Class Attributes

What they are: Class attributes (also called fields or instance variables) hold object state. There are two broad kinds:

• Instance fields — belong to each object instance.
• Static (class) fields — belong to the class itself, shared across all instances.

Key points:

• Default values: numeric→0, boolean→false, reference→null.
• Visibility controlled by access modifiers (private, protected, public, package-private).
• Prefer encapsulation: private fields + public getters/setters.

Example

public class Person {
  // instance fields
  private String name;
  private int age;

  // static field (class attribute)
  public static final String SPECIES = “Homo sapiens”;

  public Person(String name, int age)    {
    this.name = name;
    this.age = age;
  }

  // getters & setters
  public String getName() { return name; }
  public void setName(String name) { this.name = name; }

  public int getAge() { return age; }
  public void setAge(int age) { this.age = age; }
}

Inner Classes

Kinds:

1. Non-static nested (inner) class — associated with an instance, can access outer instance members.
2. Static nested class — like a top-level class but nested; cannot access non-static members of outer without an instance.
3. Local class — defined inside a method.
4. Anonymous class — expression to create an instance of an unnamed class (covered later).

Example (non-static and static nested)

public class Outer {
  private int outerValue = 10;

  // non-static inner class
  public class Inner {
    public void print() {
      System.out.println(“outerValue = ” + outerValue);
    }
  }

  // static nested class
  public static class StaticNested {
    public void show() {
      System.out.println(“I am static nested”);
    }
  }

  public static void main(String[] args) {
    Outer o = new Outer();
    Outer.Inner inner = o.new Inner();
    inner.print();

    Outer.StaticNested nested = new Outer.StaticNested();
    nested.show();
  }
}