Last Updated on September 5, 2024

With the introduction of string templates in Java 21 JEP 430, as a preview feature, proposed to be finalized with JEP 465, the language takes another significant step forward in simplifying and enhancing string manipulation. This feature, inspired by similar capabilities in other languages, promises to make string formatting more intuitive, readable, and less error-prone.

String templates represent a paradigm shift in how Java developers can construct and manipulate strings. They offer a more expressive and concise syntax compared to traditional string concatenation or formatting methods. By allowing developers to embed expressions directly within string literals, string templates reduce boilerplate code and improve code clarity.

We’ll examine how this feature addresses common pain points in string handling, its impact on code readability and maintainability, and the ways it can enhance developer productivity.

Traditional String Manipulation

Concatenation

String name = "Alice";int age = 30;
String traditional = name + " is " + age + " years old.";

String.format()

String name = "Alice";int age = 30;
String formatted = String.format("%s is %d years old.", name, age);

StringBuilder

String name = "Alice";int age = 30;
StringBuilder sb = new StringBuilder();
sb.append(name).append(" is ").append(age).append(" years old.");
String built = sb.toString();

As we will see, in each case, the string template version is more concise and directly shows the structure of the resulting string, improving readability and reducing the potential for errors.

Overview of Built-in Templates

Java’s built-in string templates introduce a new syntax that allows for direct embedding of expressions within string literals. This feature is designed to simplify common string manipulation tasks while providing a more intuitive and readable approach to string formatting.

STR processor

Java uses the STR prefix followed by a string literal containing placeholders for expressions. These placeholders are denoted by \{expression}.

Here’s a simple example:

String name = "Alice";
int age = 30;
String message = STR."\{name} is \{age} years old.";
System.out.println(message); // Output: Alice is 30 years old.

This syntax eliminates the need for explicit concatenation or formatting methods, making the code more concise and easier to read.

String templates can include various types of expressions within the placeholders

You can call methods within the placeholders.

String upperName = STR."\{name.toUpperCase()} is \{age} years old.";

Perform calculations directly within the template.

String futureAge = STR."In 5 years, \{name} will be \{age + 5} years old.";

Access object properties or methods.

Person person = new Person("Bob", 25);
String info = STR."\{person.getName()} is \{person.getAge()} years old.";

String templates support multiline strings, making it easier to format complex text:

String multiline = STR."""
    Name: \{name}
    Age: \{age}
    Occupation: \{occupation}
    """;

FMT (Formatted Strings) Processor

The FMT processor is used for creating formatted strings, similar to String.format() but with a more intuitive syntax. It allows for precise control over the formatting of numbers, dates, and other types.

double price = 19.99;
String formatted = FMT."The price is $%.2f\{price}";
System.out.println(formatted); // Output: The price is $19.99

The FMT processor supports all the formatting options available in String.format(), making it a powerful tool for creating precisely formatted strings.

RAW (Raw Strings) Processor

The RAW processor treats the template as a raw string, not processing any embedded expressions. This is useful when you need to include text that contains characters that might otherwise be interpreted as part of a template expression.

String raw = RAW."This is a \{literal} string with \{braces}";
System.out.println(raw); // Output: This is a \{literal} string with \{braces}

Benefits of Built-in Templates

  1. Improved Readability: String templates make the code more readable by clearly showing where variables or expressions are inserted into the string.
  2. Reduced Errors: By eliminating the need for index-based placeholders (as in String.format()), string templates reduce the likelihood of formatting errors.
  3. Type Safety: The Java compiler can perform type checking on the expressions within the templates, catching potential type mismatches at compile-time rather than runtime.
  4. Performance: String templates are optimized by the Java compiler, potentially offering better performance compared to concatenation or StringBuilder in certain scenarios.

Custom String Template Processors

While the built-in template processors cover many common use cases, Java also allows developers to create custom string template processors. This feature enables you to define your own logic for processing string templates, opening up a wide range of possibilities for specialized string manipulation.

To create a custom template processor, you need to implement the StringTemplate.Processor interface. This interface defines a single method, process, which takes a StringTemplate object as its parameter and returns the processed result.

public class CustomProcessor implements StringTemplate.Processor<String, RuntimeException> {
    @Override
    public String process(StringTemplate stringTemplate) {
        // Custom processing logic goes here
        return processedString;
    }
}

Let’s create a simple custom processor that converts all embedded expressions to uppercase.

public class UppercaseProcessor implements StringTemplate.Processor<String, RuntimeException> {
    @Override
    public String process(StringTemplate stringTemplate) {
        StringBuilder result = new StringBuilder();
        for (var fragment : stringTemplate.fragments()) {
            result.append(fragment);
            if (stringTemplate.values().size() > result.length() - fragment.length()) {
                String value = stringTemplate.values().get(result.length() - fragment.length()).toString();
                result.append(value.toUpperCase());
            }
        }
        return result.toString();
    }
}

To use this custom processor:

var UPPER = new UppercaseProcessor();
String name = "Alice";
int age = 30;
String result = UPPER."Hello, \{name}! You are \{age} years old.";
System.out.println(result); // Output: Hello, ALICE! You are 30 years old.

Considerations When Creating Custom Processors

  1. Performance: Custom processors may have performance implications, especially for complex operations. Always profile and optimize your custom processors for your specific use case.
  2. Error Handling: Consider how your custom processor will handle invalid input or edge cases. You may want to throw custom exceptions for better error reporting.
  3. Thread Safety: If your custom processor maintains any state, ensure it’s thread-safe if it will be used in a multi-threaded environment.
  4. Composability: Consider designing your custom processors in a way that allows them to be composed or chained with other processors for more complex operations.

Conclusion

Java string templates represent a significant leap forward in the language’s string manipulation capabilities.

This feature, introduced as a preview in Java 21, addresses long-standing pain points in string handling and formatting, offering developers a more intuitive, readable, and flexible approach to working with strings.

The introduction of string templates in Java marks a paradigm shift in how developers construct and manipulate strings. By allowing direct embedding of expressions within string literals, this feature simplifies common string operations, reducing boilerplate code and improving code clarity. The syntax is more expressive and less error-prone compared to traditional methods like string concatenation or String.format().

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