Classes as Blueprints, Objects as Instances

A class is a description, not a thing. It lists the instance variables every object of that type will carry, and the methods every object can call. Write class Student with fields name and score, and you have written a pattern, not a student. Nothing exists in memory yet.

Memory shows up when you call new. Each new Student(...) builds one object on the heap with its own name and score, independent of every other Student object. Build three students — say Ana with score 92, Ben with score 78, Carlos with score 85 — and you get three separate blocks of memory, each holding its own pair of values. Change Ben's score and Ana's is untouched, because they are not sharing storage, only sharing a design.

This is why the class itself can't hold instance data: it's the plan for the fields, not a place to store a value in them. You can't write Student.score = 92 for a non-static field, because score only comes into existence inside an object once new runs. This same split explains why every String you ever create can call substring — the String class wrote that method once, and every String object, no matter its characters, inherits the ability to use it on its own data.