Master Java 21 Template Expressions for Clean String Interpolation

When writing Java code, string manipulation is common, and historically developers have used concatenation with +, StringBuffer / StringBuilder, String::format / String::formatted, or java.text.MessageFormat. Java 21 adds a new approach called Template Expressions (STR), which supports string interpolation and can also transform structured text into objects.

Template Expressions

Template Expressions allow developers to embed expressions inside a string using the syntax \{expression}. The STR processor evaluates these expressions and produces the final string.

String blog = "blog.didispace.com";
String str = STR."My blog is \{blog}";

The example above contains three parts: the STR processor ( STR), the template with an embedded expression ( \{blog}), and the dot operator that combines them. At runtime the result is My blog is blog.didispace.com.

Multi‑line Template Expressions

Similar to Java text blocks, multi‑line templates let developers compose HTML, JSON, XML, etc., in a readable way.

var json = STR.""
{
    "user": "\{name}",
    "age": \{age}
}"";

STR Processor Capabilities

Beyond simple interpolation, STR can perform arithmetic, call methods, and access object fields.

int x = 10, y = 20;
String s = STR."\{x} + \{y} = \{x + y}"; // results in "10 + 20 = 30"

String s = STR."My blog is \{getMyBlog()}"; // assuming getMyBlog() returns "blog.didispace.com"

User u = new User("didi", "blog.didispace.com");
String s = STR."\{u.name}的博客地址为：\{u.blog}"; // results in "didi的博客地址为：blog.didispace.com"

FMT Processor

The FMT processor provides the same interpolation ability as STR but also supports left‑aligned formatting for each column, useful for generating aligned tables.

String table = FMT.""
Description     Width    Height     Area
%-12s\{zone[0].name}  %7.2f\{zone[0].width}  %7.2f\{zone[0].height}  %7.2f\{zone[0].area()}
%-12s\{zone[1].name}  %7.2f\{zone[1].width}  %7.2f\{zone[1].height}  %7.2f\{zone[1].area()}
%-12s\{zone[2].name}  %7.2f\{zone[2].width}  %7.2f\{zone[2].height}  %7.2f\{zone[2].area()}
%".repeat(28) Total %7.2f\{zone[0].area() + zone[1].area() + zone[2].area()}
"";

The formatted output aligns each column neatly, producing a clean table of descriptions, widths, heights, and areas.

Java 21's String Templates are still in preview, and further capabilities such as custom processors are expected. Readers are encouraged to explore the official documentation for deeper insights.