CODEWITHSUNDEEP
javaobjecttypes
codealive
codealive

Reputation: 25

Question About the Type of an object

I'm learning java and have a question about object types.

Take the following as an example:

class A { }

class B extends A { }

class C {A myObject = new B();}

What is the accepted way to describe the type of myObject?

Would you say it is Type A, Type B, or Type A and B?

Thanks.

Upvotes: 2

Views: 136

Answers (4)

Kissaki
Kissaki

Reputation: 9247

myObject is a reference of type A (your variable is declared to be of type A).

The object itself, which is referred to by myObject, is of type B. The instance is of type B.

It’s a matter of context and focus. When you use myObject in other places you may not know it is an instance of class B, only that it is of type A.

Upvotes: 6

Jesper
Jesper

Reputation: 207006

Inheritance implies an "is a" relationship - an instance of a subclass is an instance of a superclass. In your example, since B extends A, an instance of B is also an instance of A - that's why you can assign a new B to a variable of type A without casting.

The variable myObject is of type A, but it refers to an object which is really of type B.

Upvotes: 0

Michael Borgwardt
Michael Borgwardt

Reputation: 346536

myObject is field with the declared compile-time type A. At runtime, it will refer to an object of type B.

This distinction between the declared (or compile-time) type of variables and the runtime type of objects is crucial to understanding how polymorphism works in a statically typed language like Java. Some things (like the invocation of overriden methods) are determined by the runtime type, othersy (like the invocation of overloaded or static methods) by the compile-time type.

In general: the declared type determines what you can do with the object, while the runtime type determines how it will behave.

Upvotes: 1

khachik
khachik

Reputation: 28703

It is of type A, since type means operations you can do with it, but it is an instance of B, so all the operations you do will be in B-manner.

Upvotes: 1

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