Charlotte Au Charlotte Au - 3 months ago 9
Java Question

How to add an empty arrayList to another arrayList created from a class

I am trying to add an empty arrayList "patternList" to the "treeList" arrayList created from the treeNode class but I am having trouble populating the treeList arrayList with an empty patternList arrayList shown in the last line of the code. Basically I want an empty arrayList for each item in the treeList that I will be populating later on. Thanks.

import java.util.*;
import java.util.ArrayList;
import java.util.List;

public class testJava{

public static class treeNode {
String nodeName;
String pNodeName;
String fNodeName;
boolean begNode;
boolean targetNode;
int nodeNumber;
//String pattern;
ArrayList<String> patternList = new ArrayList<String>();
public treeNode (String nodeName, String pNodeName, boolean begNode, boolean targetNode, int nodeNumber, ArrayList<String> patternList)
{
this.nodeName = nodeName;
this.pNodeName = pNodeName;
this.begNode = begNode;
this.targetNode = targetNode;
this.nodeNumber = nodeNumber;
this.patternList = patternList;
//this.pattern = pattern;
}
}

public static void main (String[] args){
ArrayList<treeNode> treeList = new ArrayList<treeNode>();
ArrayList<String> openList = new ArrayList<String>();
ArrayList<String> closeList = new ArrayList<String>();
ArrayList<String> treePath = new ArrayList<String>();
String currentNode = "";
String targetNode = "";
int smallestNodeCount = 0;
int tempNodeCount = 0;
int openListCount = 0;
String currentPattern = "" ;

treeList.add(new treeNode ("S 1", null, true, false, 1, ArrayList<string> patternList));

Answer

To create a new empty ArrayList, the syntax would be new ArrayList<String>().

Since you're immediately passing the newly created list as a parameter to your TreeNode constructor, there's no need for a variable name.

Your last line should therefore be:

treeList.add(new treeNode ("S 1", null, true, false, 1, new ArrayList<String>()));

Also, by convention, Java class names are in UpperCamelCase, not in lowerCamelCase. That's why it's ArrayList rather than arrayList.

Your class names should be TreeNode and TestJava, with an upper-case T.

Another thing to pay attention to is your variable types; more often than not, you want your variable types to use the interface when available, rather than being explicitly tied to a particular implementation class. So instead of defining your list using ArrayList<String> openList = new ArrayList<String>();, consider making the type of openList simply List. This only applies on the left-hand side of the declaration. On the right-hand side, you still need the concrete class (i.e. ArrayList) when creating the new instance. Your declaration therefore becomes:

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