Re: this

On 12-04-28 09:33 AM, mojde wrote:


One way of understanding "this" is as part of a larger picture, which is
understanding what everything is doing at runtime, i.e. when your code
is executing.

Once you've fired up a Java application, and execution of code starts
with a proper main() method in a designated main class [1], objects with
various lifetimes start to be created. Code in _instance_ methods (as
opposed to static/class methods) runs in the context of a single object.

It's this single object that you can refer to as "this". Understand also
that often, by well-defined rules, you can omit the explicit "this".

A "this" reference (implicit or explicit) is how you affect the state of
the object that your instance methods are executing in. See above: when
an instance method executes it is in the context of one object. Getters
and setters, for example, wouldn't be very useful unless they could
access the state of the particular object. Getters and setters are
simple instance methods themselves.

Without the "this" reference you effectively lose encapsulation. You'd
have to make your member fields public, and there really wouldn't be
instance methods anymore as you'd have to pass object references around
and that would be useless.

As an example, consider a rudimentary class Vehicle as

public class Vehicle {
  private Double speed;

  public Double getSpeed() { return speed; }
  public void setSpeed(Double speed) { this.speed = speed; }
}

Those 2 accessor methods are instance methods - whenever that code
executes they have a context of a single, instantiated Vehicle object.
So for example,

Vehicle vehicle1 = new Vehicle();
vehicle1.setSpeed(15.5);

When you call setSpeed() on "vehicle1", the 'this' reference in the
method body is how the code can access the member field "speed", by
'this.speed', and actually set the value.

The getter _implicitly_ uses 'this'.

If you didn't have 'this', things would degrade to

public class Vehicle {
  public Double speed;
}

and

Vehicle vehicle1 = new Vehicle();
vehicle1.speed = 15.5;

There is another usage of 'this' for invoking another constructor from a
constructor of the same class, but I won't cover that right now.
Although in a conceptual sense it's yet another mechanism for accessing
something belonging to the current object.

AHS

1. A codebase can have any number of "main" classes. As many of those
that the codebase has, is how many individual Java applications the
codebase supports.
--
A fly was very close to being called a "land," cause that's what they do
half the time.
-- Mitch Hedberg