CODEWITHSUNDEEP
java
gallly
gallly

Reputation: 1211

in java I am confused on when to make a class public

I am going through head into java and came across this example

interface Nose{
    public int iMethod();
}

abstract class Picasso implements Nose{
    public int iMethod(){
        return 7;
    }
}

class Clowns extends Picasso{}

class Acts extends Picasso{
    public int iMethod(){
        return 5;
    }
}

Because nothing is declared public, doesn't that mean that none of these classes can be called from another file? I have another file 

public class Of76 extends Clowns{
    public static void main(String[] args) {
        Nose [] i = new Nose[3];
        i[0] = new Acts();
        i[1] = new Clowns();
        i[2] = new Of76();
        for(int x = 0; x < 3; x++) {
            System.out.println(i[x].iMethod()+" "+i[x].getClass());
        }

    }
}

So in this example class Of76 can make classes out of another file that doesn't have any public classes. I am confused on why the first file can have all those classes and why they are not in separate classes. I read that a class that is not public is private by default, and can only be called within the same class. So everything in the Nose file can only be called inside that Nose file?

Upvotes: 2

Views: 1958

Answers (2)

scottb
scottb

Reputation: 10084

The answer to this question is a matter of perspective.

If you are a new Java learner and you are learning language features and the object-oriented paradigm, then a reasonable answer is "you should always make classes public" because visibility restrictions matter to nobody but you. Visibility is an issue only for production code or when you are exporting an API for other clients to use. It is not unreasonable to make classes public until you know how to write good classes (I would NOT extend this advice to fields of classes, however).

However...

If you are maintaining or modifying production code -or- developing an API that you intend to export for other programmers to use, then the best answer is "you should never make classes public unless your clear intent is that your clients should be able to access your class and use it in their own client code.

Visibility is one of the most important security and encapsulation mechanisms in Java and you should never be cavalier about it in a production environment. Everything in your API should have the lowest possible visibility.

There are some important exceptions that can be imposed upon you by other API's that you may be using. For example, if you are developing a controller class for JavaFX, then in JavaFX 2.2 you are required to make your class public, whether you want to or not. Even so, these exceptions do not break the rule that "everything in your API should have the lowest possible visibility."

TL;DR: If the code you're working on will be accessed only by you, then you need never make classes anything but public. If you're working on production code, code that will become available to others, or an API that you intend to export for client use, then you should never make classes public unless it is a clearly indicated part of your design.

  • public MyClass this class will be visible to classes in your package and classes in other packages.
  • protected MyClass this class will be visible to classes in your package and only other classes that are subclasses of MyClass.
  • MyClass this is "default access" or "package-private" access. MyClass is visible only to classes in the same package.
  • private MyClass this class is not visisble to other classes except those that are in the same .class file.

Default access and Private access are considered non-exported visibility levels. Classes with these visibility levels may be freely modified by you as their implementation details are not exported as part of any API.

Public and protected access are considered exported visibility levels. Generally, once you've exported a class as part of your API you are expected to continue to support it "forever".

Upvotes: 2

Daniel Kaplan
Daniel Kaplan

Reputation: 67514

When you exlude public and don't have anything else there like protected or private this is called "package protected". This should explain the differences to you: In Java, difference between default, public, protected, and private

Upvotes: 7

Related Questions