尝试在Java8中使用Map<类、Function<T、R>>作为instanceOf的替代，无法放入映射

import java.util.HashMap;
import java.util.Map;
import java.util.function.Function;

public class InstanceOfMapper<T, R> {
    private Map<Class<? extends T>, Function<T, R>> typeMap;

    public InstanceOfMapper() {
        typeMap = new HashMap<>();
    }

    public void addMapping(Class<? extends T> clazz, Function<T, R> function) {
        typeMap.put(clazz, function);
    }

    public R apply(T obj) {
        for (Class<?> clazz : typeMap.keySet()) {
            if (clazz.isInstance(obj)) {
                return typeMap.get(clazz).apply(obj);
            }
        }
        // 没有匹配的类型
        return null;
    }
}

public class Main {
    public static void main(String[] args) {
        InstanceOfMapper<Number, String> mapper = new InstanceOfMapper<>();

        // 添加类型和对应的处理函数
        mapper.addMapping(Integer.class, num -> "整数：" + num.intValue());
        mapper.addMapping(Double.class, num -> "浮点数：" + num.doubleValue());
        mapper.addMapping(Long.class, num -> "长整数：" + num.longValue());
        mapper.addMapping(Byte.class, num -> "字节：" + num.byteValue());

        // 测试不同类型的对象
        Number num1 = 100;
        Number num2 = 3.14;
        Number num3 = 9999999999L;
        Number num4 = (byte) 8;

        System.out.println(mapper.apply(num1));
        System.out.println(mapper.apply(num2));
        System.out.println(mapper.apply(num3));
        System.out.println(mapper.apply(num4));
    }
}

整数：100
浮点数：3.14
长整数：9999999999
字节：8