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someDerivedClass myObject = new someDerivedClass()
someParentClass myObject = new someDerivedClass()
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Deer d = new Deer(); Cat c = new Cat(); Shark s = new Shark(); Animal a; a = d; //Valid, because a Deer is an Animal object a= c; //Valid, because a Cat is an Animal object a = s; //Valid, because a Shark is an Animal object s = a; //Invalid. An Animal is not a Shark
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Back to topclass ParentClass // extends java.lang.Object
{
// ALL CLASSES IMPLICITLY EXTEND OBJECT!
// This means that all objects have access to the
// Object class methods, such as toString and hashCode.
public void parentMethod()
{
System.out.println("Printing from the parent.");
}
}
class ChildOne extends ParentClass
{
public void childOneMethod()
{
System.out.println("Printing from child one.");
}
}
class ChildTwo extends ParentClass
{
public void childTwoMethod()
{
System.out.println("Printing from child two.");
}
// What follows is called "Overriding", which is when a child class
// implements a method of the same name and supplied arguments as
// a method in the parent class. You cannot override private methods,
// as the compiler will never allow access to private methods in the
// parent, nor can you override final methods, the compiler will cause
// a compile error.
public void parentMethod()
{
childTwoMethod();
}
}
public static void someMethod(ParentClass someObject)
{
someObject.parentMethod();
}
You can pass any ParentClass object, or better, any subclass of the ParentClass object as the argument to someMethod. Let's try it. public static void main(String[] args)
{
ChildOne one = new ChildOne();
ChildTwo two = new ChildTwo();
someMethod(one);
someMethod(two);
}
You'll notice this will first print the version of parentMethod in the ParentClass object, since ChildOne does not override parentMethod, but then it will print from the ChildTwo derived class since ChildTwo does override parentMethod. And that is polymorphism!Edited by ZekeDragon, 23 April 2011 - 01:59 PM.
Programming Professional
public static void main(String[] args)
{
ParentClass one = new ChildOne();
ParentClass two = new ChildTwo();
someMethod(one);
someMethod(two);
}
will the output be the same?
Writes binary right handed and hex left handed
System.out.println("Printing from the parent.");
While ChildTwo doeschildTwoMethod(); //--> System.out.println("Printing from child two.");
Newbie
public static void main(String[] args)
{
ParentClass one = new ChildOne();
ParentClass two = new ChildTwo();
someMethod(one);
someMethod(two);
}
public static void main(String[] args)
{
ChildOne one = new ChildOne();
ChildTwo two = new ChildTwo();
someMethod(one);
someMethod(two);
}
wim DC said:
System.out.println("Printing from the parent.");
While ChildTwo doeschildTwoMethod(); //--> System.out.println("Printing from child two.");Printing from the parent. Printing from child two. Process completed.
Edited by scottbomb, 26 April 2011 - 08:43 AM.
Writes binary right handed and hex left handed
class ParentClass // extends java.lang.Object
{
public void parentMethod()
{
System.out.println("Printing from the parent.");
}
}
class ChildOne [B]extends ParentClass[/B]
{
public void childOneMethod()
{
System.out.println("Printing from child one.");
}
}
It extends ParentClass, so you could actually see it as:
class ChildOne [strike][B][COLOR="red"]extends ParentClass[/COLOR][/B][/strike]
{
public void childOneMethod()
{
System.out.println("Printing from child one.");
}
[B][COLOR="blue"]public void parentMethod()
{
System.out.println("Printing from the parent.");
}[/COLOR][/B]
}
ChildOne one = new ChildOne(); one.parentMethod(); OUTPUT: Printing from the parent
class ChildTwo extends ParentClass
{
public void childTwoMethod()
{
System.out.println("Printing from child two.");
}
public void parentMethod()
{
childTwoMethod();
}
}
Which, as ZekeDragon mentioned has 'Overridden' the parentMethod(). It's common to also have an annotation there to make it more clear, like followed:
class ChildTwo extends ParentClass
{
public void childTwoMethod()
{
System.out.println("Printing from child two.");
}
[B]@Override[/B]
public void parentMethod()
{
childTwoMethod();
}
}
That annotation doesn't really change anything, it just makes it more clear for the user so he/she knows that method is overridden.
class ChildTwo [B][COLOR="red"][strike]extends ParentClass[/strike][/COLOR][/B]
{
public void childTwoMethod()
{
System.out.println("Printing from child two.");
}
public void parentMethod()
{
childTwoMethod();
}
[B][COLOR="blue"]public void parentMethod()
{
System.out.println("Printing from the parent.");
}[/COLOR][/B]
}
Now, you see there is a conflict there, ChildTwo now has 2 methods with the same name and parameters: parentMethod().
class ChildTwo [B][COLOR="red"][strike]extends ParentClass[/strike][/COLOR][/B]
{
public void childTwoMethod()
{
System.out.println("Printing from child two.");
}
public void parentMethod()
{
childTwoMethod();
}
}
With only his own parentMethod.ChildTwo two = new ChildTwo(); two .parentMethod();outputs:
OUTPUT: Printing from child two.
me: ChildOne one = new ChildOne(); ZekeDragon: ParentClass one = new ChildOne();And both code worked.
public void childOneMethod()
{
System.out.println("Printing from child one.");
}
And ZekeDragon can't access that one, because it's declared as a ParentClass object.ChildOne one = new ChildOne(); ParentClass one2 = new ChildOn(); one.childOneMethod(); //No problem here one2.childOneMethod(); //Big problem here
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