Within a class, a field that has the same name as a field in the superclass hides the superclass's field.
public class Test {
public static void main(String[] args) {
Father father = new Son();
System.out.println(father.i); //why 1?
System.out.println(father.getI()); //2
System.out.println(father.j); //why 10?
System.out.println(father.getJ()); //why 10?
System.out.println();
Son son = new Son();
System.out.println(son.i); //2
System.out.println(son.getI()); //2
System.out.println(son.j); //20
System.out.println(son.getJ()); //why 10?
}
}
class Son extends Father {
int i = 2;
int j = 20;
@Override
public int getI() {
return i;
}
}
class Father {
int i = 1;
int j = 10;
public int getI() {
return i;
}
public int getJ() {
return j;
}
}
Can someone explain the results for me?
In java, fields are not polymorphic.
Father father = new Son();
System.out.println(father.i); //why 1? Ans : reference is of type father, so 1 (fields are not polymorphic)
System.out.println(father.getI()); //2 : overridden method called
System.out.println(father.j); //why 10? Ans : reference is of type father, so 2
System.out.println(father.getJ()); //why 10? there is not overridden getJ() method in Son class, so father.getJ() is called
System.out.println();
// same explaination as above for following
Son son = new Son();
System.out.println(son.i); //2
System.out.println(son.getI()); //2
System.out.println(son.j); //20
System.out.println(son.getJ()); //why 10?
As per Overriding and Hiding Methods
The version of the hidden method that gets invoked depends on whether it is invoked from the superclass or the subclass.
i.e. when you invoke a method which is overridden in subclass via a super class reference the super class method is invoked and it access super class members.
This explains following as the reference used is of superclass:
System.out.println(father.i); //why 1?
System.out.println(father.j); //why 10?
System.out.println(father.getJ()); //why 10?
Similarly for the following:
System.out.println(son.getJ()); //why 10?
since getJ()
is not defined in Son
a Father
version is invoked which sees member defined in the Father
class.
If you read Hiding Fields; they specifically don't recommend such method of coding as
Generally speaking, we don't recommend hiding fields as it makes code difficult to read.
来源:https://stackoverflow.com/questions/12244670/hiding-fields-in-java-inheritance