Constructors & Object Birth

A constructor is a special method that runs at new time, shaping every object of the class as it is born:

class Account {
    String owner;
    double balance;

    Account(String owner, double balance) {   // same name as class, no return type
        this.owner = owner;
        this.balance = balance;
    }
}

Account acc = new Account("Ada", 100.0);   // fields set at birth

Three constructor rules:

• The name matches the class exactly, and there is no return type — not even void.
• If you write no constructor at all, Java writes you a silent, do-nothing default (fields get 0/false/null).
• If you write ANY constructor, the free default disappears: new Account() stops compiling unless you also write it.

Overloading constructors gives callers convenience; the classic pattern chains with this(...) so the real logic lives in exactly one place:

Account(String owner) {
    this(owner, 0.0);        // delegate: everyone starts at zero
}

Account(String owner, double balance) {
    if (balance < 0) {
        throw new IllegalArgumentException("balance must be >= 0");
    }
    this.owner = owner;
    this.balance = balance;
}

Constructors are also the natural place to validate: refuse bad births (null owner, negative balance) immediately, so the rest of the class can assume its invariants hold. This "validate at the door" habit is the beginner form of what professionals call an invariant — a guarantee every object keeps for its whole life.

Next: guarding fields — encapsulation.