Generic class

Learn to define generic classes with <T>.

Generic class

A generic class is a template declared with a type parameter. Writing class Box<T> { T value; } makes T a type parameter; you supply a concrete type when you use it: Box<String> b = new Box<>();.

The same class then works for String, Integer and other types in a type-safe way. No cast is needed and ClassCastException risk drops. By convention type parameters are single letters: T (type), E (element), K/V (key/value).

Using a generic class

To use a generic class with a concrete type, write Box<String> b = new Box<>();. The right side <> can stay empty; the compiler infers the type from the left side (diamond operator).

Field assignment looks like b.value = "Hello";. When you write String s = b.value; no cast is needed; the compiler knows value is a String. Giving a different type is a compile error; this is the main strength of generics.

Exercise Define the Box<T> class

Define class Box<T> with a T value; field inside. In main, create a box with Box<String> b = new Box<>();, assign b.value = "Book"; and print it with System.out.println(b.value);.

Show example solution

class Box<T> {
    T value;
}

public class Main {
    public static void main(String[] args) {
        Box<String> b = new Box<>();
        b.value = "Book";
        System.out.println(b.value);
    }
}

Test yourself

What does Box<String> specify?
- That an array is defined
- That Box is final
- That String is a subclass
- That T becomes String
Show answer

That T becomes String — The type inside < > is the concrete type for T.

Complete the generic class declaration

class Box____ {
    T value;
}

Show answer

class Box<T> {
    T value;
}

Match the generic class concepts

<T>

Type parameter declaration
Box<String>

Concrete type usage
<>

Diamond operator
T value

Generic field

Next lesson: Generic method

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