Constructors

Key points

• A constructor runs once, at creation. Its job is to leave the object in a valid state, or refuse to build it.
• Same name as the class, no return type. Write none and Java gives you an empty no-arg constructor for free.
• Write any constructor and that free one disappears. Add it back yourself if callers still need it.
• Overload for convenience, then chain with this(...) so the real work lives in exactly one constructor.
• Validate arguments here, not later. An IllegalArgumentException now beats a half-built object roaming loose.
• this.x = x simply distinguishes the field from the parameter. Nothing more magical than that.

Example

public class Main {
    static class Account {
        private final String owner;
        private double balance;

        Account(String owner, double balance) {
            if (owner == null || owner.isBlank()) {
                throw new IllegalArgumentException("owner is required");
            }
            this.owner = owner;          // 'this' = the object being built
            this.balance = balance;
        }

        Account(String owner) {
            this(owner, 0.0);            // delegate; must be the first statement
        }

        @Override public String toString() {
            return owner + ": " + balance;
        }
    }

    public static void main(String[] args) {
        System.out.println(new Account("Ada", 250.0));
        System.out.println(new Account("Linus"));

        try {
            new Account("   ");
        } catch (IllegalArgumentException e) {
            System.out.println("rejected at birth: " + e.getMessage());
        }
    }
}

A constructor guards the door: no object should ever exist in an invalid state.