Family rules: final, one parent, Object¶
☕ Java 17 · Building Classes on Top of Other Classes · 2/3
You can now build a child class and know what it inherits. Three rules shape every hierarchy.
final: no children¶
A final class can never be extended:
public class Subscription { }
public final class PremiumPlan extends Subscription { } // ✅ final, with a parent
public class FamilyPlan extends PremiumPlan { } // ❌ PremiumPlan is final
final stops a class from having children, not from having a parent. PremiumPlan extends Subscription just fine; it simply ends its branch of the tree.
Other class modifiers (abstract, sealed, non-sealed) come in later sections.
One direct parent¶
Every class has exactly one direct parent:
class Movie extends Content { } // ✅ one parent
class Documentary extends Movie { } // ✅ several levels
class Trailer extends Movie, Clip { } // ❌ two parents
This is single inheritance: one class name after extends, never two. A parent can have many children (Movie and Series both extend Content), and the tree can be as deep as you need.
A class can, however, implement several interfaces: that's how Java offers multiple types without multiple parents. Interfaces come in a later chapter.
Object: the ancestor of every class¶
A class without extends still has a parent:
// what you write
public class User { }
// what the compiler sees
public class User extends java.lang.Object { }
The compiler adds extends java.lang.Object automatically. That's why this works, even though User declares no method:
Object is the only class with no parent: every hierarchy ends there. Primitives like int and boolean aren't classes, so they're outside this tree; their wrappers (Integer, Boolean) are inside it.
A full hierarchy for our platform looks like this:
The lab builds exactly this tree.