How To Define A Generic Class

A generic type can be defined for a class or interface.

A concrete type must be specified when using the class to create an object or using the class or interface to declare a reference variable.

 

package DefineGenericClassAndInterface;
​
import java.util.ArrayList;
​
public class GenericStack<E> {
​
    ArrayList<E> arrayList = new ArrayList<E>();
​
    public void push(E e) {
        arrayList.add(e);
​
    }
​
    public E pop() {
        E element = arrayList.get(arrayList.size() - 1);
        arrayList.remove(arrayList.size() - 1);
        return element;
    }
​
    public boolean isEmpty() {
        return arrayList.size() == 0 ? true : false;
    }
}

Call it

package DefineGenericClassAndInterface;
​
public class CallingGenericStack {
    public static void main(String[] args) {
        GenericStack<String> stack = new GenericStack<String>();
        stack.push("Andrew");
        stack.push("Programming");
        stack.push("is great");
​
        System.out.println(stack.pop());
        System.out.println(stack.isEmpty());
        System.out.println(stack.pop());
        System.out.println(stack.pop());
        System.out.println(stack.isEmpty());
    }
}

Questions

• What is the generic definition for java.lang.Comparable in the Java API?

• Since you create an instance of ArrayList of strings using new ArrayList<String>(), should the constructor in the ArrayList class be defined as public ArrayList<E>()

• Can a generic class have multiple generic parameters?

• How do you declare a generic type in a class?