Generic classes
- TypeScript course
- Intermediate
- Lesson 17/89
- 4 min
Learn type-safe collections with generic classes.
Generic class definition
A generic class takes a type parameter in <T> right after the class name. The parameter can be used in all fields and methods. Each instance fixes the type, so a Kutu<string> only holds strings.
class Kutu<T> {
private icerik: T;
constructor(icerik: T) {
this.icerik = icerik;
}
al(): T { return this.icerik; }
}
const k = new Kutu<string>("merhaba");
console.log(k.al()); // merhabaType-safe collections
Generic classes are ideal for writing type-safe collections. Stack, queue, tree structures can be written once as a generic class and used with any type. new Depo<number>() creates an instance that stores numbers, new Depo<string>() one that stores strings.
Practice Type-safe storage
Write a generic class Depo<T>. It should have a private field ogeler: T[] starting as an empty array. Method ekle(o: T): void pushes an item; method al(i: number): T returns the item at the given index. Then create a Depo<string> instance, add "elma", and print the result of al(0).
Starter code
// Depo<T> sınıfını yaz
// private ogeler: T[] = []
// ekle metodu öge eklesin, al metodu indeksle döndürsün
// Depo<string> oluştur, "elma" ekle, al(0) yazdırSample solution
class Depo<T> {
private ogeler: T[] = [];
ekle(o: T): void {
this.ogeler.push(o);
}
al(i: number): T {
return this.ogeler[i];
}
}
const d = new Depo<string>();
d.ekle("elma");
console.log(d.al(0));Expected output
elma
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Where is the type parameter written in a generic class?
- Right after the class name, inside angle brackets Correct answer
- At file top with a type T declaration
- As a constructor parameter
- After the method name
Explanation: In a generic class the type parameter is written inside angle brackets right after the class name.
2 Generic class instance
class Kutu<T> {
private deger: T;
constructor(deger: T) { this.deger = deger; }
al(): T { return this.deger; }
}
const k = new Kutu<____>(42);class Kutu<T> {
private deger: T;
constructor(deger: T) { this.deger = deger; }
al(): T { return this.deger; }
}
const k = new Kutu<number>(42);Explanation: Since 42 is a number, the type argument must be number. string or boolean cause a compile error; T is undefined at the instantiation site.
3 Generic class parts
| Term | Meaning |
|---|---|
class Kutu<T> |
Generic class declaration |
new Kutu<number>(5) |
Generic class instance |
private icerik: T |
Type parameter as field |
al(): T |
Type parameter as return type |