While coding I was trying to declare a class that can create an arraylist of arraylists, but soon enough I found it hard to define a proper constructor for my class. I wanted to define some methods for me to handle the huge outer arraylist(1000*1000), but I might be affected by C and always tried to use something like structdef.
How should I define my class? I guess declaring every lines seperatedly is not a wise choice, and I don't want to use 2D arraylist directly. Besides, how should I define a constructor to get an object that is an 2D arraylist?
//Update here
Below is my code example:
class farbicMap {
//attribute
ArrayList<Integer> farbicUnit = new ArrayList<Integer>();
//constructor
farbicMap () {
for (int i=0;i<1000;++i) {
farbicUnit.add(0);
}//this gives an arraylist with size of 100
//I want to use the above arraylist to construct another list here
}
//method
setUnitValue(int v) {
...
}
}
Seems that I didn't really understand the concept of class... I wanted to use the class to represent a map with some nodes. Now that's much clearer to me.
This is how I understood your consern:
class Test {
public static void main(String[] args) {
Board board = new Board(1000, 1000);
board.put(1, 2, "X");
Object x = board.get(1, 2);
System.out.println("x = " + x);
}
}
class Board {
private final int xSize;
private final int ySize;
private ArrayList<ArrayList<Object>> board = new ArrayList<>();
public Board(int xSize, int ySize) {
this.xSize = xSize;
this.ySize = ySize;
for (int i = 0; i < xSize; i++) {
board.add(getListOfNulls());
}
}
public Object get(int x, int y) {
return board.get(x).get(y);
}
public void put(int x, int y, Object toAdd) {
List<Object> xs = board.get(x);
if (xs == null) {
xs = getListOfNulls();
}
xs.add(y, toAdd);
}
private ArrayList<Object> getListOfNulls() {
ArrayList<Object> ys = new ArrayList<>();
for (int j = 0; j < ySize; j++) {
ys.add(null);
}
return ys;
}
}
You should use Array if size is fixed.
Related
So I have this code in the main class
public class OneDArrays
{
public static int[] create (int size)
{
int[] a1 = new int[size];
for (int i = 0; i < size; i++)
{
a1[i] = i*2+1;
}
return a1;
}
public int sumSome (int[] b1, int howmany)
{
int sum = 0;
if (howmany <= b1.length)
{
for (int i = 0; i < howmany; i++)
{
sum = sum + b1[i];
}
}
else
{
sum = -1;
}
return sum;
}
public int[] grow (int[] c1, int extra)
{
int[] newArray = new int[c1.length+extra];
for (int i = 0; i < newArray.length; i++)
{
while (i <= c1.length)
{
newArray[i] = c1[i];
i++;
}
newArray[i] = 0;
}
return newArray;
}
public void print (int[] d1)
{
for (int i = 0; i < d1.length; i++)
{
System.out.println (d1[i] + ", ");
}
}
}
And then I have my tester class,
public class OneDArraysTester
{
public static void main (String[] args)
{
int[] test1;
test1.create (5);
}
}
How do retrieve the method from the first class? I get the error that "create" is an undeclared method. If the "create" method were a constructer, I know I could just type create test1 = new create (5) but I don't see a way to turn it in to a constructer, so what's the way of doing that but for a method?
You invoke a static method with the classname. Literally className.methodName. Like,
int[] test1 = OneDArrays.create(5);
You have made a class named OneDArrays so you can call it's methods by creating an instance or object of that class.
like this :
OneDArrays ObjectOfClass = new OneDArrays();
int test1[] = ObjectOfClass.create(5);
similarly you can also call other methods of that class by accessing methods of this newly created object ObjectOfClass.
like :
sumOfArray = ObjectOfClass.sumSome(test1,3);
int biggerTest1[] = ObjectOfClass.grow(test1,10);
If you want to make create method works as a constructor than you can but you cannot return value from a constructor so you cannot return your array from that constructor.
Since you have declared the create method as static, #ElliotFrisch is the best way. But, it is not always a good idea to make methods static. So another way to achieve what you want would be to make the create method non-static.
public int[] create (int size){/*Method Body*/};
And then create an object of the OneDArray class to access the method.
OneDArrays oneDArrays = new OneDArrays();
int[] test1 = oneDArrays.create(5);
or,
int[] test1 = new OneDArrays().create(5);
I'm trying to test out my program to see how it works, but I'm not sure how to call upon it in the main method. I've tried doing Assignment5Solution.findOrder() but it does not work. Any help with this issue would be greatly appreciated. The code is supposed to take the number of classes a student has to take along with the prerequisites for each course if there are any, and put the correct order of what classes the student should take.
package Assignment5;
import java.lang.reflect.Array;
import java.util.*;
/**
*
* #author harpe
*/
class Assignment5Solution {
public int[] findOrder(int numCourses, int[][] prerequisites) {
int E = prerequisites.length;
Graph G = new Graph(numCourses);
for (int i = 0; i < E; i++) {
G.addEdge(prerequisites[i][1], prerequisites[i][0]);
} // Graph is constructed
DFS d = new DFS(G); // depth first search
return d.reverseDFSorder();
}
public class DFS {
private boolean[] marked;
private int[] courseOrder; // i.e., reverse post order
private boolean hasCycle;
private int index; // index for the array courseOrder, index 0 is for the course taken first, …
private HashSet<Integer> callStack; // used to detect if there are cycles on the graph
DFS(Graph G) {
marked = new boolean[G.V()];
courseOrder = new int[G.V()];
index = courseOrder.length - 1; // index 0 of courseOrder will be course taken first, lastIndex will be taken last
callStack = new HashSet<Integer>(); // HashSet is a hash table, for O(1) search
for (int v = 0; v < G.V(); v++) { // to visit each node, including those on islands or isolated
if (!marked[v] && !hasCycle) {
dfs(G, v);
}
}
}
private void dfs(Graph G, int v) {
marked[v] = true;
callStack.add(v); // use HashSet to simulate callStack
for (int w : G.adj(v)) {
if (!marked[w]) {
dfs(G, w);
} else if (callStack.contains(w)) // search in HashSet is O(1)
{
hasCycle = true; // this is a cycle!
break;
}
}
callStack.remove(v);
courseOrder[index--] = v; // index starts from array length -1, decrease by 1 each time, and then ends at 0
}
public int[] reverseDFSorder() {
if (hasCycle) {
return new int[0]; // return an empty int array (with size 0)
}
return courseOrder;
}
} // end of class DFS
public class Graph {
private int V;
private List[] adj;
Graph(int V) // constructor
{
this.V = V;
adj = new List[V];
for (int i = 0; i < V; i++) {
adj[i] = new ArrayList<Integer>();
}
}
public void addEdge(int v, int w) {
adj[v].add(w);
}
public Iterable<Integer> adj(int v) {
return adj[v];
}
public int V() {
return V;
}
} // end of class Graph
} // end of class Solution
public int[] findOrder(int numCourses, int[][] prerequisites) {}
would need to be:
public static int[] findOrder(int numCourses, int[][] prerequisites) {}
The static keyword means you do not need to a declare an object of the class to use it. So you can use it using:
Assignment5Solution.findOrder(numCourses, prerequisites)
//numCourses and prerequisites can be any int and int[][] respectively.
EDIT: Another note too, depending on where your main method is you may need to make class Assignment5Solution a public class with:
public class Assignment5Solution {
It currently is package protected so it will only be able to be used if it is in the same package.
EDIT2:
If you want to use it as a nonstatic method you need to do something like this(change null and 0 to the real values):
Assignment5Solution test = new Assignment5Solution() {};
int numCourses = 0;
int [][] prereqs = null;
int[] reverseOrder = test.findOrder(numCourses, prereqs);
I am trying to learn ArrayList and Vector.
If for example I have private Vector cardsInMyHand; can I change it to ArrayList. As you see or I am wrong.
private Vector numbers;
.
.
.
Vector studentNumbers;
studentNumbers = new Vector();
public int getNumbers(Vector studentNumbers, int x)
{
if (x >= 0 && x < studentNumber.size())
{
return ((Integer)hand.elementAt(x)).intValue();
} else
{
return 0;
}
}
change to private ??????; ...... ArrayList<String> studentNumbers = new ArrayList<String>();
Can I do this ?
public int getNumbers(ArrayList studentNumbers, int x)
{
if (x >= 0 && x < studentNumber.size())
{
return ((Integer)hand.elementAt(x)).intValue();
} else
{
return 0;
}
}
Yes, but your syntax has a few typos. And you should not use Raw Types. Finally, I suggest you use the List interface and read about Autoboxing and Unboxing.
public int getNumbers(List<Integer> studentNumbers, int x) {
if (x >= 0 && x < studentNumbers.size()) {
return studentNumbers.get(x);
} else {
return 0;
}
}
Check the below example with type safety.
public ArrayList<String> convertVectorToList(Vector<String> studentNoVec){
return new ArrayList<String>(studentNoVec);
}
public static void main(String[] args) {
Vector<String> v= new Vector<String>();
v.add("No1");
v.add("No2");
Test a = new Test();
for(String no : a.convertVectorToList(v)){
System.out.println(no);
}
}
First, Java Vector class considered obsolete or deprecated. If you want to convert Vector to an ArrayList use the below mentioned way. Simple and easy.
// convert vector to araryList
public ArrayList convertVectorToList(Vector studentNoVec){
return new ArrayList(studentNoVec);
}
I've been searching a lot for this problem and I can't find a solution. I'm trying to build a mini-game and I have a method for creating platforms. I have a class with every platform parameters, and I made a class array so i can have multiple platforms at the same time.
Problem: When I try to call the method for constructing the platform by sending the parameters I want, it gives me a NullPointerException. The method was working before, but with everything static and so i couldnt have multiple instances of that class, and now I removed the static fields from the platform class and it gives me the NullPointerException every time I call the method.
I copied the part of the code that gives me the error, the error goes the following way:
public static void main(String[] args) {
Game ex = new Game();
new Thread(ex).start();
}
In Game class:
public Load_Stage load = new Load_Stage();
public Game() {
-other variables initializatin-
Initialize_Items();
load.Stage_1(); // <--- problem this way
In Load_Stage class:
public class Load_Stage {
public Platforms plat = new Platforms();
public void Stage_1(){
Stage_Builder.Build_Platform(200, 500, 300, plat.platform1);
Stage_Builder.Build_Platform(100, 200, 100, plat.platform1);
}
}
And inside the Stage_Builder class:
public class Stage_Builder {
public static final int max_platforms = 10;
public static Platform_1[] p1 = new Platform_1[max_platforms];
public static boolean[] platform_on = new boolean[max_platforms];
public Stage_Builder() {
for (int c = 0; c < platform_on.length; c++) {
platform_on[c] = false;
}
}
public static void Build_Platform(int x, int y, int width, ImageIcon[] type) { // BUILDS A PLATFORM
for (int b = 0; b < max_platforms; b++) {
if (platform_on[b] == false) {
p1[b].Construct(x, y, width, type); // <-- NullPointerException here
platform_on[b] = true;
break;
}
}
}
}
Thanks beforehand.
EDIT: Here's the Platform_1 class (sorry for forgetting about it):
public class Platform_1 {
private int platform_begin_width = 30;
private int platform_middle_width = 20;
public int blocks_number = 0;
public ImageIcon[] platform_floors = new ImageIcon[500];
private int current_width = 0;
public int [] platform_x = new int [500];
public int platform_y = 0;
public int platform_width = 0;
public void Construct(int x, int y, int width, ImageIcon [] type) {
platform_width = width;
platform_y = y;
for (int c = 0; current_width <= platform_width; c++) {
if (c == 0) {
platform_x[c] = x;
platform_floors[c] = type[0];
current_width += platform_begin_width;
} else if ((current_width + platform_middle_width) > platform_width) {
platform_floors[c] = type[2];
blocks_number = c + 1;
platform_x[c] = current_width + x;
current_width += platform_middle_width;
} else {
platform_floors[c] = type[1];
platform_x[c] = current_width + x;
current_width += platform_middle_width;
}
}
}
}
And the Platforms class:
public class Platforms {
public ImageIcon[] platform1 = {new ImageIcon("Resources/Sprites/Stage_Objects/Platform1/begin.png"),
new ImageIcon("Resources/Sprites/Stage_Objects/Platform1/middle.png"),
new ImageIcon("Resources/Sprites/Stage_Objects/Platform1/end.png")};
}
The problem and solution are both obvious.
public static Platform_1[] p1 = new Platform_1[max_platforms];
After this line of code executes, p1 is an array of references of type Platform_1 that are all null.
Executing this line of code tells you so right away:
p1[b].Construct(x, y, width, type); // <-- NullPointerException here
The solution is to intialize the p1 array to point to non-null instances of Platform_1.
Something like this would work:
for (int i = 0; < p1.length; ++i) {
p1[i] = new Platform1();
}
I'm not seeing where you put things in the p1 array in the Stage_Builder class.
Another possibility (unlikely, but possible if you haven't shown everything) is that something in the Platform class, which you didn't show, is not initialized, and is breaking when you call Construct.
Also, the following seems problematic
public static Platform_1[] p1 = new Platform_1[max_platforms];
public static boolean[] platform_on = new boolean[max_platforms];
public Stage_Builder() {
for (int c = 0; c < platform_on.length; c++) {
platform_on[c] = false;
}
}
it appears you declare static variables p1 and platform_on, but you only populate platform_on in a constructor. So the first time you create a Stage_Builder instance, you populate one static array with all false, and don't put anything in the other static array...
Populate those static variables in a static block
// static var declarations
static {
// populate static arrays here.
}
The array you are calling a message on was never filled.
You have
public static Platform_1[] p1 = new Platform_1[max_platforms];
so p1 is
p1[0] = null
p1[1] = null
.
.
.
p1[max_platforms] = null
You try to call
p1[b].Construct(x, y, width, type);
which is
null.Construct(...);
You need to initialize that index on the array first.
p1[b] = new Platform_1();
p1[b].Construct(...);
First of all, your problem is that p1[b] is most likely null, as duffymo pointed out.
Secondly, you are using arrays in a really weird way. What about
a) delete Stage_Builder
b) Instead, have an ArrayList somewhere
c) The equivalent of Build_Platform1() look like this, then:
p1.add(new Platform1(x, y, width, type);
d) no if on[i], no max_platforms, no for loop to add a platform (the latter is a bad performance problem if you actually have a few hundret platforms)
Let me give some code so you can see what I'm doing with the following java code for android. Say for example I have the following two classes, one extended from the other:
class MyClassOne {
protected float x, y;
MyClassOne(float x, float y) {
this.x = x; this.y=y;
}
public void printY(){
System.out.print(y);
}
}
class MyClassTwo extends MyClassOne {
protected String stringSpecificToThisClass;
private long longSpecificTothisClass;
MyClassTwo(float x, float y, String s, long l) {
this.x=x; this.y=y;
this.longSpecificTothisClass= l; this.stringSpecificTothisClass=s;
}
}
These classes are then initialized in the following way
private ArrayList<MyClassOne> mClassOne = new ArrayList<MyClassOne>();
private ArrayList<MyClassTwo> mClassTwo = new ArrayList<MyClassTwo>();
for(int i=0;i<5;i++){
Random random = new Random(10);
mClassOne.add(new MyClassOne(i*12, random.nextInt()));
mClassTwo.add(new MyClassOne(i*11, random.nextInt()));
}
now, what I want to do is compare and sort both arraylists according to the value of y.
The way i do this for a single list is like so:
private Object[][] mSort(){
Object[][] mSort = new Object[mClassOne.size()][2];
for(int i = 0; i<mClassOne.size(); i++){
mSort[i][0] = i;
mSort[i][1] = mClassOne.get(i).y;
}
Arrays.sort(mSort, new Comparator<Object[]>(){
#Override
public int compare(Object[] obj1, Object[] obj2){
Float comp1 = (Float)obj1[1]; Float comp2 = (Float) obj2[1];
return comp1.compareTo(comp2);
}
});
return mSort;
}
Object[][] mSort = mSort();
for(int i=0;i<mClassOne.size();i++){
int z = (Integer)mSort[i][0];
mClassOne.get(z).printY();
}
which could output something like this:
2, 4, 5, 6, 9
Hopefully the code above is clear enough so others can see what I'm trying to do; the question is:
"How could I combine both ArrayLists then sort them by their respective y value."
The Answer I was looking for
ArrayList<MyClassOne> mTest = new ArrayList<MyClassOne>();
mTest.addAll(mClassOne);
mTest.addAll(mClassTwo);
Collections.sort(mTest, new Comparator<MyClassOne>(){
#Override
public int compare(MyClassOne obj1, MyClassTwo obj2) {
return (int) (obj1.getY() - obj2.getY()); }
}
);
// mTest is now sorted, verified by ~ foreach(mTest) {print mTest.getY(); }
Well, you could use a Comparator<MyClassOne> which should be able to handle MyClassTwo instances as well, since MyClassTwo extends MyClassOne.
Just create a single List<MyClassOne>, add all elements of the other lists and sort.
In cases where the classes don't extend each other, introduce a common interface.