So, I'm trying to practice my java skills by applying it to some math homework and making a frequency distribution chart using inheritance. In my head, I envision it as a frequency distribution (parent class = FreqDist) that can have multiple "MyStatClasses" (in the form of the MyStatClass array). Each FreqDist has variables that span across all MyStatClasses which is why I put them in the parent class. However, when I call the MyStatClass constructor, my program gets a StackOverflowError. I think this is because the super(s, i) line calls back to the FreqDist constructor and starts over, causing an infinite loop. Assuming this is the case, how would I fix this?
Ideally, I'd like to access my MyStatClass array and grab values that only apply to that MyStatClass, but I cannot get it to work.
public class FreqDist {
private MyStatClass[] freqClasses;
private double[] dblValuesArray;
private int intNumberOfClasses;
private double dblMax;
private double dblMin;
private int intClassWidth;
public FreqDist(String strValues, int intNumOfClasses) {
System.out.println("This is the beginning of the FreqDist constructor...");
dblValuesArray = getDoubleValues(strValues);
intNumberOfClasses = intNumOfClasses;
dblMin = dblValuesArray[0];
dblMax = dblValuesArray[dblValuesArray.length - 1];
intClassWidth = (int)Math.ceil((dblMax - dblMin) / intNumberOfClasses);
freqClasses = new MyStatClass[intNumberOfClasses];
for (int x = 0; x < freqClasses.length; x++) {
freqClasses[x] = new MyStatClass(strValues, intNumOfClasses);
}
}
public double[] getDoubleValues(String strValues) {
String[] strValuesArray = strValues.trim().split(" ");
dblValuesArray = new double[strValuesArray.length];
for (int x = 0; x < strValuesArray.length; x++) {
dblValuesArray[x] = Double.parseDouble(strValuesArray[x]);
}
Arrays.sort(dblValuesArray);
return dblValuesArray;
}
public int getNumberOfClasses() {
return intNumberOfClasses;
}
public double getMin() {
return dblMin;
}
public double getMax() {
return dblMax;
}
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
System.out.print("What are the values? ");
String values = scan.nextLine();
System.out.print("How many classes? ");
int classes = scan.nextInt();
FreqDist fd = new FreqDist(values, classes);
}
}
public class MyStatClass extends FreqDist {
public MyStatClass(String s, int i) {
super(s, i);
}
}
Ok so this is mostly an issue with a flaw in your design.
From what I understand FreqDist is a class that should contain an array of MyStatClass. You want them to have the same properties so you make MyStatClass extend FreqDist. However when you call FreqDist it MyStatClass which Calls a new MyStatClass over and over and over.
One way to solve this is to create a new class that has the shared properties you want FreqDist and MyStatClass to have, and have those two classes inherit from said class. Then create separate constructors for FreqDist and MyStatClass.
A parent type should never refer to its own subtypes, as yours does. Her the parent initializes subtype instances, which require that each initialize the parent type, which initializes subtype instances, which initialize their parent type, which initializes... KABLOOEY!
Related
I am a novice Java programmer trying to use classes defined in a different file. So, I've written these two .java files:
First, there's MyLibrary.java:
package mymainprogram;
public class MyLibrary {
public class MyRecord {
int number;
char letter;
}
public static int TriplePlusThree(int input_number) {
return ((input_number*3) + 3);
}
}
Then, MyMainProgram.java:
package mymainprogram;
import java.util.Scanner;
public class MyMainProgram {
public static void main(String[] args) {
Scanner keyread = new Scanner(System.in);
System.out.print("Enter Number to Process: ");
int num = keyread.nextInt();
int result = MyLibrary.TriplePlusThree(num);
System.out.println("3x + 3 = "+result);
String letters = "ABCDEFGHIJ";
MyLibrary.MyRecord[] TenRecs = new MyLibrary.MyRecord[10];
for (int i = 0; i < 10; i++) {
TenRecs[i].number = i; //NullPointerException here
TenRecs[i].letter = letters.charAt(i);
}
}
}
I had no problem getting the method to work just fine; now my goal is to create an array where each member of the array has an integer and character. (Note: I'm not looking for better ways to accomplish this objective; I'm merely using this trivial example to try to get this working).
When I tried to run my program, I got:
java.lang.NullPointerException
I researched this, and found this page, which says:
If we try to access the objects even before creating them, run time errors would occur. For instance, the following statement throws a NullPointerException during runtime which indicates that [this array] isn't yet pointing to [an] object. The objects have to be instantiated using the constructor of the class and their references should be assigned to the array elements in the following way.
studentArray[0] = new Student();
So, I tried to do that in my Main Program:
MyRecordArray[0] = new MyLibrary.MyRecord();
but that gives this error:
an enclosing instance that contains MyLibrary.MyRecord is required
That error message led me to this Stack Exchange question, which says:
you have to create an object of X class (outer class) and then use objX.new InnerClass() syntax to create an object of Y class.
X x = new X();
X.Y y = x.new Y();
So, in accordance with that answer, I've added these two lines to my program:
MyLibrary mylibrary = new MyLibrary();
MyLibrary.MyRecord myrecord = mylibrary.new MyRecord();
Those lines don't give any warnings or compilation errors, so I feel like I'm one step closer, but I'm still trying to figure out how to make an array. I know if I wanted to make an array of integers, I would simply do this:
int[] TenInts = new int[10];
So, I've tried things like:
myrecord[] TenRecs = new myrecord[10];
MyRecord[] TenRecs = new MyRecord[10];
But nothing is working, and I feel like I'm grasping at straws now. I get the feeling that the right set of eyes could solve this pretty quickly.
You need to declare the inner class as static.
You can modify the code as follows to suit your requirements:
This is the code for MyLibrary
public class MyLibrary {
public static class MyRecord{
int number;
char letter;
public MyRecord(){
number = 0;
letter = '\0';
}
public MyRecord(int number, char letter){
this.number = number;
this.letter = letter;
}
}
public static int TriplePlusThree(int input_number){
return (input_number * 3 + 3);
}
}
This is the code for the MyMainProgram
import java.util.Scanner;
public class MyMainProgram {
public static void main(String[] args){
Scanner in = new Scanner(System.in);
System.out.println("Enter number to process");
int num = in.nextInt();
System.out.println("3x + 3 = " + MyLibrary.TriplePlusThree(num));
String letters = "ABCDEFGHIJ";
MyLibrary.MyRecord[] TenRecords = new MyLibrary.MyRecord[2];
for (int i=0; i<TenRecords.length; i++){
TenRecords[i] = new MyLibrary.MyRecord();
TenRecords[i].number = i;
TenRecords[i].letter = letters.charAt(i);
}
// Printing class records
for (int i=0; i<TenRecords.length; i++){
System.out.println("Printing records of record " + i + " : ");
System.out.println("Number : " + TenRecords[i].number);
System.out.println("Letter : " + TenRecords[i].letter);
}
in.close();
}
}
You can create the instance of the inner class as follows:
TenRecords[i] = new MyLibrary.MyRecord();
Hope this helps.
The nested class MyRecord contains a hidden reference to the outer class MyLibrary and therefore must be associated with an instance of MyLibrary. This way MyRecord can access private members of MyLibrary.
MyLibrary.MyRecord myrecord = mylibrary.new MyRecord();
Wow, this is funny syntax. In all my years of java programming, I never used such a construct. Typically, you would create objects of inner classes (MyRecord) within the outer class (MyLibrary). Another common thing is to declare the inner class as static which would eliminate the need for an instance of the outer class.
MyRecord[] TenRecs = new MyRecord[10];
This will create an array where all the elements are NULL. You have to initialize each of them (e.g. with a loop).
If you initialize MyRecord[10] the array has null objects. You still have to initialize each element in the array to a new MyRecord object. Otherwise you will get the NPE.
one way to do is : List<MyRecord> TenRecs = new ArrayList<MyRecord>();
TenRecs.add( new MyRecord() );
or for ( int i = 0; i < 10; i++ ) TenRecs[i] = new MyRecord();
also if you add an import statement : import mymainpackage.MyLibrary.MyRecord; You don't need to do mylibrary.new MyRecord(); just do new MyRecord();
You have to create each object in array before initialize. Refer to this link.
Create each object like this.
MyLibrary outer = new MyLibrary();
TenRecs[i] = outer.new MyRecord();
Full code:
MyLibrary.MyRecord[] TenRecs = new MyLibrary.MyRecord[10];
for (int i = 0; i < 10; i++) {
MyLibrary outer = new MyLibrary();
TenRecs[i] = outer.new MyRecord();
TenRecs[i].number = i;
TenRecs[i].letter = letters.charAt(i);
}
There are several points you need to note.
First, difference between a instance inner class and a static inner class.
An instance inner class, declared without static modifier,
public class OutterClass {
public class InstanceInnerClass {}
}
should be created like this:
OutterClass outter = new OutterClass();
InstanceInnerClass iInner = outter.new InstanceInnerClass();
while a static inner class, declared with static modifier,
public class OutterClass {
public static class StaticInnerClass {}
}
should be created like this:
StaticInnerClass sInner = new OutterClass.StaticInnerClass();
Secondly, you accessed an array entry before it is filled
MyLibrary library = new MyLibrary();
MyLibrary.MyRecord[] TenRecs = new MyLibrary.MyRecord[10];
for (int i = 0; i < 10; i++) {
// Create new instance
TenRecs[i] = library.new MyRecord();
TenRecs[i].number = i;
TenRecs[i].letter = letters.charAt(i);
}
class anyName
{
int Tcol = 0;
int fc = 0;
int x = 0;
float randx = (random(1, 1000));
float randy = (random (0, 600));
int Tsizes = 1;
{
if (fc >= x) { //Random Ellipse 3
stroke (Tcol);
fill (Tcol);
ellipse (randx, randy, Tsizes, Tsizes);
}
}
}
anyName ranx1 = new anyName();
ranx1.x = 100;
Hi, I am trying to add a class/object to my code and it is not working. This is the class I have so far, but when I instantiate one object from that class (ranx1), and then try change one of the variables inside it (x), it says there is a error. Is there anything I need to change? I would really appreciate any help.
Since I instantiated an object from that class, how would I change the variables for the new object? For example, if in the class x = 0, then I made a copy and this time I want x to = 100, but all the other variables such as Tcol and fc to stay the same. I know this is possible because my teacher taught it, but it is not working right now for me.
ranx1.x = 100;
You need to declare your variables as "public" if you are trying to access from a class that is not in the same package.
I guess your problem is you are trying to access to that attribute from a class that is in another package, you should declare the atrributes as public to gain access to them. But this solution wouldn't be totally correct, a better approach is declaring them as private and creating public getters and setters to access/modify them.
Said that, you should post a working example, that piece code does not compile because you are trying to execute code that is out of any class... and I'm not sure what you are trying to do with the curly braces before the if clause.
when you careate a class, every member has a control access.
when you don't state the control access like:
public x;
protected fc;
private Tcol;
they all get default private.
you can't access private members from outside the class.
do:
class anyName
{
public int Tcol = 0;
public int fc = 0;
public int x = 0;
public float randx = (random(1, 1000));
public float randy = (random (0, 600));
public int Tsizes = 1;
{
if (fc >= x) { //Random Ellipse 3
stroke (Tcol);
fill (Tcol);
ellipse (randx, randy, Tsizes, Tsizes);
}
}
}
whoever, i must point out that most times it's not recommended to set members access as public and you should learn about getter and setters.
now i hope the rest of your code is in a main function and in a main class, but if not it should be like so:
public class Main
{
public static void main(String[]args){
anyName ranx1 = new anyName();
ranx1.x = 100;
}
}
First of all you cannot access sub class veriables like that. If you want to access you should make this
public class ExampleApp {
class anyName
{
int x = 0;
}
public static void main(String[]args){
ExampleApp ea = new ExampleApp();
ExampleApp.anyName ranx1= ea.new anyName();
ranx1.x =100;
}
}
Or you can use them inside of the class with method
public void method() {
anyName ea = new anyName();
ea.x=100;
}
you cannot use your veriables private if you want to access them.Secondly if you want to use this libraries you cant use innerclass. Inside of innerclass you cannot access libraries because java sees it in diffrent package. If you make it public class and import Math you can use it otherwise you should make method for your ellipse, random .
I'm looking for a better way to organize my class.
Right now my code looks like this:
mainMethod:
-number1 input
-call method1 with number1 als value
method1:
-do "stuff" with input
-call method2 with new "stuff" as value
method2:
-do stuff
-call method3
etc...
So i start with user input in my main method and my whole class works like domino, the first method needs to be called to run the next method.
I would rather have method1 return a value and save it in some global variable in my class which can be used by method2 and so on.
Here is my Code with exactly this problem: (it calculates sqrt)
package wurzel;
import java.util.Scanner;
import wurzel.Fraction;
public class wurzel {
public static void main(String[] args) {
Scanner s = new Scanner(System.in);
System.out.println("Eingabe:");
int in = s.nextInt();
s.close();
bisection(in);
}
private static int bisection(int N){
int counter = 0;
for(int i = 0; i < N;i++){
if((counter*counter) > N){
counter--;
break;
}
else if(counter*counter == N){
break;
}
else counter++;
}
calculateSequence(counter,N);
return counter;
}
static int[] calculateSequence(int vKomma, int look){
int m = 0;
int d = 1;
int a = vKomma;
int[] intarr = new int[4];
intarr[0] = vKomma;
for(int i = 1; i <= intarr.length; i++){
if(i == intarr.length )
break;
else{
m = (d*a) - m;
d = (look - (m*m)) / d;
a = (vKomma + m) / d;
intarr[i] = a;
}
}
calculateApproximation(intarr);
return intarr;
}
static double calculateApproximation(int[] sequence ){
Fraction result = new Fraction((sequence.length)-1);
for(int dcounter = sequence.length; dcounter > 0; dcounter--){
result = result.reciprocal().add(sequence[dcounter-1]);
}
System.out.println("Approximation als double: " +result.doubleValue());
System.out.println("Approximation als Bruch: " +result);
return result.doubleValue();
}
}
You should also seperate code into different classes. E.g. have a generic MathHelper class which has these methods in it.
This helps keep code seperate and easy to read.
You mentioned that you wanted to store some data in a global variable for the class as a whole. The way to do this is by making the class use instance methods rather than class methods. With class methods, you have to use class variables. But, when you use instance methods, you can define instance variables which can be accessed by any method within the class.
To do this, change all your methods (apart from main()) by removing the keyword static from each method. Then, define the instance variable that is global for the class. e.g. if your global variable is of type int then it would be:
private int myInstanceVariable;
This instance variable, or global variable, can be accessed by a getter and setter.
It is quite normal methods to call each other and to form long chains of calls.
So I would not worry about that. Actually in real world, in enterprise code,
you have call stack going through tens of calls. So I think global variables would
be worse practice compared to what you did.
I have extended a class in hope to store a global array (make the array within the class be seen by another object) by using the set1sub(sub.local) method
public class BattleShipsClient extends ShipLocations implements Runnable, BattleShipConstants
{
BattleShipsClient()
{
}
public void placeBoard(int player) throws IOException// this is only called once and not updated when clicked again
{
while(getLocations())
{
while(isWaiting())//true
{
toServer.writeInt(player);
int row = getRowSelected();
int col = getColumnSelected();
int choice=Integer.parseInt(JOptionPane.showInputDialog("Please 1 for a sub, 2 for a battleship and 3 for a destroyer"));
clickCount ++;
if(clickCount >2)
{
setLocation(false);
continueToPlay=true;
}
if (choice ==1 &&sub.placed==false)
{
String orientation =JOptionPane.showInputDialog("please enter your orientations");
if(orientation.equalsIgnoreCase("h"))
{
//row os the same
//column number will be sent, then plus one on the other side
sub.local = new int[2][2];
sub.local[0][0] =row;
sub.local[0][1]=col;
sub.local[1][0]=row;
sub.local[1][1] =col+1;
toServer.writeInt(row);
toServer.writeInt(col);
toServer.writeChar('s');
sub.placed=true;
setp1sub(sub.local);
/*setp1sub(new int[][]{{row,col},
{row,col+1}});*/
grid[row][col+1].setToken(getMyToken());
}
I then have a ship Locations class however when i create a new object of the ship locations class and try to read this array it always is set to [[0, 0], [0, 0]], ive tried making it static and atomic
public class ShipLocations {
int [][] p1sub;
public ShipLocations()
{
p1sub = new int[2][2];
}
public int[][] getp1sub()
{
return p1sub;
}
public void setp1sub(int[][] local) {
for (int i = 0;i <local.length;i++)
{
for(int j = 0;j<local.length;j++)
{
p1sub [i][j]= local[i][j];
}
}
}
}
Whenever you create a new instance of ShipLocations(or a subclass) the constructor is called, which in your case, reinitializes the p1sub array.
In your design, you are overusing inheritance. You should not inherit from a class just to use its methods and variables.
To store a global variable in a class:
public class ShipLocations {
static int [][] p1sub;
static{
p1sub = new int[2][2];
}
public static void setp1sub(...){...}
public static int[][] getp1sub(){...}
}
And then use it by class name instead of creating instances:
int [][] x = ShipLocations.getp1sub();
Though the use of global variables shoud be avoided when possible. It is considered bad design and might be a problem when reusing the code.
The correct way of doing this is to have the ShipLocations object as a local variable in BattleShipsClient and set it when initializing new instance. You will then first create a common ShipLocation object and hand it to every client that should see the same array.
I am creating a text based game and I am having some issues.. This is what I have so far. So far I have a Combat Class, and two Classes for two different Weapons. I am trying to assign hit points to the weapons themselves. But my biggest issue is in the Combat class. I am trying to create it to were there will be random weapon drops at random times and also random Weapons. So far in the Combat class I have this:
public class Combat {
final int chanceOfDrop = 3;
static Weapons[] wepArray = {new M4(), new M16()}
static boolean[] hasWeapon = {false, true};
public static int ranNumberGen(int chanceOfDrop) {
return (int) (Math.random()*1);
}
private void enemyDead() {
boolean canDrop = false;
if(ranNumberGen(chanceOfDrop)==0){
canDrop = true;
}
if(canDrop == true){
givePlayerWeapon(Weapon[Combat.ranNumberGen(Weapons.length)]);
}
private static void givePlayerWeapon(int w) {
hasWeapon[w] = true;
for w <(Weapons.length-1) {
if has weapon[w] {
System.out.println(wepArray[w].getWeaponName);
}
}
}
}
}
}
I have issues when I am creating the new M4(), and the new M16() it says Type mismatch: cannot convert form M4 to Weapons. I do have a class named Weapons, could that be the problem?
And here is my M4 Class, both M4 and M16 Classes are identical
public abstract class M4 {
private Integer weaponDamage = 5;
private Integer weaponAmmo = 25;
private String weaponName = "M4";
public M4(String name, int ammo, int damage) {
name = weaponName;
ammo = weaponAmmo;
damage = weaponDamage;
}
public String getWeaponName() {
return weaponName;
}
public Integer getAmmo() {
return weaponAmmo;
}
public Integer getDamage() {
return weaponDamage;
}
}
I don't think I have any issues here. Maybe my problem lies within this though. Although, I have a Weapons class, but nothing in it. Do I need that?
A few things to fix at first sight:
Create a generic Weapon class that defines some properties that apply to each weapon, like name, damage, ammo, scope multiplier, etc... Then create subclasses for Weapon, like M4 and M16, that specify the properties and eventually add weapon-specific properties.
Add brackets to this line:
System.out.println(wepArray[w].getWeaponName); // Change to getWeaponName()
Remove the abstract keyword from M4.
Fix the ranNumberGen method because it will always return 0 right now. Math.random() returns a float in the range [0,1[. This means that casting it to an int will always result in 0. Multiply it by n to have a random int in the range of [0, n[. You probably want this:
public static int ranNumberGen(int max) {
return (int) (Math.random() * max);
}
Change this line:
givePlayerWeapon(Weapon[Combat.ranNumberGen(Weapons.length)]);
to:
givePlayerWeapon(wepArray[Combat.ranNumberGen(wepArray.length)]);
The syntax of a for-loop is like this:
for (variable-initialization; condition; increment)
So in your case, you want:
for (int i = 0; i < hasWeapon.length; ++i)
{
if (hasWeapon[i]) System.out.println(wepArray[i].getWeaponName());
}
You might want to revisit your decision to use an inheritance-style heirarchy for game objects before it is too late.
In practice, I've found a component-entity model and/or prototype model to be much more effective. You could take a look at the code in my old Java roguelike game Tyrant for inspiration:
Weapon definitions: mikera/tyrant/Weapon.java (Github is down right now so can't find the exact link, but should be easy enough to Google)
The idea is that you make your objects by setting properties / composing compoenents in a Map-like game object rather than using static inheritance.
When you want to create a random weapon in this model, you can just get a list of all the possible weapon prototypes, and clone one of them at random to make a new weapon.
the mean of abstract in "public abstract class M4" is that you cannot make a new object with this class.
So you can put all commons fields of your weapons in the weapon class and make m4 and m16 extends the weapon and you code would compile.