How and when the type gets resolved for generic methods in java?

I expect following Unit Test to fail with ClassCastException, but its passing.

The class has a generic method whose second parameter and return value are of type V.

On calling the method second time, with type of second parameter V as Integer, the return type should be Integer. But at runtime it actually returns String value.

import java.util.HashMap;
import java.util.Map;

import org.junit.Test;

public class GenericMethodTest {

  private class NonGenericClass {

    private final Map<Object, Object> myMap = new HashMap<>();

    <K, V> V addGeneric(K key, V value) {

      V existingV = (V) myMap.get(key);
      // why no ClassCastException on this above line, when type of V is Integer, but myMap.get(key) returns value of
      // type String?

      if (existingV == null) {
        myMap.put(key, value);
        return value;
      }
      return existingV;
    }
  }

  @Test
  public void test() {
    NonGenericClass nonGenericClass = new NonGenericClass();

    nonGenericClass.addGeneric("One", "One");
    // String valueString = (String) nonGenericClass.addGeneric("One", Integer.valueOf(1));
    // Compiler error as expected, if above line uncommented - Cannot cast from Integer to String.

    // But no error at run-time, and below call returns value of type String.
    nonGenericClass.addGeneric("One", Integer.valueOf(1));
  }

}