Java Generics and Functions

Question

I have a function

public static void printTreeMap (TreeMap <Object, ArrayList> map, PrintStream ps)

and a TreeMap:

TreeMap <Integer, ArrayList<MyClass>> tm = new TreeMap<>();

When I'm trying to call printTreeMap like that: printTreeMap(tm, System.out);, I get an exception

The method printTreeMap(TreeMap<Object,ArrayList>, PrintStream) in the type task_v7 is not applicable for the arguments (TreeMap<Integer,ArrayList<MyClass>>, PrintStream) Java(67108979)

How can I fix it?

Answer 1

I would suggest this approach. Note that for general methods of this type there is no a priori knowledge of those types. Therefore, this relies solely on the types involved overriding toString().

public static <T,U> void printTreeMap(TreeMap<T,  ArrayList<U>> tm, PrintStream st) {
     tm.forEach((k,v)-> {
          st.println(k);
          for (U e : v) {
              st.println("  " + e);
          }
     });
}

Note that T and U need not be different as both could be the same type.

Here is an example.

TreeMap<Integer, ArrayList<MyClass>> tm = new TreeMap<>();
tm.put(10, new ArrayList<>(
        List.of(new MyClass("Alpha"), new MyClass("Beta"))));
tm.put(30, new ArrayList<>(
        List.of(new MyClass("Gamma"), new MyClass("Delta"))));

printTreeMap(tm, System.out);

Prints

10
  Alpha
  Beta
30
  Gamma
  Delta

The class

class MyClass {
    String a;
    
    public MyClass(String a) {
        this.a = a;
    }
    
    public String toString() {
        return a;
    }
}

A more versatile approach for any Map of Lists would be the following:

public static <L extends List<?>, M extends Map<?,L>> void printMapOfLists(M tm,
        PrintStream st) {
    tm.forEach((k, v) -> {
        st.println(k);
        for (Object e : v) {
            st.println("  " + e);
        }
    });
}