StringConcatFactory Example

Main.java

//https://stackoverflow.com/questions/65139541

import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.MethodType;
import java.lang.invoke.StringConcatException;
import java.lang.invoke.StringConcatFactory;
import java.time.LocalTime;
import java.time.temporal.ChronoField;
import java.util.spi.ToolProvider;

public class Main {
    public static void main(String[] args) throws Throwable {

        String time = switch(LocalTime.now().get(ChronoField.HOUR_OF_DAY) / 6) {
            case 0 -> "night"; case 1 -> "morning"; case 2 -> "afternoon";
            case 3 -> "evening"; default -> throw new AssertionError();                
        };

        System.out.println("Hello "+System.getProperty("user.name")+", good "+time+"!");

        String tmp = "prefix \1 "+time+" \2 suffix";

        invokeManually(time);

        showBytecode();
    }

    private static void invokeManually(String time) throws Throwable {
        MethodHandle mh = StringConcatFactory.makeConcatWithConstants(
                MethodHandles.lookup(), // normally provided by the JVM
                "foobar", // normally provided by javac, but meaningless here
                // method type is normally provided by the JVM and matches the invocation
                MethodType.methodType(String.class, String.class),
                "Hello \2, good \1!", // recipe, \1 binds a parameter, \2 a constant
                System.getProperty("user.name") // the first constant to bind
        ).getTarget();

        // we can now use the handle to perform a concatenation
        // the argument types must match the MethodType specified above
        String result = (String)mh.invokeExact(time);

        System.out.println(result);
    }

    private static void showBytecode() {
        System.out.println();
        ToolProvider.findFirst("javap")
            .ifPresent(tp -> tp.run(System.out, System.err, "-v", "-c", Main.class.getName()));
    }

    private Main() {}
}