Develop Reference

How to call abstract methods from an ArrayList traversing objects?

I am trying to use a loop to traverse an Arraylist of objects, but when I call abstract methods to be printed I get the symbol cannot be found error For example:
ArrayList<Shape> al = new ArrayList<Shape>();
Shape triangle = new Triangle(3.0, 2.5, 2.0);
Shape rectangle = new Rectangle(2.0, 4.0);
Shape circle = new Circle(1.0);
al.add(triangle);
al.add(rectangle);
al.add(circle);
for(int i = 0; i < al.size(); i++)
{
System.out.println(al.get(i), al.calculateArea(), al.calculatePerimeter(), al.toString());
}
}
Full rectangle class
public class Rectangle extends Shape
{
private double length, width;
public Rectangle(double length, double width)
{
double length1 = length;
double width1 = width;
}
public double calculateArea()
{
double rectangleArea = length * width;
return rectangleArea;
}
public double calculatePerimeter()
{
double rectanglePerimeter = (length * 2) + (width * 2);
return rectanglePerimeter;
}
public String toString()
{
// put your code here
return super.toString() + "[length=" + length + "width=" + width + "]";
}
toString() and get(i) seem to work fine, but in calling the abstract methods implemented in triangle, circle etc subclasses the symbol error comes up. I tried overriding these methods in the subclasses but I get the same error.

Here:
al.calculateArea()
You invoke the methods on your list al, not on a List element!
And of course, the List itself doesn't know anything about the methods your list elements provide! Because the list object is of type List (respectively ArrayList). A List is not a Shape, thus you can't call Shape methods on the list.
You need
al.get(i).calculateArea()
for example! Or even simpler:
for (Shape aShape : al) {
shape.calculateArea();...
In other words: you don't pay with your wallet, you pay by fetching your money out of the wallet, and then you pay with that money!

Manually translate polygon

public class Hexagon extends JPanel {
// Team that controls the hexagon
public int controller; // 0 neutral // 1 is team 1 // 2 is team 2
// Color of the hexagon
// /* All attributes of the board, used for size an boarder etc... */ Board board
// /* Determines where the hexagon sits on the game board */ int position
public static void main(String args[])
{
JFrame j = new JFrame();
j.setSize(350, 250);
for(int i = 0; i < 121; i++)
{
Hexagon hex = new Hexagon();
j.add(hex);
}
j.setVisible(true);
}
#Override
public void paintComponent(Graphics shape)
{
super.paintComponent(shape);
Polygon hexagon = new Polygon();
// x, y coordinate centers, r is radius from center
Double x, y;
// Define sides of polygon for hexagon
for(int i = 0; i < 6; i++)
{
x = 25 + 22 * Math.cos(i * 2 * Math.PI / 6);
y = 25 + 22 * Math.sin(i * 2 * Math.PI / 6);
hexagon.addPoint(x.intValue(), y.intValue());
}
// Automatic translate
hexagon.translate(10, 10);
// How do I manually control translate?
shape.drawPolygon(hexagon);
}
}
How do I manually translate a polygon? I need to do it to create a game board. So far I've only accomplished automatic translation of polygons which is definitely what I don't need.

I mean like parameterize it so that it's not always 10.
Then you need parameters for your class:
Create a method in your class like `setTranslation(int x, int y) and save the x/y values to instance variables.
In the paintComponent() method you reference these instance variables
Then when you create the Hexagon you can manually set the translation.
Something like:
public void setTranslation(int translationX, int translationY)
{
this.translationX = translationX;
this.translationY = translationY;
}
...
//hexagon.translate(10, 10);
hexagon.translate(translateX, translateY);
...
Hexagon hex = new Hexagon();
hex.setTranslation(10, 10);
Or, you can just pass the translation values as parameters to the constructor of your Hexagon class. The point is you need to have custom properties in your Hexagon class if you want each Hexagon to have a different translation.

Single array to store multiple variables in each element

EDIT: to clarify, one of my requirements is to use a single array.
I am having trouble storing multiple variables to a single element in an array. We are creating a really simple program to mimic Microsoft Paint. One of the requirements is to store each element I draw in to an array so that the 'paint' method repaints the drawings each time the windows is minimized and then redisplayed. Here are the requirements
We are to assume a max size for the array is 20.
Each element should include 5 variables:
char shape (l for line, r for rectangle, c for circle)
Start x value
Start y value
width (rectangle), or ending x (line), or radius (circle)
height (rectangle), or ending y (line), or radius (circle)
Here is my code for the array class:
class storeDraws {
final int MAXSIZE = 20;
static int S[];
static int n; //number of draws user makes
static char shape;
static double px, py, w, h;
storeDraws () {
S = new int [MAXSIZE];
n = 0;
shape = 'l';
px = 0;
py = 0;
w = 0;
h = 0;
}
}
I have read a few places that I can input the array as (using the mouseReleased(MouseEvent e) method:
storeDraws[] input = new storeDraws{value, value, value, value, value};
But I don't think that would work for what I am trying to do with the 'paint' method to redraw the shapes. I thought I could somehow pass it using the standard format of S[n] = (char, double, double, double, double), but I get warning that this is illegal.
Edit 8:30 am
I got this part working. In my class here is my code now.
class storeDraws {
static char shape;
static int px, py, w, h;
storeDraws () {
shape = 'l';
px = 0;
py = 0;
w = 0;
h = 0;
}
}
I then declared this in the DrawPanel class:
private storeDraws[] store = new storeDraws[20];
private int n = 0;
And mouseReleased method of DrawPanel:
public void mouseReleased(MouseEvent e) {
if (drawShape == "line") {
store[n].shape = 'l';
store[n].px = p1.x;
store[n].py = p1.y;
store[n].w = p3.x;
store[n].h = p3.y;
n++;
}
And paint:
public void paint(Graphics g) {
for (int i = 0; i < n; i++) {
if (store[i].shape == 'l')
g.drawLine(store[n].px, store[n].py, store[n].w, store[n].h);
But if I draw 6 lines it only repaints the last line.

I think you need to separate some of the functionality you want. You can have a class for each of the elements and then store instances of the class in an array of DrawingElement objects.
So you'd do something like this:
DrawingElement[] drawing = new DrawingElement[20];
DrawingElement circle = new DrawingElement('c', 10, 10, 10, 10);
DrawingElement rect = new DrawingElement('r', 20, 10, 10, 10);
drawing[0] = circle;
drawing[1] = rect;
Note: If you need to be able to get the number of objects in the array (variable n in your code) you may want to use some implementation of
a Linked List (which has a size() method) and do some check when adding elements to make sure you don't add past the max of 20.
Example with LinkedList:
LinkedList<DrawingElement> drawing = new LinkedList<DrawingElement>();
DrawingElement circle = new DrawingElement('c', 10, 10, 10, 10);
DrawingElement rect = new DrawingElement('r', 20, 10, 10, 10);
drawing.add(circle);
drawing.add(rect);
int n = drawing.size(); //will be 2
The Drawing Element Class:
public class DrawingElement
{
char shape;
double px, py, w, h;
public DrawingElement(char shape, double px, double py, double w, double h)
{
this.shape = shape;
this.px = px;
this.py = py;
this.w = w;
this.h = h;
}
//Add getters and setters (accessors and mutators) for class variables
}

I think you should use collection for this purpose.Create array, store values
then add each array object in collection(list).

you can use arraylist instead of array. this will help you to store different variable.
If there is need of using only array you can use object array
Sample Code
object a[]= {'a',10,10.2,10.3,10.4,10.5}

Please refer ArrayList
In your case it would be better to use Java Collection(ArrayList) rather than Array.

First point, We cannot store different types of variables values into an Array.
An array is a container object that holds a fixed number of values of a single type.
Here you are trying to store multiple type of variables into integer array.
see this link:
https://docs.oracle.com/javase/tutorial/java/nutsandbolts/arrays.html
To overcome this drawback, We have Collection API introduced with JDK1.5
Refer this link:
http://docs.oracle.com/javase/tutorial/collections/index.html

Is there a method that computes a smaller rectangle out of two given rectangles?

One of the exercises in my Java textbook says "Consult the API documentation to find methods for:
Computing the smallest rectangle that contains two given rectangles. • Returning a random floating-point number."
I've looked at the Java API for class Rectangle, but I can't find one that computes the smaller rectangle. The closest methods I've found are union and bounds, but I don't think that's correct.
I found min from the Java Math class and wrote a test program to see if it would work, but min cannot have arguments of rectangles.
Here's the code I wrote:
import java.awt.Rectangle;
public class RectangleSize {
public static void main(String[] args)
{
Rectangle a = new Rectangle(5, 5, 10, 10);
Rectangle b = new Rectangle(5, 5, 20, 20);
int min = Math.min(a, b); //In Eclipse, I get an error.
System.out.println(min);
}
}

I am studying java recently using the textbook of Big Java Early Object. I found the same question in chapter 2. Eventually I found the answer by looking at Java SE8 API list.
First use method .add() to combine 2 rectangles.
Note: actually, this combined one is already the one you need. But you can get a new rectangle (same size and location) at step 2.
Them use .getBounds() to get the smallest rectangle containing the combined one.
.
import java.awt.*;
public class RectangleTester01 {
public static void main(String[] args) {
Rectangle box1 = new Rectangle(10, 20, 40, 40);
Rectangle box2 = new Rectangle(20, 30, 60, 60);
box1.add(box2);
System.out.println(box1);
Rectangle box3 = box1.getBounds();
System.out.println(box3);
}
}
output is:
java.awt.Rectangle[x=10,y=20,width=70,height=70]
java.awt.Rectangle[x=10,y=20,width=70,height=70]

You want to use Rectangle.contains . You would be given many rectangles. You'd need to loop through all the rectangles and see if it contains the two rectangles given. If it does you should calculate the size of that rectangle. In the end you take the rectangle with the smallest size.
public Rectangle getSmallest(Rectangle one, Rectangle two, Rectangle[] rectangles) {
Rectangle smallest = null;
double area = Double.MAX_VALUE;
for (Rectangle r: rectangles) {
if (r.contains(one) && r.contains(two)) {
calculatedArea = r.getWidth() * r.getHeight();
if (calculatedArea < area) {
area = calculatedArea;
smallest = r;
}
}
}
return r;
}

I think the method you're looking for is Rectangle2D.createUnion. It combines two rectangles to make a bigger one that contains both with a minimum of extra space.

Here is another question to find a rectangle which contains a list of rectangles.find-smallest-area-that-contains-all-the-rectangles.
Here is my brute answer which is not accuracy and did not try the union function of Rectengle and also the createUnion(Rectengle2D r) as well.
Math.min can help find the minimum number in a number array. You may write a similar one for Rectangle. This is something from 1D to 2D in my mind. BTW, it would be much interesting if you extends it to 3D or nD objects calculation.
package com.stackoverflow.q26311076;
import java.awt.Rectangle;
public class Test {
public static void main(String[] args) {
Rectangle a = new Rectangle(5, 5, 10 , 10);
Rectangle b = new Rectangle(5, 5, 20 , 20);
Rectangle min = getMin(a, b) ;
// ...
}
public static Rectangle getMin(Rectangle a, Rectangle b) {
//find the min range in X.
{
double x1 = a.getX();
double x2 = a.getX() + a.getWidth();
double x3 = b.getX();
double x4 = b.getX() + b.getWidth();
double minX1 =Math.min( Math.min(x1, x2), Math.min(x3, x4)) ;
double maxX1 =Math.max( Math.max(x1, x2), Math.max(x3, x4)) ;
}
//find the min range in Y.
{
double y1 = a.getY();
double y2 = a.getY() + a.getHeight();
double y3 = b.getY();
double y4 = b.getY() + b.getHeight();
double minY1 =Math.min( Math.min(y1, y2), Math.min(y3, y4)) ;
double maxY1 =Math.max( Math.max(y1, y2), Math.max(y3, y4)) ;
}
//build new rectangle with X & Y
Rectangle r = new Rectangle();
r.setRect(minX1, minY1, maxX1 - minX1, maxY1 - minY1);
return r;
}
}

How to use constructor in Java

Can someone please tell me what's wrong with this simple program? I'm getting output as "0".
package myConst;
public class Doconstructor
{
int length,width;
Doconstructor(int x, int y)
{
int area;
area = length * width;
System.out.println("area ="+area);
}
}
class work
{
public static void main(String args[])
{
Doconstructor d1 = new Doconstructor(10, 15);
}
}

Doconstructor d1 = new Doconstructor(10, 15);
// you are assigning values for x and y
But
Doconstructor (int x,int y)
{
int area; // you are never use x and y values for calculation
area = length *width; // so area remain 0 since current length and width is 0
System.out.println("area ="+area);
}
You need to change your code as follows.
Doconstructor (int x,int y)
{
int area;
this.length=x;
this.width=y;
area = length *width;
System.out.println("area ="+area);
}

Edit like this:-
package myConst;
public class Doconstructor
{
int length,width;
Doconstructor(int x, int y)
{
int area;
this.length=x;//Using this for current object
this.width=y;//Using this for current object
area = length * width;
System.out.println("area ="+area);
}
}
class work
{
public static void main(String args[])
{
Doconstructor d1 = new Doconstructor(10, 15);
}
}
Your output will be:
area =150
Must read this :
http://docs.oracle.com/javase/tutorial/java/javaOO/thiskey.html

You are not setting the values of length and width and by default they are both 0. You might have to do this:
Doconstructor(int x, int y){
int area;
area = x * y;
length = x;
width = y;
System.out.println("Area = "+area);
}

You're not using the variable values you pass to the constructor in it but rather the length and width values that have been initialized to 0. You want area = x * y; instead.

The length and width fields are implicitly initialized to 0. Multiply them and you get 0.
I think what you want is
length = y ;
width = x ;
int area = length * width ;
System.out.println("area ="+area);

You have this:
public class Doconstructor {
int length,width;
Doconstructor (int x,int y)
{
int area;
area = length *width;
System.out.println("area ="+area);
}
}
At no point do you set length or width equal to anything. Their initial values are 0 and your program is doing precisely what you told it to do. area = length * width = 0 * 0 = 0.
You also are not doing anything with the x or y that you passed to the constructor, but this probably was not your intention. When writing programs, you basically need to clearly instruct the computer to do what you want to do. It's not going to guess what you want. If you ignore x and y, and don't assign any values to length or width, then that is precisely what will happen and you cannot be surprised when you see the results you see.

you are writing int length,width at class level so length and width are set to 0 as default.
After that in the constructor you are not setting any values to length and width so you are the values for length and width are 0.Hence area is also 0
Please check this link for list of default values

Constructors are used to create objects and to set the attributes. You are not setting the attributes in your constructor. Here is how your constructor should look like.
Doconstructor(int x, int y){
length = x;
width = y;
}
Secondly you are mixing the logic of a constructor and a method. You are doing the calculation of area, which seems to be a perfect fit for another method in your class. so better move that logic in a separate method:
public int calculateArea() {
int area;
area = x * y;
return area;
}
Finally create an object using constructor to set the attributes length and width. And then call calculateArea method to do the business logic of calculating area.
public static void main(String args[]){
Doconstructor d1 = new Doconstructor(10, 15); // create object and set length & width
d1.calculateArea();
}

you are not assigning the value of x and y to the variables width and length. The default value of width and length are (int) 0. Thats why you are getting the output (0*0=0). First assign the values to the variables or use "area=x*y;" .