在java构造函数中声明ArrayList

我正在研究一个项目,并且我被教导在构造函数中实例化变量.我在使用ArrayList思想时遇到了一些麻烦.您能否提出一些最佳实践,我是否需要使用实例变量定义ArrayList,或者我可以在构造函数中执行此操作.谢谢你的建议！我有一个我正在谈论的内容的例子：

//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

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

list = new ArrayList<String>();