Java – questions about polymorphism and overloading

I try to understand the concepts of polymorphism and overloading I have the following code as an experiment However, I can't figure out why the program didn't run (it failed because of mobj. Foo (STR)) Use polymorphism to define mobj, and from the content I can collect, it should be myderivedclass type If this is true, will this line not work properly?

Why is that line invalid?

class MyBaseClass {
  protected int val;
  public MyBaseClass() { val = 1; }
  public void foo() { val += 2; }
  public void foo(int i) { val += 3; }
  public int getVal() { return val; }
}

class MyDerivedClass extends MyBaseClass {
  public MyDerivedClass () { val = 4; }
  public void foo() { val += 5; }
  public void foo(String str) { val += 6; }
}

class Test {
  public static void main(String[] args)
  {
    MyBaseClass mobj = new MyDerivedClass();
    String str = new String("hello");
    mobj.foo();
    mobj.foo(str);
    mobj.foo(4);
    System.out.println("val = " + mobj.getVal());
  }
}

Solution

Polymorphism is only valid when overriding methods defined by the parent, and mobj Foo (STR) is not Mybaseclass does not implement a class with a signature foo (string) So foo (string) implemented in myderivedclass does not cover anything Remember that Java distinguishes methods by name and parameter