Why must constructor arguments be stored manually?

[Daimoth]

Say some simple class takes three arguments, cat, dog, and mouse. I often see the "this" keyword in the constructor to store them all as variables.

I understand that it's necessary to store them for use inside the class, but why is it necessary? Why can't they be used immediately?

This is something I do, but don't understand.

[KevinWorkman]

Using the this Keyword (The Java™ Tutorials > Learning the Java Language > Classes and Objects)

[Daimoth]

Thank you, but I understand the basics of that keyword. I understand how to store arguments as variables, but what I don't understand is why it's necessary to store them at all. Why aren't they available for immediate use? Can arguments not be treated like variables?

[KevinWorkman]

You're asking a couple different questions which don't immediately have to do with one another.

You use the this keyword to refer directly to a class variable when a local variable has the same name and is therefore hiding the class variable. If the class variable has a different name than the local variable, you don't have to use the this keyword. That's in the tutorial I linked for you.

The other thing you're asking about is the scope of a variable. Class variables have scope inside the class. Arguments and variables declared inside a method have scope only inside a method (so you can't use it inside a different method). Variables declared inside an if statement or loop only have scope inside that block. You set a variable with a wider scope equal to a value with a smaller scope if you want to use that value outside of the scope in which it was declared.

[JosAH]

Thank you, but I understand the basics of that keyword. I understand how to store arguments as variables, but what I don't understand is why it's necessary to store them at all. Why aren't they available for immediate use? Can arguments not be treated like variables?

No, arguments (parameters) are gone when the method or constructor has gone too; there is nothing magical about constructors and even more: a constructor can do what it wants with the parameters, i.e.
it doesn't have to store them verbatim in its class variables.

kind regards,

Jos

[Daimoth]

So if two variable names aren't stepping on each others' toes, I don't need to use "this" at all?

[JosAH]

So if two variable names aren't stepping on each others' toes, I don't need to use "this" at all?

Indeed; the name of a parameter has nothing to do with the name of a member variable; if there's a clash a simple this.member solves the problem.

kind regards,

Jos

[Daimoth]

And they are stored because arguments are no longer accessible after the constructor has run?

[KevinWorkman]

And they are stored because arguments are no longer accessible after the constructor has run?

If they are stored, that's probably the reason for storing them.

You might want to throw together a few example programs to test out what we're talking about.

[Daimoth]

Thanks guys, you were both a lot of help.