String Templates

NOTE: This is a preview feature NOT enabled by-default in JDK-21.

String Templates are introduced as per java specification JPE-430.

They provide a way to introduce variables into strings which are resolved during runtime.

public class StringTemplate {

	private static String name = "T2P";
	private static String message = STR."Greetings from \{name}!";
	
	public static void main(String[] args) {
		System.out.println(message);	//Expected Output: Greetings from T2P!
	}
}

Lets say we run this program via command prompt

java StringTemplate.java

This code will not compile and we’ll get below errors

StringTemplate.java:4: error: string templates are a preview feature and are disabled by default.
        private static String message = STR."Greetings from \{name}!";
                                            ^
  (use --enable-preview to enable string templates)
1 error
error: compilation failed

So in order to resolve it, we need to enable the preview feature using --enable-preview for jdk 21 using --source 21

java --enable-preview --source 21 StringTemplate.java

Now we can see the program compiles correctly and produces the below output

Note: StringTemplate.java uses preview features of Java SE 21.
Note: Recompile with -Xlint:preview for details.
Greetings from T2P!

Finally we got our code running 🤩.

Few more points on String Templates

1. STR template processor is the part of the language and is automatically included in every class.
2. Not only local variables but methods and instance variables are also supported.

    String message = STR."Greetings from \{name}!";        //local variable
     String message = STR."Greetings from \{getName()}!";   //method
     String message = STR."Greetings from \{this.name}!";   //instance variable
   

3. Mathematical Expressions too.

    int a = 10, b = 30;
     String result = STR."\{a+b}";	//40
   

4. Double quotes need not to be escaped in String Templates

    boolean test = true;
     String msg = STR."The result is \{ test? "true" : "false" }.";	//The result is true.
   

5. Multi-line expressions are allowed too.

    import java.time.LocalDateTime;
     String currentDateTime = STR."Current Date and Time is: \{
     LocalDateTime.now()
     }";
     //Current Date and Time is: 2023-10-07T08:55:36.874386100