Constructors and Default Values

Every object needs a moment where it gets set up before anyone can use it. That moment is the constructor. It runs exactly once, at the instant new creates the object, and its whole job is to take the arguments you pass in and store them into the object's fields so the object starts life in a sensible state. Without that, you'd have an object with fields sitting empty, waiting to be filled in by hand after the fact, which invites bugs where someone forgets a step.

Mechanically, a constructor looks like a method named exactly after the class, with parameters but no return type at all. Inside the body, you typically assign each parameter to the matching field, often using this.fieldName = parameterName when the names match. If a constructor never touches a particular field, that field is not left undefined. Java fills it with a default automatically: 0 for int and double, false for boolean, and null for any object reference like a String. So a forgotten field doesn't crash the program, it just quietly holds a default value, which can itself cause bugs if you expected something else.

The missing return type trips people up because new Student(...) obviously hands back a Student. The resolution is that new does the returning, not the constructor. new allocates memory for the object, calls the constructor purely to initialize fields, then new itself returns the reference. That's also why constructors connect directly to the private fields and encapsulation ideas sitting right next to this topic: the constructor is usually the only piece of code trusted to set fields directly, before getters and setters take over.