Created
August 19, 2025 19:28
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Named parameters with lambdas
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| package example; | |
| import java.io.Serializable; | |
| import java.lang.invoke.SerializedLambda; | |
| import java.lang.reflect.Method; | |
| import java.lang.reflect.Parameter; | |
| import java.util.Objects; | |
| import static example.Example.MyClass.*; | |
| import static java.util.Arrays.asList; | |
| public class Example { | |
| public static void main(String... args){ | |
| // arg1=null | |
| // arg2=Point[x=0, y=0] | |
| foo(o -> { }); | |
| //arg1=bar | |
| //arg2=Point[x=0, y=0] | |
| foo(o -> o.arg1 = "bar"); | |
| //arg1=bar | |
| //arg2=Point[x=5, y=2] | |
| foo(o -> { o.arg1 = "bar"; o.arg3 = new Point(5,2); }); | |
| //foo | |
| //null | |
| bar(o -> o.arg1 = "foo"); | |
| //null | |
| //foo | |
| bar(o -> o.arg2 = "foo"); | |
| } | |
| static class MyClass { | |
| public record Point(int x, int y){} | |
| public static class FooParams{ | |
| public String arg1 = null; | |
| public Point arg3 = new Point(0,0); | |
| } | |
| public static class BarParams{ | |
| String arg1 = null; | |
| String arg2 = null; | |
| } | |
| static void foo(Args<FooParams> args){ | |
| System.out.println( "arg1=" + args.args().arg1) ; | |
| System.out.println( "arg2=" + args.args().arg3) ; | |
| } | |
| public static void bar(Args<BarParams> o){ | |
| System.out.println(o.args().arg1); | |
| System.out.println(o.args().arg2); | |
| } | |
| } | |
| interface Args<T> extends MethodFinder { | |
| void accept(T args); | |
| default T args() { | |
| try { | |
| T t = (T) parameter(0).getType().newInstance(); | |
| accept(t); | |
| return t; | |
| } catch (Exception e) { | |
| throw new RuntimeException(e); | |
| } | |
| } | |
| } | |
| public interface MethodFinder extends Serializable { | |
| default SerializedLambda serialized() { | |
| try { | |
| Method replaceMethod = getClass().getDeclaredMethod("writeReplace"); | |
| replaceMethod.setAccessible(true); | |
| return (SerializedLambda) replaceMethod.invoke(this); | |
| } catch (Exception e) { | |
| throw new RuntimeException(e); | |
| } | |
| } | |
| default Class<?> getContainingClass() { | |
| try { | |
| String className = serialized().getImplClass().replaceAll("/", "."); | |
| return Class.forName(className); | |
| } catch (Exception e) { | |
| throw new RuntimeException(e); | |
| } | |
| } | |
| default Method method() { | |
| SerializedLambda lambda = serialized(); | |
| Class<?> containingClass = getContainingClass(); | |
| return asList(containingClass.getDeclaredMethods()) | |
| .stream() | |
| .filter(method -> Objects.equals(method.getName(), lambda.getImplMethodName())) | |
| .findFirst() | |
| .orElseThrow(UnableToGuessMethodException::new); | |
| } | |
| default Parameter parameter(int n) { | |
| return method().getParameters()[n]; | |
| } | |
| class UnableToGuessMethodException extends RuntimeException {} | |
| } | |
| } |
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