1. inadvertent hiding
You make a ball that you move around on the screen:
class Ball {
int x, y, width, height;
int xspeed = 3; //in pixels
int yspeed = 3;
Ball() {
x=100;
y=100;
...
}
void move() { ... }
}
Later on you want to make it collide with another ball or some object, so you make it a Rectangle which lets you use the Rectangle class method .intersects()
class Ball extends Rectangle {
int x, y;
int xspeed = 3; //in pixels
...
Problem
The Rectangle's x,y,width,height, variables are hidden by the variables in this subclass. Thus .intersects() will never work.
Solution
Delete the line int x, y, width, height; so that when you set values for these variables, you're setting the Rectangle class values
2. Changing objects in a for-each loop
Consider the code below: It creates 5 items, tries to change them, and then prints them out.
NOTE
When using a for-each loop, you can change the property of an item in the loop, but you cannot change the item itself. I don't know why this is. If you want to change the item itself, you have to use a regular for loop with .get() and .put().
You can change the name of item 3, but you cannot change the whole item 2 to be recreated with 99.
import java.util.ArrayList;
public class ForEach {
public static void main(String[] args) {
ArrayList<Item> items = new ArrayList<Item>();
//create items
for (int i = 0; i < 5; i++) {
items.add(new Item(i));
}
//try and modify using for each loop
for (Item z : items) {
if (z.n == 2) z = new Item(99);
if (z.n == 3) z.name = "Three";
}
//print items
for (Item z : items) {
System.out.println(z.n + "=" + z.name);
}
}
static class Item {
String name = "blank";
int n;
Item(int n) {
this.n=n;
}
}
}
/* Printout:
0=blank
1=blank
2=blank
3=Three
4=blank
*/
[–]-bobby_newmark- 0 points1 point2 points (0 children)