Object - Root of Everything

java.lang.Object is the mother of all classes. Every class extends Object.

Object

Object sits at the top of the class hierarchy. Every class implicitly extends Object.

// These are equivalent:
class MyClass { }
class MyClass extends Object { }

toString()

Returns string representation. Default: ClassName@hashcode. Override for meaningful output.

@Override
public String toString() {
    return "Person{" + name + "}";
}

equals()

Default: reference equality (==). Override for value equality.

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;
    Person p = (Person) o;
    return age == p.age && Objects.equals(name, p.name);
}

hashCode()

For hash-based collections (HashMap, HashSet).

@Override
public int hashCode() {
    return Objects.hash(name, age);
}

Contract

• If equals() true, hashCode() must be same.
• If hashCode() same, equals() may be false.
• Always override both together. Use IDE generation.

IDE Generation

Right-click -> Generate -> equals() and hashCode(). Select fields that define identity. IDEs (IntelliJ, Eclipse) generate correct, reliable code.

✅ Итоги урока

• Object is the root superclass of all Java classes.
• toString(): string representation (override for meaningful output).
• equals(): value equality (override for logical comparison).
• hashCode(): hash value (MUST override when equals is overridden).
• Contract: equal objects must have equal hashCodes.
• IDE generates equals/hashCode reliably.
• String pool: literals are interned by default.