ArrayList构造函数并在方法中使用

Question

Hi I am a novice n just learning java. I was studying ArrayList n came accross this code for example {CODE1}. I would like to use the same code but add a ArrayListDemo constructor n create methods such as displayList and removeElement. I tried to find such examples but i did not understand them.

This is the code that i tried {CODE2} With my modifications please tell me where m going wrong.

*** CODE1 {Example Code} ** * *

import java.util.ArrayList;

public class AraryListDemo {

  public static void main(String[] args) {

    ArrayList al = new ArrayList();
    System.out.print("Initial size of al :  " + al.size());
    System.out.print("\n");

    //add.elements to the array list
    al.add("C");
    al.add("A");
    al.add("E");
    al.add("B");
    al.add("D");
    al.add("F");
    al.add(1,"A2");//inserts objects "A2" into array at index 1

    System.out.print("size of al after additions " + al.size());
    System.out.print("\n");

    //display the array list
    System.out.print("contents of al: " +  al );
    System.out.print("\n");

    //Remove elements from the array list
    al.remove("F");
    al.remove(2);

    System.out.print("size of after deletions : " + al.size());
    System.out.print("\n");
    System.out.print("contents of al:" + al);

  }

}

** * ** * ** CODE 2 {My Modifications} ** * ** * ** * ****

class ArrayListDemo

{
ArrayList<String> al;//variable declared




ArrayListDemo() throws IOException//try constructor for this
{
    al = new ArrayList<String>();
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    System.out.println("\n Enter Student Names");
    for(int i=0;i<=5;i++)// will dispaly 
    {
        al.add(br.readLine());
    }



}
void dispList(ArrayList <String> al)
{
    System.out.println("\n Display Student Names");
    for(String str : al)
    {
        System.out.println("\t Name :  "+str+"\n");
    }
}
}
class DisplayArrayList
{
public static void main(String []args) throws IOException
{

    ArrayList <String> al = new ArrayList <String>();
    ArrayListDemo e = new ArrayListDemo();
    e.dispList(al);

}
}

Answer 1

ArrayList <String> al = new ArrayList <String>();
ArrayListDemo e = new ArrayListDemo();
e.dispList(al);

In the above code, you are creating a new ArrayList al , and passing the same to dispList() method, which doesn't iterate, because the al has no elements .

I guess you wanted to iterate through the elements which you created within ArrayListDemo . So you may want to write dispList() method as below, which will now use ArrayList defined within the class

void dispList() //method parameter "al" is removed now and, al is the al of ArrayListDemo
{
    System.out.println("\n Display Student Names");
    for(String str : al) //here al refers to ArrayList defined within the class
    {
        System.out.println("\t Name :  "+str+"\n");
    }
}

Answer 2

It's not clear what exactly you're asking, but I note that you have a problem with your declarations (plural) of al : You have one ArrayList named al in your main , and you have another one that belongs to ArrayListDemo . You're reading values into the second one and then printing out the (empty) first one.

You really don't need a separate class with a constructor here. You can just have two static methods readList(List<String> al) and dispList(List<String> al) . If you really do want to have a separate class, pick one place to store the List (either in main or in the class).

As a note, it's generally a good idea to use the most general type for variables and method parameters that you can. You're declaring an ArrayList , which is fine, but if you make your variable and parameters List s, your code is more flexible.

Answer 3

The easiest (but not a prefered) solution to make your effort work is to pass the array to the displist() method that was filled by the constructor.

public static void main(String []args) throws IOException
{
    ArrayListDemo e = new ArrayListDemo();
    e.dispList(e.al);
}

Answer 4

Your code runs as following :-

• ArrayList <String> al = new ArrayList <String>(); // Initialise an ArrayList of type string

• ArrayListDemo e = new ArrayListDemo(); // Initialised class ArrayListDemo

• class constructor reads data from user input and add to ArrayList a1 by br.readLine()

• e.dispList(al); iterates the ArrayList instance a1 and print its output.