declaring ArrayList in java Constructor

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

Don't confuse declaration and initialization. Also careful that you don't shadow the variable. (Those are all keywords.) — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Mar 29, 2014 at 19:56
Sotiros I just did that for brevities sake. You're totally right and in practice I don't! Rohit I was taught that in a default constructor anything that needs to have a value to be accessed later should be instantiated in the default constructor instead of adding default values before that, this prevents you from having issues later on with random values floating around. — demuro1
– demuro1, Commented Mar 29, 2014 at 20:02
Rohit is referring to a concept called shadowing. If you have a field declared with the name list and then declare a new variable called list within the constructor, when referring to list within the constructor, you'll be referring to the local variable rather than the field. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Mar 29, 2014 at 20:03

casperw · Accepted Answer · 2014-03-29 19:57:50Z

5

Change

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

to

list = new ArrayList<String>();

answered Mar 29, 2014 at 19:57

casperw

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Comments

Connor Cartwright · Accepted Answer · 2014-03-29 20:01:54Z

5

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).

edited Mar 29, 2014 at 20:01

answered Mar 29, 2014 at 19:57

Connor Cartwright

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skiwi · Accepted Answer · 2014-03-29 19:56:33Z

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.

answered Mar 29, 2014 at 19:56

skiwi

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faeophyta · Accepted Answer · 2014-03-29 20:00:32Z

2

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>(); }

answered Mar 29, 2014 at 20:00

faeophyta

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Roshan Shahukhal · Accepted Answer · 2014-03-29 20:11:03Z

2

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

answered Mar 29, 2014 at 20:11

Roshan Shahukhal

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2 Comments

demuro1 Over a year ago

That method isn't finished, It will eventually be something like public void setValues(List<String> list, String... strings) { for(String s : strings){ list.add(this.s); } } or something like that

Roshan Shahukhal Over a year ago

ya... that sounds ok now !! and btw, using variable names as of the predefined keywords is bad practice !

Matej Špilár · Accepted Answer · 2014-03-29 19:59:05Z

0

java offers you also an Initializing Fields

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

answered Mar 29, 2014 at 19:59

Matej Špilár

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1 Comment

E A Over a year ago

where did it mention java prefers to initializing the list as a field?

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