Java Packages

Package Concept

Packages group related classes together
Provide an abstraction barrier for code organization
Help manage the namespace and avoid naming conflicts
Enable access control between different parts of code

Package Naming Convention

Hierarchical dot notation (e.g., com.google.common.math, java.io)
Usually follows reverse domain name of the organization
```
com.company.project.module
```
Prevents package name collisions across different organizations

Package Structure and File System

Java maps package names to directory hierarchy
Class a.b.C is looked for in path a/b/C.class

Example:

package com.example.project;

public class MyClass { ... }

// This class would be stored in: com/example/project/MyClass.class

Default Package

Classes without a package declaration belong to the "default" package
All classes in the same directory without package declarations share this namespace

Package Access Control

Modifier	Class	Package	Subclass (same pkg)	Subclass (diff pkg)	World
public	✅	✅	✅	✅	✅
protected	✅	✅	✅	✅	❌
(no modifier)	✅	✅	✅	❌	❌
private	✅	❌	❌	❌	❌

Example of Protected Access

// In file A.java (package com.example)
package com.example;

public class A {
    protected int x;  // protected data field
}

// In file B.java (package com.example)
package com.example;

public class B {
    public void accessX() {
        A a = new A();
        a.x = 10;  // Accessible: same package
    }
}

// In file C.java (package com.example.other)
package com.example.other;

import com.example.A;

public class C extends A {
    public void accessX() {
        x = 20;  // Accessible: subclass from different package
    }
}

// In file D.java (package com.example.other)
package com.example.other;

import com.example.A;

public class D {
    public void accessX() {
        A a = new A();
        // a.x = 30;  // ERROR: Cannot access protected field from unrelated class
    }
}

Creating and Using Packages

Declare package as the first line in your Java file:

package cs2030s.fp;

public interface BooleanCondition<T> {
    boolean test(T t);
}

To use classes from other packages:

// Option 1: Full qualification
cs2030s.fp.BooleanCondition<Integer> isEven = x -> x % 2 == 0;

// Option 2: Import and use
import cs2030s.fp.BooleanCondition;
BooleanCondition<Integer> isEven = x -> x % 2 == 0;

Best Practices

Organize related classes in the same package
Use hierarchical packages for large projects
Always declare packages in production code
Use meaningful package names that reflect code purpose
Keep package structure aligned with project architecture