收集器：ArrayList & List

Collection 利用 Generic 來實作的好處：

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class ArrayListDemo {
    public static void main(String[] args){
    
        List partList = new ArrayList<>();
        partList.add(new Integer(1111));
        partList.add(new Integer(2222));
        partList.add(new Integer(3333));
        partList.add("Hello World");  //Compile Error : Not Integer
        
        // 利用 Iterator 來遍訪 ArrayList 內的 elements 
        Iterator elements = partList.iterator();  
        while (elements.hasNext()){
            Integer partNumberObject = elements.next();
            int partNumber = partNumberObject.intValue();
            
            System.out.println("Part Number: " + partNumber);
        }
    }
}