在java构造函数中声明ArrayList

Question

I am working on a project, and I was taught to instantiate variables in constructors. I'm having some trouble doing this with an ArrayList thought. Can you suggest some best practices, do I need to define the ArrayList with the instance variables or can I do it in the constructor. Thanks for your suggestions! I have an example of what I'm talking about below:

//imports
import java.util.*;
import java.lang.*;

public class ArrayListConstructorDemo
{
//instance variables/attributes

String string;
List<String> list;// for example does this line need to say List<String> list = new ArrayList<String>();

//constructors
public ArrayListConstructorDemo()
{
    String string = "null";
    List<String> list = new ArrayList<String>();//is there anyway I can do this here instead of 6 lines up?
}//end default constructor
public ArrayListConstructorDemo(String string,List<String> list)
{
    this.string = string;
    this.list = list;
}//end generic constructor

//observers/getters/accessors
 public String getString(){return string;}//end method getString()
 public List<String> getList(){return list;}//end method getList()

//transformers/setters/mutators
     public void setTable(String string){this.string = string;}
     public void setValues(String list)
     {

    //  for(String s : test) 
    //  {
            list.add(this.list);
    //  }
     }
public String toString()
{
    return "this is a generic toString method for the class ArrayListConstructorDemo";
}//end toString

public static void main(String[] args)  
{
    ArrayListConstructorDemo alcd = new ArrayListConstructorDemo();
    System.out.println(alcd.list.size());

//test Lists in general
    List<String> bleh = new ArrayList<String>();
    bleh.add("b1");
    System.out.println(bleh.get(0));
}//end method main()
}//end class ArrayListConstructorDemo

Answer 1

Change

List<String> list = new ArrayList<String>();

to

list = new ArrayList<String>();

Answer 2

If you want to declare it in the constructor, then you (most likely) want to declare the outer field, so you want:

list = new ArrayList<String>();

Currently you are shadowing the List<String> list class variable, meaning that you are creating a new instance of list , rather than that you are initializing the list instance variable.

I personally prefer initializing it at declaration time though, so what you previously had. I prefer this to make the code more concise and you most likely won't end up forgetting to initialize it if you teach yourself that habbit.

Answer 3

If you want to just declare it in the constructor you can have the code:

     ArrayList<String> name = new ArrayList<String>();

Otherwise you can declare it as a field, and then initialize it in the constructor.

   private ArrayList<String> name;

And then in the constructor:

    name = new ArrayList<String>();

Making it a field would be useful, as you would then be able to create accessor/mutator methods in order to retrieve and use the List from different classes, without having to declare it public (which is rarely a good thing).

Answer 4

Generally the practice is to declare before the constructor, and initialize in the constructor. Here's an example:
class myClass ArrayList<String> strings public myClass() { strings=new ArrayList<String>(); }

Answer 5

java offers you also an Initializing Fields

http://docs.oracle.com/javase/tutorial/java/javaOO/initial.html see Initializing Instance Members

Answer 6

How can you do this ??

public void setValues(String list) {

    // for(String s : test)
    // {
    list.add(this.list);
    // }
}

There is no method like add() to manipulate Strings, Instead you would have done this :

public void setValues(List<String> list) {

    // for(String s : test)
    // {
    list.add(this.list);
    // }
}

And regarding declaring ArrayList in the constructors you can do like this :

String string;
List<String> list;// for example does this line need to say List<String>
                    // list = new ArrayList<String>();

// constructors
public ArrayListConstructorDemo() {
    string = "null";
    list = new ArrayList<String>();// is there anyway I can do this here
                                    // instead of 6 lines up?
}// end default constructor