Java Generic

allows us to control parameter type for class or method and change it according our needs.

Generic Class

public static class MyClass<T> {
  private T myProp;
  
  public T myFunction() {
    return myProp;
  }
}

public static void main(String[] args) {
  // pass "String" as parameter type
  MyClass<String> myClass = new MyClass<>();
  
  String data = myClass.myFunction();
}

Generic index (just as behavior, not mandatory) for parameter type :

E - Element (Collection or data structure)
K - Key
N - Number
T - Type
V - Value
S, U, V, etc - 2nd, 3rd, 4th types ...

Generic Method

public <T> int count(T[] datas) {
  return datas.length;
}

int[] numbers = {1, 2, 3, 4, 5};
int numLength = <int>count(numbers);

// or without define type, automatically detect int by array of number
int numLength = count(numbers);

// <T> will be String, according on parameter type (datas);
String[] texts = {"Marsha", "Lenathea", "Lavia"};
int textLength = count(texts);

Invariant

means that we cant substitute subtype (child) with supertype (parent).

// code below will error, cause String != Object, 
   although String is an instance of Object.
   
MyData<String> dataString = new MyData<>("Marsha");
❌ MyData<Object> dataObject = dataString;

MyData<Object> dataObject = new MyData<>(100);
❌ MyData<Integer> dataInteger = dataObject;

Covariant

means that we can substitute subtype (child) with supertype (parent), with keyword :

?extends ParentClass

⚠️ Covariant is read-only, so we cant write data generic.

public static void main(String[] args) {
  MyData<String> data = new MyData<>("Marsha");
  process(data);
}

public static void process(MyData<? extends Object> data) {
  // we can do this with extends ParentClass
     String -> Object
     
  Object object = data.getData(); // read only
  ❌ data.setData("Lenathea"); // cant do write data
}

Contravariant

it's the opposite of Covariant, which we can substitute supertype (parent) with subtype (child), with keyword :

?super SubClass

⚠️ we can do read-write in Contravariant, but we must be careful when get (read) data.

public static void main(String[] args) {
  MyData<Object> objectData = new MyData<>("Marsha");
  ✅ MyData<?super String> stringData = objectData;
}

public static void process(MyData<? super String> myData){
  myData.setData("Lenathea");
}