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Consider the following code:
public class MyThread extends Thread {
@Override
public void run(){
System.out.println("Testing thread.");
}
}
public class App
{
public static void main( String[] args )
{
MyThread thread = new Thread();
}
}
Why does the syntax checker not accept line 5 in 'App.java' (The one with the MyThread Reference Type)? If MyThread is aThread, then why is it incorrect to create a new Thread out of it? I know I'm supposed to replace it with new MyThread, but why is Thread not allowed? Doesn't 'extend' mean 'inherits all properties etc.'?
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CsTreval wrote: If MyThread is aThread, then why is it incorrect to create a new Thread
out of it?
Your code is stating that 'Thread' is a 'MyThread' which it isn't.
The correct code would be..
Thread thread = new MyThread();
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You should be creating the same type as you have declared thus:
MyThread thread = new MyThread();
One of these days I'm going to think of a really clever signature.
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Yes, but there is also polymorphism as jschell said: Thread thread = new MyThread. I just got the assignment backwards. To me it is more logical to think of a=b instead b=a.
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Thread thread = new MyThread();
...works, cause Thread is the parental to MyThread.
Using that leaves you with the basic functionality that is given in Thread (unless you cast Thread to a MyThread, then you'll be able to access the additional functions).
You could add additional functionality to any Object when you extend it - which is the basic meaning of extension.
So when initializing the Object you will have to choose the right one - the parental Object does not own the additional methods, does not know them.
You would not be able to access them.
Example:
public class Vehicle{
}
public class Bus extends Vehicle{
public int getSeatQuantity(){
}
}
A vehicle can be anything from a bike up to a race car, truck, even a train.
It's not important for a bicycle or a race car how many seats it has. But it's important for a bus.
So when you want a Bus (...and later probably know how much seats are available) you will have to create a Bus and not just a Vehicle.
Simple said: it's more specific.
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You've got the idea now, but consider this as another way of looking at it:
a MyThread is a Thread and then some. If you had added extra class members - code or data - in the definition of MyThread, it would have stood out like the proverbial.
Another clue is in Java's use of the keyword extends.
Cheers,
Peter
Software rusts. Simon Stephenson, ca 1994. So does this signature. me, 2012
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A super class reference variable has the capability to hold its sub class object but a sub class does not have that capability.
As MyThread is subclass of Thread, you can write
Thread t=new MyThread();
But not
MyThread m=new Thread();
That's false.
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