help me.. how to apply inheritance&polymorphsm in GUI

[ice]

hello,

all i know, GUI is only an interface..
what code should i use/add/change to implement inheritance& polymorphsm in GUI?
can i use same way to implement linkedlist in GUI?

thank you..

[Belisarius]

All inheiritence is is when you extend a class and add some functionality. It's very straight forward - simply use the "extends" keyword when defining a class. It's covered under Learning the Java Language.

Polymorphism is a very simple concept that I can't remember the text-book definition of right now. Basically you start to use it naturally and don't think of it as ploymorphism. I have to run but when I get the chance I'll look up what it means and give you a few examples.

[Belisarius]

Polymorphism is basically accessing a property of an object without knowing exactly what object that is. The most basic example in Java would be the "toString()" method implemented by all Objects. Take this code for instance (note, this isn't really legal Java code, it's just enough to illustrate the point):

// Define some example classes

class A {
  public String toString(){
    return "foo";
  }
}

class B {
  public String toString(){
    return "bar";
  }
}

// Define a method in some class

public void print(Object test){
  System.out.println(test);
}

// Put this in your main method

A a = new A();
B b = new B();

print(a);

Let's look at what is going on. First, look at the "print()" method. We told it to accept all objects of type "Object". Well, in Java, all objects are implicitly (that is they are by default) Objects in addition to their own types. That means A is an "Object" and B is and "Object". That also means that they can call all methods defined for "Object", one of which is "toString()".

Now, we wanted the "toString()" method to work differently in A than how it works in "Object" or B. So we "overrode" it, or rewrote it. This new "toString()" method only effects objects of type "A". Same goes for the "toString()" method defined in "B". But, because it was defined in the parent class "Object", Java knows it will *always* be defined. If we hadn't written a "toString()" method in "A" and "B", then it would have simply inheirited the "toString()" method written for "Object". Because Java knows that method will always be there, in our "print()" method, we can safely call it no matter what kind of "Object" we pass to it.

Now, you might be wondering how Java knows that the Object that we pass to "print()" is an A or B object, and which "toString()" method to call. Java keeps track of this all internally so you don't have to. If you tell a method to accept a certain object, it will accept not only that object, but all of its children (classes that inheirt/extend it) as well. However, only those methods and properties defined in the parent class (in this case, Object) will be accessible. That is, if we defined a "exampleMethod()" class in "A", in "print()" we could not say "test.exampleMethod()". We would have to "cast" the "test" object to "A", but that is outside the scope of this discussion.

To summarize, polymorphism (in Java at any rate) is the ability to access properties of a given object by calling on what is defined in the parent class without knowing exactly what the child class is.

[destin]

Originally Posted by Belisarius

// Define some example classes

class A {
  public String toString(){
    return "foo";
  }
}

class B {
  public String toString(){
    return "bar";
  }
}

// Define a method in some class

public void print(Object test){
  System.out.println(test);
}

// Put this in your main method

A a = new A();
B b = new B();

print(A);

In Java 5, there is annotation to override methods:

class A {
    @Override public String toString() {
        return "foo";
    }
}

Personally, I like this. It lets you know when you are actually overriding a method. Prior to this, you would write out the method and wouldn't be sure if you were overriding the method. If, for example, you write toStirng() instead of toString(), the code would compile, but not run as expected. With the override annotation you will receive an error telling you that there is no method toStirng() to override.

@Belisarius
I think you meant that last bit to be print(a);, rather than print(A);.

[Belisarius]

You're right, I corrected the code. Just a typo. And I didn't realize that was a change in 1.5, although it should accept the old way as it's suppose to be backwards compatable.

[destin]

Originally Posted by Belisarius

And I didn't realize that was a change in 1.5, although it should accept the old way as it's suppose to be backwards compatable.

It does accept the old way; I was just pointing this out.

[ice]

Originally Posted by Belisarius

All inheiritence is is when you extend a class and add some functionality. It's very straight forward - simply use the "extends" keyword when defining a class. It's covered under Learning the Java Language.

Polymorphism is a very simple concept that I can't remember the text-book definition of right now. Basically you start to use it naturally and don't think of it as ploymorphism. I have to run but when I get the chance I'll look up what it means and give you a few examples.

i did learn polymorphsm but i do not know how to apply it in interface(GUI) that i created.
for example, i create an interface (GUI) for user to enter his personal data, his D.O.B and address. my program allow many users to enter their data, how can i ensure that i can store the previous data and retrieve anyone's data?


thank you..

[Belisarius]

That's not inheirently a polymorphism problem, but you could turn it into one by saying that there are different types of users.

The simplest way to store the information would be to create an object to store all the properties. For example, create a Person class with variables for the DOB, address, etc. You can then write the object to a file in order to store it and read the file in order to load it back up. A simple way to do this would be to use Serialization.

To showcase ploymorphism, you could create different users, such as, say, an InetPerson, which would contain all the information to contact them online - it would be an extension of the Person object.