Experiment with mixins

Author: Nicolai Parlog

src/main/java/org/codefx/demo/java10/lang/var/Mixins.java

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+package org.codefx.demo.java10.lang.var;
+
+import java.math.BigDecimal;
+import java.util.function.Supplier;
+
+public class Mixins {
+
+	/*
+	 * DOWNSIDES
+	 *
+	 *  - requires considerable amount of boilerplate
+	 *  - has a non-trivial structure
+	 *  - delegating interface has less flexibility than a class
+	 *  - Object's methods can not be default-implemented,
+	 *    so toString, equals, hashCode, ... can never be forwarded
+	 *    to existing implementation
+	 *
+	 * ALTERNATIVES
+	 *
+	 *  - extension type collecting all needed methods
+	 *  - utility methods
+	 */
+
+	public void report(Megacorp megacorp) {
+		// without `var` there would be no way to declare a variable
+		// of type `IsSuccessful` and `IsEvil`
+		var corp = (MegacorpDelegate & IsSuccessful & IsEvil) () -> megacorp;
+		System.out.printf(
+				"Corporation %s is %s and %s.\n",
+				corp.name(),
+				// relying on `IsSuccessful`
+				corp.isSuccessful() ? "successful" : "a failure",
+				// relying on `IsEvil`
+				corp.isEvil() ? "evil" : "a failure"
+		);
+	}
+
+	// important domain concept, used throughout the system
+	interface Megacorp {
+
+		String name();
+
+		BigDecimal earnings();
+
+		BigDecimal taxes();
+
+	}
+
+	// created right next to `Megacorp` to allow easy extension
+	// throughout the system
+	@FunctionalInterface
+	interface MegacorpDelegate extends Megacorp {
+
+		// there can only be this one abstract method
+		Megacorp delegate();
+
+		@Override
+		default String name() {
+			return delegate().name();
+		}
+
+		@Override
+		default BigDecimal earnings() {
+			return delegate().earnings();
+		}
+
+		@Override
+		default BigDecimal taxes() {
+			return delegate().taxes();
+		}
+
+		// because `Object` methods can not have default implementations,
+		// there can be no delegations for toString, equals, hashCode, ... :(
+
+	}
+
+	// these are concepts that are only useful in a very narrow part of the system
+	// and so they are not added to the original interface to prevent polluting it
+	// with too many methods;
+	// these mixins must not have abstract methods
+
+	interface IsSuccessful extends Megacorp {
+
+		final BigDecimal SUCCESS_BOUNDARY = new BigDecimal("500000000");
+
+		default boolean isSuccessful() {
+			return earnings().compareTo(SUCCESS_BOUNDARY) > 0;
+		}
+
+	}
+
+	interface IsEvil extends Megacorp {
+
+		default boolean isEvil() {
+			return true;
+		}
+
+	}
+
+}