Java generic classes

Question

I understand that in the fields of a class, you can make the field type the name of another class and then in the constructor initialise that field by calling a new class of that type, i.e.

public class Auction {
    private Bid bid;
}
public Auction {
    bid = new Bid();
}

The main reason for doing this is, as I understand it, to access the methods of that class.

My question is I've noticed in some methods that there are local variables created that have a type of a different class with a variable name. What is the purpose of assigning a local variable name with a type of another class? Is this another way of just accessing those methods directly, even if it hasn't been done in the fields or constructor?

What you ask is unclear, can you elaborate (with some pseudo code) on "what is the purpose of assigning a local variable name with a type of another class?" — , Oct 08 '16 at 08:14
This has nothing to do with generics, you're setting a field to have a value which is the type of *another* class. Java `String` is a common class (I presume) you're used to using as fields. It's like that. — Elliott Frisch, Oct 08 '16 at 08:15
Your code is invalid and I'm not 100% sure whether you're asking *"How to invoke instance methods without a instance?"* here. — fabian, Oct 08 '16 at 08:16
so say in this method //close a bid public void close { Person p; Bid bid; for{Lots lot: lot} {.......The fact that both local variables p and bid have been given types that are of class Person and class bid, why is that? — Jimmyu, Oct 08 '16 at 08:35

score 0 · Answer 1 · edited Oct 08 '16 at 08:18

0

You can do this assignment only if type of the right part is-a type of the left part. So, for example, you can use methods of left part' type with realization from right part' type.

Number number = new Integer(10);

See also OOP in Java and Polymorphism in Java.

edited Oct 08 '16 at 08:18

OneCricketeer

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answered Oct 08 '16 at 08:18

Filipp Voronov

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Please use `Integer.valueOf()`, see http://stackoverflow.com/questions/9030817/differences-between-new-integer123-integer-valueof123-and-just-123 – Oct 08 '16 at 08:21

score 0 · Answer 2 · answered Oct 08 '16 at 08:35

First of all, you need to learn some basics about OOP. We use Objects to model the problems and solve them in Object Oriented Programming.

These "type of a different class with a variable name" is an instance of a class, which is called an Object. We can assign a variable name to an object in order to use that object and its behaviors.

Java generic classes

2 Answers2