从Parent类型的实例调用child的方法

Question

I have to make the following equals method:

public boolean equals(ParentClass pa) {
    if (!(pa instanceof ChildClass))
        return false;
    boolean sameID = ID.equals(pa.getID());
    return super.equals(pa) && sameID;
}

The class ChildClass is a child of the class ParentClass, and this would be it's equals method. The method getId() is only in the ChildClass, but I need to know how to get the method pa.getID() to work, is there any way to do a cast here? since i get an error, the compiler can't be sure if pa is an instance of Child or not.

This is the erro that i get:

Error:(19, 73) java: cannot find symbol
  symbol:   method getID()
  location: variable pa of type ParentClass

I need the method to be of type ParentClass, because the function should return false when an object of type ParentClass instead of an error.

Any help would be appreciated.

Answer 1

After you check the type of object and ensure that it's instance of ChildClass just convert it to ChildClass type and use it:

public boolean equals(ParentClass pa) {
if (!(pa instanceof ChildClass))
    return false;
ChildClass child = (ChildClass) pa;
boolean sameID = ID.equals(child.getID());
return super.equals(child) && sameID;
}

Answer 2

There's a common pattern that's useful to follow when overriding equals . I'll provide a version with the corrections here, and outline what those corrections were below:

@Override
public boolean equals(Object o) {
    if (this == o) { return true; }
    if (!super.equals(o) 
        || o == null 
        || getClass() != o.getClass()) { 
            return false; 
    }
    ChildClass other = (ChildClass) o;
    return this.getID().equals(other.getID());
}

First, the stuff you can generally copy-paste between implementations of equals :

• Always @Override the equals method - this will help identify issues where you're accidentally overloading it (same method name, different argument types). Overloading equals is a bad practice that will cause problems.

• Start with == check - if this passes, no need to waste CPU cycles doing any more logic. Though this is really just a minor optimization, it's a useful best practice.

• If you are extending a class other than Object , check for super equality.

• Check to make sure the classes are the same . Note that this is not an instanceof check.

  • Remember, equals must be symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
  • If you use instanceof , you'll likely run into case where parent.equals(child) == true but child.equals(parent) == false - that's bad.
  • There are cases where instanceof is ok, but checking class equality is safer and more correct most of the time.

Now, the class specific stuff:

• Cast the Object parameter to your class type, and compare the ones you care about for equality.
  • Note that the calls on this should match the calls on other - in your case, I changed both to call getID() method. Alternatively, both can refer to the private member ID - but they should be consistent.

Finally, don't forget that equals and hashCode are a pair - you should always implement them together . (I know you didn't specifically ask about it, but I'm pointing it out as it's a common error people make when implementing equals )

• If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result. If you only override equals() and not hashCode() your class violates this contract.

I hope this helps you and anyone else with problems implementing equals .