Classes and Objects

A class defines fields, methods, constructors, nested types, and initialization behavior. Each object has identity and instance state; static members belong to the class. Keep fields private and expose operations that preserve invariants.

A focused model

public final class BankAccount {
    private final String owner;
    private long balanceCents;

    public BankAccount(String owner, long openingCents) {
        this.owner = Objects.requireNonNull(owner);
        if (openingCents < 0) throw new IllegalArgumentException();
        balanceCents = openingCents;
    }

    public void deposit(long cents) {
        if (cents <= 0) throw new IllegalArgumentException();
        balanceCents = Math.addExact(balanceCents, cents);
    }
}

this refers to the receiver. Static factories can name construction choices and return subtypes. Override equals and hashCode together for value semantics; provide a useful toString without secrets. Records are concise immutable data carriers but still need invariant validation. Favor composition and single-purpose classes.