How to create an 2D ArrayList in java?
I want to create a 2D array that each cell is an ArrayList!
If you want to create a 2D array of ArrayList
.Then you can do this :
ArrayList[][] table = new ArrayList[10][10];
table[0][0] = new ArrayList(); // add another ArrayList object to [0,0]
table[0][0].add(); // add object to that ArrayList
Two dimensional array list
You would use
List<List<String>> listOfLists = new ArrayList<List<String>>();
And then when you needed to add a new "row", you'd add the list:
listOfLists.add(new ArrayList<String>());
I've used this mostly when I wanted to hold references to several lists of Point in a GUI so I could draw multiple curves. It works well.
For example:
import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.RenderingHints;
import java.awt.Stroke;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.util.ArrayList;
import java.util.List;
import javax.swing.*;
@SuppressWarnings("serial")
public class DrawStuff extends JPanel {
private static final int PREF_W = 400;
private static final int PREF_H = PREF_W;
private static final Color POINTS_COLOR = Color.red;
private static final Color CURRENT_POINTS_COLOR = Color.blue;
private static final Stroke STROKE = new BasicStroke(4f);
private List<List<Point>> pointsList = new ArrayList<List<Point>>();
private List<Point> currentPointList = null;
public DrawStuff() {
MyMouseAdapter myMouseAdapter = new MyMouseAdapter();
addMouseListener(myMouseAdapter);
addMouseMotionListener(myMouseAdapter);
}
@Override
public Dimension getPreferredSize() {
return new Dimension(PREF_W, PREF_H);
}
@Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2 = (Graphics2D) g;
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
g2.setStroke(STROKE);
g.setColor(POINTS_COLOR);
for (List<Point> pointList : pointsList) {
if (pointList.size() > 1) {
Point p1 = pointList.get(0);
for (int i = 1; i < pointList.size(); i++) {
Point p2 = pointList.get(i);
int x1 = p1.x;
int y1 = p1.y;
int x2 = p2.x;
int y2 = p2.y;
g.drawLine(x1, y1, x2, y2);
p1 = p2;
}
}
}
g.setColor(CURRENT_POINTS_COLOR);
if (currentPointList != null && currentPointList.size() > 1) {
Point p1 = currentPointList.get(0);
for (int i = 1; i < currentPointList.size(); i++) {
Point p2 = currentPointList.get(i);
int x1 = p1.x;
int y1 = p1.y;
int x2 = p2.x;
int y2 = p2.y;
g.drawLine(x1, y1, x2, y2);
p1 = p2;
}
}
}
private class MyMouseAdapter extends MouseAdapter {
@Override
public void mousePressed(MouseEvent mEvt) {
currentPointList = new ArrayList<Point>();
currentPointList.add(mEvt.getPoint());
repaint();
}
@Override
public void mouseDragged(MouseEvent mEvt) {
currentPointList.add(mEvt.getPoint());
repaint();
}
@Override
public void mouseReleased(MouseEvent mEvt) {
currentPointList.add(mEvt.getPoint());
pointsList.add(currentPointList);
currentPointList = null;
repaint();
}
}
private static void createAndShowGui() {
JFrame frame = new JFrame("DrawStuff");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.getContentPane().add(new DrawStuff());
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
createAndShowGui();
}
});
}
}
Java - Filling multidimensional (2d) ArrayList like a 2d array
Unlike arrays, you can't initialize an entire ArrayList directly. You can specify the expected size beforehand (this helps performance when you are using very large lists, so it is a good practice to do it always).
int boardSize = 3;
ArrayList<ArrayList<Character>> board = new ArrayList<ArrayList<Character>>(boardSize);
for (int i = 0; i < boardSize; i++) {
board.add(new ArrayList<Character>(boardSize));
for (int j = 0; j < boardSize; j++){
board.get(i).add('0');
}
}
How to create a Multidimensional ArrayList in Java?
ArrayList<ArrayList<String>> array = new ArrayList<ArrayList<String>>();
Depending on your requirements, you might use a Generic class like the one below to make access easier:
import java.util.ArrayList;
class TwoDimentionalArrayList<T> extends ArrayList<ArrayList<T>> {
public void addToInnerArray(int index, T element) {
while (index >= this.size()) {
this.add(new ArrayList<T>());
}
this.get(index).add(element);
}
public void addToInnerArray(int index, int index2, T element) {
while (index >= this.size()) {
this.add(new ArrayList<T>());
}
ArrayList<T> inner = this.get(index);
while (index2 >= inner.size()) {
inner.add(null);
}
inner.set(index2, element);
}
}
2D Array List of different types in Java
Instead of a List
use a Map
: That can "map" a value to another, and store it in a collection.
Something like this:
Map<String, Room> adjacentRooms = new HashmMap<>();
adjacentRooms.put("north", room);
adjacentRooms.get("east");
You may want to use constants, to make sure the values are "discrete".
It has a drawback, tho: it cannot assign more than 1 value to a key, that is more than 1 Room
s to a direction...
How to convert 2Darray into 2D ArrayList in java?
I would suggest 3 improvements in your code.
use
array2D[i].length
instead ofarray2D[1].length
.use
eachRecord.add(String.valueOf(array2D[i][j]));
instead ofeachRecord.add(String.valueOf(array2D[j]));
.
array2D[index]
returns a total array. array2D[indexRow][indexCol]
returns the object at those indexes.
- Instead of clear() initiate the eachRecord list inside the loop.
List<String> eachRecord = new ArrayList<String>();
inside the first for
loop.
String [][]array2D = {{"A", "B"}, {"C", "D"}, {"E", "F"}};
List<List<String>> arrayList2D = new ArrayList<List<String>>();
for (int i = 0; i < array2D.length; i++) {
List<String> eachRecord = new ArrayList<String>();
for (int j = 0; j < array2D[i].length; j++) {
eachRecord.add(String.valueOf(array2D[i][j]));
}
arrayList2D.add(eachRecord);
}
System.out.println(arrayList2D);//[[A, B], [C, D], [E, F]]
If you want to add whole array you could use Arrays#asList
method.
String[][] array2D = { { "A", "B" }, { "C", "D" }, { "E", "F" } };
List<List<String>> arrayList2D = new ArrayList<List<String>>();
for (int i = 0; i < array2D.length; i++) {
List<String> eachRecord = Arrays.asList(array2D[i]);
arrayList2D.add(eachRecord);
}
System.out.println(arrayList2D);
Related Topics
Refreshing Static Content with Spring MVC and Boot
When Do You Need to Explicitly Call a Superclass Constructor
Accessing Java Static Final Variable Value Through Reflection
Display Numbers from 1 to 100 Without Loops or Conditions
How to Copy Hashmap (Not Shallow Copy) in Java
Using Sha1 and Rsa with Java.Security.Signature VS. Messagedigest and Cipher
How to Achieve Conditional Resource Import in a Spring Xml Context
Differencebetween Canonical Name, Simple Name and Class Name in Java Class
Is Doing a Lot in Constructors Bad
Java Enum Methods - Return Opposite Direction Enum
Use Cases for Rxjava Schedulers
Java/Jaxb: Unmarshall Xml to Specific Subclass Based on an Attribute
"Faceted Project Problem (Java Version Mismatch)" Error Message