Classes, Objects, State and Identity

Blueprints and instances

A class is a blueprint that defines what data (fields or attributes) and operations (methods) its objects have. An object is a concrete instance of a class, created at run time, typically with new in Java or by calling the class in Python. Every object has state (the current values of its fields), behaviour (its methods) and identity (it is a distinct object, even if another object has identical state). In Java, C#, Python and JavaScript, variables of class types hold references to objects that live on the heap; assigning one variable to another copies the reference, not the object, so both variables refer to the same object. Methods are called on objects with dot notation (account.deposit(500)), and inside a method the current object is available as this (Java, C++, C#) or self (Python). Classes can also have static members that belong to the class itself rather than any one object, such as a counter of how many objects have been created.

Objects, references and identity

Two references to one object versus two equal but separate objects.

class Student {
    String name;
    int marks;

    Student(String name, int marks) {
        this.name = name;
        this.marks = marks;
    }
}

Student a = new Student("Asha", 91);
Student b = a;                  // copies the reference: same object
b.marks = 95;
System.out.println(a.marks);    // 95, because a and b refer to one object

Student c = new Student("Asha", 95);
System.out.println(a == c);     // false: same state, different identity

A house plan and houses

A class is the architect's plan; objects are the houses built from it. Many houses share the plan but each has its own address (identity) and its own furniture (state). Giving a friend your address (a reference) does not build them a new house.

Quick check: After `Student b = a;` in Java, what does b refer to?

• A new copy of the object
• Nothing until it is initialised
• The Student class itself
• The same object that a refers to