Generics

CSCI 1913 – Introduction to Algorithms, Data Structures, and Program Development
Adriana Picoral

Polymorphisms in Java

Overloading – methods in with the same name (different parameters) in the same class
Overriding – methods with the same signature in different (super-sub pair) classes
Parametric polymorphism – Generics

“Parametric” – flexible, in this case flexibility for type

The problem to be solved

Imagine we want to write a class called Box that holds one item of any type.

public class BoxString {
    private String item;
    
    public void set(String item) {
        this.item = item;
    }
    
    public String get() {
        return item;
    }
}

The problem to be solved

Imagine we want to write a class called Box that holds one item of any type.

public class BoxPolygon {
    private Polygon item;
    
    public void set(Polygon item) {
        this.item = item;
    }
    
    public Polygon get() {
        return item;
    }
}

The problem to be solved

Imagine we want to write a class called Box that holds one item of any type.

public class BoxCard {
    private Card item;
    
    public void set(Card item) {
        this.item = item;
    }
    
    public Card get() {
        return item;
    }
}

Flawed solution

We can use the Object class as type:

public class Box {
    private Object item;
    
    public void set(Object item) {
        this.item = item;
    }
    
    public Object get() {
        return item;
    }
}

Flawed solution

Box box = new Box();
box.set("Hello");
String str = (String) box.get();  // need to cast - annoying!

box.set(42);
String oops = (String) box.get();  // compiles fine, crashes at runtime!

Type casting every time we retrieve something
Compiler can’t catch type errors - code crashes at runtime
No type safety

Generics

Enables code reuse and flexibility
Code works independently of the types of values it operates on

Generics allow us to write code that works with multiple types while maintaining type safety.

Parametrically polymorphic methods are called generic methods
Parametrically polymorphic data types are called generic data types

Box with generics

public class Box<T> {
    private T item;
    
    public void set(T item) {
        this.item = item;
    }
    
    public T get() {
        return item;
    }
}

Box with generics

Box<String> stringBox = new Box<>();
stringBox.set("Hello");
String str = stringBox.get();  // no cast needed!

Box<Integer> intBox = new Box<>();
intBox.set(42);
// intBox.set("Hello");  // compiler error - caught at compile time!