Switch over type in java
Before I start, I know there are a bunch of answers to this question that suggest alternate approaches. I\'m looking for assistance to this particular approach as to whethe
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Here's an example of this which uses a simple object for each case.
package mcve.util; import java.util.*; import java.util.function.*; /** * Allows switch-like statements with classes and consumers. */ public final class ClassSwitch implements ConsumerA usage example would be something like this, assuming imports of e.g.
import static mcve.util.ClassSwitch.*;:cswitch(anObject, ccase(Byte.class, b -> System.out.println("Byte")), ccase(Short.class, s -> System.out.println("Short")), ccase(Integer.class, i -> System.out.println("Integer")), ccase(Long.class, l -> System.out.println("Long")), ccase(Float.class, f -> System.out.println("Float")), ccase(Double.class, d -> System.out.println("Double")) );You could also create a reusable object:
ClassSwitch ts = new ClassSwitch(ccase(String.class, System.out::println), ccase(Double.class, System.out::println)); ts.accept(anObject);Notes:
If you want a
defaultcase, you can useObject.classas the last case.There's no way to make a case which handles
null, but it could be modified a little bit for that. You could e.g. make aclass NullCasewhosetestmethod returnsobj == null.
What you could also do is actually generate overloads instead of using varargs. This lets you associate classes with consumers using just generic method declarations. The following is a fairly simple example of this:
package mcve.util; import java.util.*; import java.util.function.*; /** * Allows switch-like statements with classes and consumers. */ public final class GeneratedClassSwitch { private GeneratedClassSwitch() {} /** * Generates overloads for a class switch to System.out. * * For example, if max=4, then 5 methods are generated: * with 0, 1, 2, 3, and 4 cases. * * @param max * the number of cases in the largest overload generated * @param indents * the number of indents to indent each generated method * @throws IllegalArgumentException * if max is negative or greater than 26, or if indents * is negative */ public static void generateFixedOverloads(int max, int indents) { if (max < 0 || max > 26) { throw new IllegalArgumentException("max=" + max); } String indent = String.join("", Collections.nCopies(indents, " ")); for (int i = 0; i <= max; ++i) { System.out.print(indent); System.out.print("public static "); if (i > 0) { System.out.print("<"); for (char ch = 'A'; ch < 'A' + i; ++ch) { if (ch != 'A') { System.out.print(", "); } System.out.print(ch); } System.out.print("> "); } System.out.print("void cswitch"); if (i > 0) { System.out.println(); System.out.print(indent + " (Object o, "); for (char ch = 'A'; ch < 'A' + i; ++ch) { if (ch != 'A') { System.out.println(","); System.out.print(indent + " "); } System.out.print("Class<" + ch + "> class" + ch); System.out.print(", Consumer action" + ch); } } else { System.out.print("(Object o"); } System.out.println(") {"); for (char ch = 'A'; ch < 'A' + i; ++ch) { if (ch == 'A') { System.out.print(indent + " "); } else { System.out.print(" else "); } System.out.println("if (class" + ch + ".isInstance(o)) {"); System.out.print(indent + " "); System.out.println("action" + ch + ".accept(class" + ch + ".cast(o));"); System.out.print(indent + " "); System.out.print("}"); if (ch == ('A' + i - 1)) { System.out.println(); } } System.out.print(indent); System.out.println("}"); } } // Generated code pasted below. public static void cswitch(Object o) { } public static void cswitch (Object o, Class classA, Consumer actionA) { if (classA.isInstance(o)) { actionA.accept(classA.cast(o)); } } public staticclassC, Consumer actionC) { if (classA.isInstance(o)) { actionA.accept(classA.cast(o)); } else if (classB.isInstance(o)) { actionB.accept(classB.cast(o)); } else if (classC.isInstance(o)) { actionC.accept(classC.cast(o)); } } public static classC, Consumer actionC, Class classD, Consumer actionD) { if (classA.isInstance(o)) { actionA.accept(classA.cast(o)); } else if (classB.isInstance(o)) { actionB.accept(classB.cast(o)); } else if (classC.isInstance(o)) { actionC.accept(classC.cast(o)); } else if (classD.isInstance(o)) { actionD.accept(classD.cast(o)); } } public static classC, Consumer actionC, Class classD, Consumer actionD, Class classE, Consumer actionE) { if (classA.isInstance(o)) { actionA.accept(classA.cast(o)); } else if (classB.isInstance(o)) { actionB.accept(classB.cast(o)); } else if (classC.isInstance(o)) { actionC.accept(classC.cast(o)); } else if (classD.isInstance(o)) { actionD.accept(classD.cast(o)); } else if (classE.isInstance(o)) { actionE.accept(classE.cast(o)); } } public static classC, Consumer actionC, Class classD, Consumer actionD, Class classE, Consumer actionE, Class classF, Consumer actionF) { if (classA.isInstance(o)) { actionA.accept(classA.cast(o)); } else if (classB.isInstance(o)) { actionB.accept(classB.cast(o)); } else if (classC.isInstance(o)) { actionC.accept(classC.cast(o)); } else if (classD.isInstance(o)) { actionD.accept(classD.cast(o)); } else if (classE.isInstance(o)) { actionE.accept(classE.cast(o)); } else if (classF.isInstance(o)) { actionF.accept(classF.cast(o)); } } public static classC, Consumer actionC, Class classD, Consumer actionD, Class classE, Consumer actionE, Class classF, Consumer actionF, Class classG, Consumer actionG) { if (classA.isInstance(o)) { actionA.accept(classA.cast(o)); } else if (classB.isInstance(o)) { actionB.accept(classB.cast(o)); } else if (classC.isInstance(o)) { actionC.accept(classC.cast(o)); } else if (classD.isInstance(o)) { actionD.accept(classD.cast(o)); } else if (classE.isInstance(o)) { actionE.accept(classE.cast(o)); } else if (classF.isInstance(o)) { actionF.accept(classF.cast(o)); } else if (classG.isInstance(o)) { actionG.accept(classG.cast(o)); } } public static classC, Consumer actionC, Class classD, Consumer actionD, Class classE, Consumer actionE, Class classF, Consumer actionF, Class classG, Consumer actionG, Class classH, Consumer actionH) { if (classA.isInstance(o)) { actionA.accept(classA.cast(o)); } else if (classB.isInstance(o)) { actionB.accept(classB.cast(o)); } else if (classC.isInstance(o)) { actionC.accept(classC.cast(o)); } else if (classD.isInstance(o)) { actionD.accept(classD.cast(o)); } else if (classE.isInstance(o)) { actionE.accept(classE.cast(o)); } else if (classF.isInstance(o)) { actionF.accept(classF.cast(o)); } else if (classG.isInstance(o)) { actionG.accept(classG.cast(o)); } else if (classH.isInstance(o)) { actionH.accept(classH.cast(o)); } } } If you want to generate overloads, for example to have more than 8 cases, you can say something like the following:
GeneratedClassSwitch.generateFixedOverloads(16, 1);That will generate methods to
System.outthat follow the general form of:public staticclassC, Consumer actionC) { if (classA.isInstance(o)) { actionA.accept(classA.cast(o)); } else if (classB.isInstance(o)) { actionB.accept(classB.cast(o)); } else if (classC.isInstance(o)) { actionC.accept(classC.cast(o)); } } Notice that we're able to map each class type to its associated consumer type, i.e.
Class<A>withConsumerA>,Class<B>withConsumerB>, and so on. This is actually impossible with varargs (as of the current version of Java, anyway, which is 10).Our usage example is now like this, again assuming imports of e.g.
import static mcve.util.GeneratedClassSwitch.*;:cswitch(anObject, Byte.class, b -> System.out.println("Byte"), Short.class, s -> System.out.println("Short"), Integer.class, i -> System.out.println("Integer"), Long.class, l -> System.out.println("Long"), Float.class, f -> System.out.println("Float"), Double.class, d -> System.out.println("Double") );(Notes about
defaultcases andnullare the same as the first example.)
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