Java Classes and Objects

A class is a blueprint. An object is a real thing built from that blueprint. The class Student describes what a student has (name, marks) and what a student can do (study). The class alone is not one student sitting in a bench. Only when you write new Student() do you get a real object in memory.

Let’s take this on the board. class Student { String name; void study() { System.out.println(name + " is studying"); } }. Then Student s = new Student(); s.name = "Asha"; s.study();. Output: Asha is studying. s is not the object. s is a remote — a reference — that points to the object on the heap. The heap is where objects live.

You can make two objects from one class: Student a = new Student(); Student b = new Student(); a.name = "Asha"; b.name = "Ravi";. Same blueprint, two people. Changing Asha’s name does not change Ravi. That is why we bother with objects instead of one global name variable.

null means the remote currently points to nothing. Student s = null; then s.study() crashes with NullPointerException. You pointed the remote at empty air and pressed a button. Always create with new (or get a real object) before you call a method.

Interview line, say it slowly: class = design, object = instance in memory via new. Don’t say “class and object are the same”. The class name Student does not store Asha’s marks. Only the object does.

Diagram

Class Student   (blueprint)
        │ new
        ▼
   s1 Asha          s2 Ravi

Exam tip

Say blueprint vs instance, then show Student s = new Student();

Example

class Student {
  String name;
}

public class Main {
  public static void main(String[] args) {
    Student s = new Student();
    s.name = "Asha";
    System.out.println(s.name);
  }
}

Java Classes and Objects — output: Asha. new Student() makes a real object; s.name is a field on that object.

Short notes

DefClass = blueprint. Object = real instance made with new.
RuleReference variable points to the heap object. null = no object.
RememberMany objects can come from one class.
TrapClass name is not the object. Class alone does not store one student’s data.

Java Classes and Objects

Diagram

Class Student   (blueprint)
        │ new
        ▼
   s1 Asha          s2 Ravi

Exam tip

Say blueprint vs instance, then show Student s = new Student();

Example

class Student {
  String name;
}

public class Main {
  public static void main(String[] args) {
    Student s = new Student();
    s.name = "Asha";
    System.out.println(s.name);
  }
}

Java Classes and Objects — output: Asha. new Student() makes a real object; s.name is a field on that object.

Short notes

DefClass = blueprint. Object = real instance made with new.
RuleReference variable points to the heap object. null = no object.
RememberMany objects can come from one class.
TrapClass name is not the object. Class alone does not store one student’s data.

Java Classes and Objects

Example

Short notes

Questions

Java Classes and Objects

Example

Short notes

Questions