polymorphic mapper, Java

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//'main' method must be in a class 'Rextester'.
//Compiler version 1.8.0_111

import java.util.function.*;
import java.util.stream.*;
import java.util.*;
import java.lang.*;

class Rextester
{  
    public static void main(String args[])
    {
        Function<A, Stream<P>> mapper = PolymorphicMapper.<A, Stream<P>> builder()
            .caseClass(B.class, b -> IntStream.range(0, b.value).mapToObj(num -> new P(String.format("%02d", num))))
            .caseClass(C.class, c -> Stream.of(new P(String.format("%s %s", c.s1, c.s2))))
            .build();
        
        List<A> inputs = Arrays.asList(new B(1), new C("hello", "world"), new B(10), new C("bonjour", "toi"));
        
        String result = inputs.stream()
            .map(mapper)
            .flatMap(Function.identity())
            .map(P::toString)
            .collect(Collectors.joining(", "));
        
        System.out.println(result);
    }
    
    static abstract class A {
        
    }
    
    static class B extends A {
        public int value;
        
        B(int value) {
            this.value = value;
        }
    }
    
    static class C extends A {
        public String s1;
        public String s2;
        
        C(String s1, String s2) {
            this.s1 = s1;
            this.s2 = s2;
        }
    }
    
    static class P {
        public String id;
        
        P(String id) {
            this.id = id;
        }
        
        @Override
        public String toString() {
            return String.format("P(id=%s)", id);
        }
    }
    
    static class PolymorphicMapper<T, R> {
        
        static <T, R> PolymorphicMapper<T, R> builder() {
            return new PolymorphicMapper<T, R>();
        }
        
        private Map<Class<?>, Function<Object, R>> mappers = new LinkedHashMap<>();
        
        private PolymorphicMapper() {
            
        }
        
        @SuppressWarnings("unchecked")
        public <K extends T> PolymorphicMapper<T, R> caseClass(Class<K> targetClass, Function<K, R> mapper) {
            mappers.put(targetClass, (Function<Object, R>) mapper);
            
            return this;
        }
        
        public Function<T, R> build() {
            return value -> {
                for (Map.Entry<Class<?>, Function<Object, R>> entry: mappers.entrySet()) {
                    if (entry.getKey().isInstance(value)) {
                        return entry.getValue().apply(value);
                    }
                }
                
                throw new IllegalArgumentException("Could not find mapper for value: " + value);
            };
        }
    }
}

[ + ] Show input
Compilation time: 1.55 sec, absolute running time: 0.37 sec, cpu time: 0.38 sec, memory peak: 25 Mb, absolute service time: 1,93 sec