OOP_2|Java Packages: How to Create, Use, and Organize Your Code for Better Visibility and Security
Notes on Java packages, including how to create and use packages, control visibility, and manage your code. Learn how the Java run-time system finds packages, and discover best practices for using packages to organize and secure your code
1. What are packages in Java?
In Java, a package is a way to group related classes, interfaces, and subpackages together. This allows you to organize your code and keep it manageable as your project grows.
Packages also provide a way to avoid naming conflicts between different code modules. For example, if you have two classes with the same name in different packages, you can refer to them by their full package names to distinguish them.
2. How do you create a package in Java?
To create a package in Java, you simply need to include a package statement at the top of your source code file, like this:
package com.example.mypackage;This statement indicates that the classes in this file are part of the “com.example.mypackage” package.
By convention, Java package names are written in all lowercase and use periods to separate the different levels of the package hierarchy. For example, you might have packages like “com.example.mypackage”, “com.example.mypackage.subpackage”, and so on.
3. How do you use a package in Java?
Once you’ve created a package, you can use it in other parts of your code by importing it. To import a package, you include an import statement at the top of your source code file, like this:
import com.example.mypackage.MyClass;This statement imports the “MyClass” class from the “com.example.mypackage” package. You can then use the class in your code as if it were defined in the same package
Note that you can also use the * wildcard character to import all the public classes in a package:
import com.example.mypackage.*;However, it’s generally considered better practice to be more specific about which classes you’re importing, to avoid name conflicts and make your code easier to understand.
4. Visibility control in Java packages
One of the main benefits of packages is that they provide a way to control visibility in your code. When you declare a class or interface within a package, you can choose whether it should be visible only within that package (using the default “package-private” visibility), or whether it should be visible to other packages as well (using the “public” visibility).
For example, suppose you have a class defined like this:
package com.example.mypackage;
class MyPackageClass {
// ...
}This class is visible only within the “com.example.mypackage” package, because it has the default visibility. Other classes within the same package can access it, but classes in other packages cannot.
If you want to make a class visible outside its package, you can declare it as public:
package com.example.mypackage;
public class MyPublicClass {
// ...
}Now, other classes in other packages can import and use this class.
5. How does the Java run-time system find packages?
When you run a Java program that uses packages, the Java run-time system needs to know where to find the class files for those packages. There are several ways it can do this:
- By default, the Java run-time system starts looking for class files in the current working directory. If your package is in a subdirectory of the current directory, it will be found automatically.
- You can set the CLASSPATH environment variable
Summary:
- Java packages are used to group related classes, interfaces, and subpackages together for organization and to avoid naming conflicts.
- To create a package in Java, use the package statement at the top of your source code file, with package names separated by periods.
- To use a package in Java, use the import statement at the top of your source code file, specifying the package and class you want to use.
- Packages provide visibility control in Java, with default package-private visibility or public visibility.
- The Java run-time system finds packages by default in the current working directory, or through the CLASSPATH environment variable or -classpath option.
