创建一个通用方法 Print，它将接受所有类型的 collections 到它

Question

Why the print method for map is not working? the method should accept all generics and print them. it works fine for list, set, Queues but problem arises in map.

public class Question6 {

    @SuppressWarnings("unchecked")
    static void print(@SuppressWarnings("rawtypes") Collection c) {
        System.out.println(c.getClass());
        c.forEach(System.out::println);
        System.out.println();
    }
    
    public static void main(String[] args) {
        List<Integer> l = new ArrayList<>();
        l.add(1);
        l.add(2);
        print(l);
            
        List<Dummy> ld = new ArrayList<Dummy>();
        ld.add(new Dummy());
        print(ld);

        Map<Integer,Integer> m = new LinkedHashMap<Integer, Integer>();
        m.put(1,1);
        print(m); // gives error?
                
    }
}

class Dummy{
    
}

Answer 1

Map is not a Collection . You can either call your print method like this:

print(map.entrySet());
// or 
print(map.keys());
// or
print(map.values());

or overload it as follows:

static void print(Collection<?> c) {
    System.out.println(c.getClass());
    c.forEach(System.out::println);
    System.out.println();
}

static void print(Map<?, ?> map) {
    System.out.println(map.getClass());
    map.entrySet().forEach(System.out::println);
    System.out.println();
}