Java : Generics

Generics

If you declare a method to take ArrayList<Animal> it can take ONLY an ArrayList<Animal>, not ArrayList<Dog> .To avoid this we use generics :

 ie.,Use ArrayList<? extends Animal> instead of ArrayList<Animal> 

(Animal is parent class)

Note : But this is not required if user is sending arrays[],

Example :

public class generics {

    public static void main(String[] args) {
       
        ArrayList<Animal> A=new ArrayList<Animal>();
        A.add(new Animal());

        ArrayList<dog> B=new ArrayList<dog>();
        B.add(new dog());
        B.add(new dog());

        System.out.println(new generics().cal(B));
        System.out.println(new generics().cal(A));
    }
       
//    try ArrayList<Animal>-------instead of-------- ArrayList<? extends Animal>
    public int cal( ArrayList<? extends Animal> A ){       
        return A.size();
    }
}

class Animal{
    private boolean wild;

    public void set_wild(boolean wild){
        this.wild=wild;
    }
   
    public boolean get_wild(){
        return this.wild;
    }   
}

class dog extends Animal{
   
    private StringBuffer dog_name=new StringBuffer();
   
    public void set_dogname(String dog_name){
        this.dog_name.append(dog_name);
    }
}

Output (ArrayList<? extends Animal>) :
1
2

Output (ArrayList<Animal>)
Error