Same generic type in two different classes in Java

Question

I have two different classes:

class A<K,V> {
  B b = new B;

  public V get(K k) {
     return b.get(k);
  }
}

class B<K,V> {
  V v;

  public V get(K k) {
     return v;
  }
}

I get an incompatible types error in the get method of class A.

Does that mean type V of class A is not the same as type V of class B? In that case, how do I fix this issue? Can I simply typecast it to V before returning?

Answer 1

Parametrize B with V in A . This way the compiler can be sure that your class B will return an object of type V .

class A<K,V> {
  B<K,V> b = new B<K,V>;

  public V get(K k) {
     return b.get(k);
  }
}

Answer 2

Change B b = new B; inside class A to B<K,V> b = new B<>();

This makes sure that the types referenced by K and V within the definition of B for the variable b match up with those within the definition of A .