a single string. When used with a double, it can provide precise control over the formatting, including the number of decimal places, leading zeros, and more.

double value = 12.34567;
String stringValue = String.format("%.2f", value);
System.out.println(stringValue);

Output

12.35

Advantages:

• Consistent Formatting: Like DecimalFormat, String.format() provides precise control over the output format, ensuring consistency across multiple values.
• Multiple Data Types: A single String.format() call can handle multiple values of different data types, making it a versatile choice for creating composite strings.
• Readability: The template provided in String.format() makes it easier to understand the desired format, especially when working with complex strings.
• Localization: String.format() can also be used with locale-specific formatting conventions, though this requires the java.util.Formatter class.

 

6. Using StringBuilder class

StringBuilder is primarily used for constructing and manipulating large strings without incurring the performance penalty associated with the immutability of the String class. While it's not a direct method for converting a double to a String, it becomes relevant in scenarios where you're building large strings that include double values among other data.

To convert a double to a String within a StringBuilder, you typically append the double value to the StringBuilder instance, and Java handles the conversion automatically.

double value = 789.0123;
StringBuilder sb = new StringBuilder();
sb.append("The value is: ").append(value);
String stringValue = sb.toString();
System.out.println(stringValue);

Output

The value is: 789.0123

Advantages:

• Performance: When constructing large or complex strings, StringBuilder is typically more efficient than using the + operator repeatedly due to its mutable nature.
• Flexibility: StringBuilder provides various methods to manipulate and build strings, such as insert, delete, and reverse, which can be useful in certain scenarios.
• Automatic Conversion: As seen in the example, appending a double to StringBuilder directly converts it to its string representation, making the process straightforward.

 

7. Using StringBuffer class

The StringBuffer class, like StringBuilder, is used for constructing and manipulating strings. The primary distinction between StringBuilder and StringBuffer is that StringBuffer is thread-safe, meaning its methods are synchronized to support concurrent access by multiple threads. This synchronization introduces some performance overhead, making StringBuffer somewhat slower than StringBuilder in single-threaded scenarios.

To convert a double to a String using StringBuffer, the approach is similar to StringBuilder: you append the double value directly, and the conversion is handled automatically by Java.

double value = 456.7890;
StringBuffer sb = new StringBuffer();
sb.append("The value is: ").append(value);
String stringValue = sb.toString();
System.out.println(stringValue);

Output

The value is: 456.789

Advantages:

• Thread-Safety: The synchronized methods of StringBuffer ensure it's safe to use in multi-threaded environments.
• Automatic Conversion: Appending a double to StringBuffer directly converts it to its string representation, making the process simple and clear.
• String Manipulation: Like StringBuilder, StringBuffer provides various methods for string manipulation, such as insert, delete, and reverse.

 

8. Using Double Constructor and toString() method

The Double class is a wrapper class for the double primitive data type. One can create a Double object by passing a double value to its constructor. After the Double object is created, you can invoke its toString() method to get the string representation of the double value.

double value = 123.456;
Double doubleObject = new Double(value);
String stringValue = doubleObject.toString();
System.out.println(stringValue);

Output

123.456

Advantages:

• Object Orientation: By converting a double to a Double object, you get access to various methods provided by the Double class. This can be useful in scenarios where you need to perform object-oriented operations on the double value.
• Consistency: Using the Double class's toString() method ensures a consistent string representation of the double value.

 

Performance Implications of Converting double to String

When converting a double to a String in Java, there are various methods available, each with its performance implications. The efficiency of each method might be relevant, especially in situations where the conversion occurs frequently, such as in loops or high-throughput systems.

1. Direct Methods (Double.toString(), String.valueOf()):

• • Implications: These methods are direct, efficient, and present minimal overhead. They are suitable for most common use cases.
  • When to Use: When the primary requirement is to quickly convert a double to a String without specific formatting needs.

1. String Concatenation Using + Operator:

• • Implications: In the background, Java uses a StringBuilder for string concatenation using the + operator. For occasional conversions, this is fine. However, inside loops or for repeated concatenation, this approach might introduce overhead.
  • When to Use: For quick and minimal conversions outside of performance-critical sections.

1. DecimalFormat Class:

• • Implications: Offers advanced formatting but comes with a performance overhead due to parsing the format pattern and applying it. The initialization of DecimalFormat can also be costly.
  • When to Use: When precise formatting is required. If used repeatedly, consider caching and reusing the DecimalFormat instance.

1. String.format() Method:

• • Implications: It's versatile but generally slower than direct methods because of the overhead introduced by parsing the format string.
  • When to Use: When there's a need for complex string formatting that includes values other than the double in question.

1. StringBuilder and StringBuffer:

• • Implications: Efficient for building large or complex strings. StringBuffer, being thread-safe, has synchronization overhead making it slower than StringBuilder in single-threaded scenarios.
  • When to Use: When constructing or modifying large strings, especially in loops. Use StringBuilder for single-threaded applications and StringBuffer when thread-safety is a concern.

1. Double Constructor:

• • Implications: Creating a new Double object has memory and performance costs. Also, the constructor approach is deprecated in favor of valueOf().
  • When to Use: Generally, it's better to avoid this method for mere conversions due to its inefficiency and use more direct methods or the recommended valueOf().

 

 

Summary

In the realm of Java programming, converting a double to a String is a frequent requirement. The double data type, a double-precision 64-bit IEEE 754 floating-point, is adept at representing decimal values. Its conversion to a string aids in tasks like user display, file operations, and more.

Several methods facilitate this conversion:

• Direct Methods: Functions like Double.toString() and String.valueOf() are quick and efficient ways to perform the transformation without any fuss.
• String Concatenation: Using the + operator for concatenation is handy but might introduce overhead when used extensively or inside loops.
• Formatting Methods: Tools like DecimalFormat and String.format() are vital when custom formatting is essential. However, their detailed control comes at a slight performance trade-off.
• String Building: For constructing complex strings, especially in threaded environments, StringBuilder and StringBuffer come into play. While the former is swift due to its non-synchronized nature, the latter ensures thread safety at a slight performance cost.
• Wrapper Class Approach: The Double constructor offers an object-oriented path to the conversion, although it's not the most performance-efficient method.

While each method has its merits, the choice boils down to the specific scenario at hand. It's crucial to weigh performance, precision, and ease of use according to the application's demands.

 

References

Converting between Numbers and Strings
DecimalFormat Class
Strings Class
StringBuilder Class
StringBuffer Class