Not quite sure why this won't work, when i try to compile and run it gives me a null pointer exception. I know it's super simple and probably a stupid question but I can't seem to figure it out!
import javax.swing.JOptionPane;
public class Whatever
{
private int age;
private String name;
private float salary;
public Whatever ()
{
String userName = JOptionPane.showInputDialog ("What is your name?");
Whatever listData[] = new Whatever [10];
listData[6].name = userName;
}
public static void main (String [] args)
{
Whatever testWhatever = new Whatever ();
}
}
Array of Whatever instances - all are null.
I would guess you'd have another problem with OutOfMemoryError as soon as you fix it, because when you call new to initialize the Whatever array elements they'll construct their own arrays and call new, and so on until you get OOM error.
I'll spell it out for you so you can get to the next error:
import javax.swing.JOptionPane;
public class Whatever
{
private int age;
private String name;
private float salary;
public Whatever () {
String userName = JOptionPane.showInputDialog ("What is your name?");
Whatever listData[] = new Whatever[10];
for (int i = 0; i < listData.length; ++i) {
listData[i] = new Whatever(); // This is where you'll get the OOM error. See why?
}
// You'll never get here.
listData[6].name = userName;
}
public static void main (String [] args)
{
Whatever testWhatever = new Whatever();
}
}
And you're putting Swing code in a constructor? Did you intend this as an example of how to write bad code?
Just for future reference, you should run your code in a good IDE - like IntelliJ, the best on the market - with debugging turned on and step through the code. You'll figure it out pretty quickly where the problem lies, faster than asking at SO will tell you.
So yes, it's a pretty stupid example. Hopefully you aren't writing anything like this for real.
With this
Whatever listData[] = new Whatever [10];
you initialized a new Array, but the elements in the Array are not initialized.
So you get a NullPointerException when you access listData[6].name.
You could try this:
for(int i = 0; i < listData.length; i++) {
listData[i] = new Whatever();
}
, but please do this not in the constructor itself.
Because then you would get OutOfMemoryException like duffymo said.
Try to do this directly in main for example.
Related
I'm not sure why, but I'm having a problem launching the code that I put together. For some reason, the code shows fatal errors when I enter a parameter within the methods. I have checked it several times but could not figure out what I did wrong.
Any advice would be appreciated.
public class songcode
{
/**
* #param args the command line arguments
*/
public class Songwriter{
private int song;
//variable for the amount of songs played
private int royalty;
//variable for the amount of payments
private int identification_number;
//id number of the song writer
public String first_name;
//string for the first name of songwriter
public String last_name;
//string for the last name of the songwriter
public int Adding_song(){
song = song + 1;
if(song > 100){
System.out.println("You are a big star!");
}
return(song);
}
public int Requesting_check(){
royalty = royalty + 10;
System.out.println("You just got payed");
return royalty;
}
public static void main(String[] args){
}
}
}
See the comments for some quick help :)
public class SongWriter {
private int song;
//variable for the number of songs played
private int royalty;
//variable for the number of payments
private int identification_number;
//id number of the song writer
public String first_name;
//string for the first name of songwriter
public String last_name;
//string for the last name of the songwriter
public SongWriter(int myFavSong){
//define default values for vars above
this.song = myFavSong;
//'This' references the Songwriter object not required but useful practice.
}
// Lower case first word upcase second : camelCase
public int addingSong(int numSongs){
song = song + numSongs;
if(song > 100){
System.out.println("You are a big star!");
}
return(song);
}
public int requestingCheck(){
royalty = royalty + 10;
System.out.println("You just got payed");
return royalty;
}
// The main method shouldn't be nested another class
public static void main(String[] args){
//In java because our variables and functions are not static
//you need a reference to your SongWrite object
SongWriter songWriter = new SongWriter();
// Notice I modified your function to take in an int as a param
songWriter.song = addingSong(5);
System.out.println(songWriter.song);
}
}
because of lack information it is a little hard for us to solve your problem. Please provide us exacly what steps did you take and what errors did you get.
From what i see now, since your are using inner class (class inside of another class), you should make it static:
static class Songwriter{/*your code here*/ }
When you got that you can create an object of that class in your main() by calling your outer class, so in your case it would be:
songcode.Songwriter name = new songcode.Songwriter();
Now you can use your methods that are within your inner class by using name.method() like:
name.Requesting_check();
for(int i = 0; i<200; i++){
name.Adding_song();
}
Personally i would create a new java file called Songwriter.java, add a constructor and work with that, but maybe that's not what you were testing here ;-)
Hope it did help you and please next time provide more detailed informations, so we can give you an exact answer without guessing what's wrong and what exacly do you want to achieve with your code.
I decided to create an account in order to ask a question I cant seem to figure out myself, or by some googling, hopefully I didn't just overlook it.
Essentially I am trying to make a text adventure game in Java, and am having a little trouble seeing how I should relate everything in the idea of objects. I have been successful in using XML stax and sending a file to the program, and using attributes and what not, to make it where the user can enter an integer associated with an option, and see if option requires an "item" or gives them an Item. I however did not take an OOP to this.
I want my new program to people able to take a string of user input in, instead of only an integer, and checking it against an array list if it exists. This is closer to the classic MUDs most may be familiar with.
I want to design it in a modular way, so I can slowly add on ideas, and more complexity to go along, so I don't want a "well it works so lets leave it alone" approach either.
Currently I simply want something close to this:
A Room object, which would have: an ID, Description, and interact-able
a Choice object (this one im not sure on) I thought about making an object to hold each rooms possible choices, both for exit, and for interact-ables
if so, the room object may need a Choice Object.
I've thought it over, tried some code, thought it over again, and every time, I keep ending up hard coding more than I feel I should, and making tons more variables than I feel are necessary, which makes me feel like i'm missing something crucial in my thinking.
I also want these rooms to be created through an inputted file, not generated in the code (so essentially the code is a story reader/crafter for any type, not one)
I have also been attempting this too long, and my solutions are becoming worse, but below was my most recent attempt at a rough Idea:
a GameManager class that takes the userInput and checks it some, before passing it along. I havent passed any data because im not sure of the approach. also im not used to regex, so some of that may also be wrong, if it is, maybe point it out, but that is not my focus
import java.util.Scanner;
public class GameManager {
private static final String EXIT_PHRASE = "exit";
public static void main(String[] args) {
Scanner userInput = new Scanner(System.in);
String userStringVal = "";
while(!userStringVal.equals(EXIT_PHRASE)){
userStringVal= userInput.nextLine();
if(checkKeywords(userStringVal)){
System.out.println("matches keyword");
}
else System.out.println("didnt match a keyword");
}
userInput.close();
}
public static boolean checkKeywords(String string){
boolean isKeyword = false;
string.toLowerCase();
if(string.matches("travel.*") || string.matches("search.*")){
System.out.println("passed first check");
String substring = string.substring(6);
if(matchDirection(substring)){
isKeyword = true;
}
}
return isKeyword;
}
public static boolean matchDirection(String string){
boolean hasDirection = false;
if(string.matches(".*\\bnorth|south|east|west|northeast|northwest|southeast| southwest|up|down")){
hasDirection = true;
}
return hasDirection;
}
}
The Room object I thought about as such:
import javax.xml.stream.XMLStreamException;
import javax.xml.stream.XMLStreamReader;
public class Room {
private String roomDescription = "";
private int roomID=0;
private int northExit=0;
private int southExit=0;
private int eastExit=0;
private int westExit=0;
private int northeastExit=0;
private int northwestExit=0;
private int southeastExit=0;
private int southwestExit=0;
private int upExit=0;
private int downExit=0;
private String[] interactables = new String[10];
private Options options = new Options();
public Room(XMLStreamReader reader) throws XMLStreamException{
setAttValues(reader);
setRoomDescription(reader);
setUpOptions();
}
public void setinteractables(XMLStreamReader reader){
int count = reader.getAttributeCount();
for(int i = 0; i < count; i++){
interactables[i] = reader.getAttributeValue(i);
}
}
public void setAttValues(XMLStreamReader reader){
int count = reader.getAttributeCount();
for(int i = 0; i < count; i++){
String att = reader.getAttributeLocalName(i);
if(att !=""){
switch(att){
case "North": northExit=Integer.parseInt(att);
case "South": southExit=Integer.parseInt(att);
case "East": eastExit=Integer.parseInt(att);
case "West": westExit=Integer.parseInt(att);
case "NorthEast": northeastExit=Integer.parseInt(att);
case "NorthWest": northwestExit=Integer.parseInt(att);
case "SouthEast": southeastExit=Integer.parseInt(att);
case "SouthWest": southwestExit=Integer.parseInt(att);
case "Up": upExit=Integer.parseInt(att);
case "Down": downExit=Integer.parseInt(att);
case "ID": roomID=Integer.parseInt(att);
}
}
}
}
public void setRoomDescription(XMLStreamReader reader) throws XMLStreamException{
roomDescription = reader.getElementText();
}
public void setUpOptions(){
options.setCardinalPointers(northExit, southExit, eastExit, westExit);
options.setIntercardinalPointers(northeastExit, northwestExit, southeastExit, southwestExit);
options.setElevationPointers(upExit, downExit);
}
}
what can I do to make sure I dont have to state so many directions with so many variables?
here is a quick and rough idea of an Option class that I thought about, but i didn't finish deciding I am already too far in the wrong direction
public class Options {
private int northPointer = 0;
private int southPointer= 0;
private int eastPointer = 0;
private int westPointer = 0;
private int northeastPointer= 0;
private int northwestPointer = 0;
private int southeastPointer = 0;
private int southwestPointer = 0;
private int upPointer = 0;
private int downPointer = 0;
private String northInteractable = "";
private String southInteractable = "";
private String eastInteractable = "";
private String westInteractable = "";
private String northeastInteractable ="";
private String northwestInteractable = "";
private String southeastInteractable = "";
private String southwestInteractable = "";
private String upInteractable = "";
private String downInteractable = "";
public Options(){
}
public void setCardinalPointers(int north, int south, int east, int west){
northPointer = north;
southPointer = south;
eastPointer = east;
westPointer = west;
}
public void setIntercardinalPointers(int northeast, int northwest, int southeast, int southwest){
northeastPointer = northeast;
northwestPointer=northwest;
southeastPointer=southeast;
southwestPointer=southwest;
}
public void setElevationPointers(int up, int down){
upPointer = up;
downPointer = down;
}
public String whatToReturn(String string){
String importantPart = "";
if(string.matches("travel.*")){
String substring = string.substring(6);
}
else {
importantPart = "Interactable";
String substring = string.substring(6);
if (substring.matches("\\bnorth\\b")) {
if(northInteractable!=0){
}
}
else if (substring.matches("\\bsouth\\b"))
else if (substring.matches("\\beast\\b"))
else if (substring.matches("\\bwest\\b"))
else if (substring.contains("northeast"))
else if (substring.contains("northwest"))
else if (substring.contains("southeast"))
else if (substring.contains("southwest"))
else if (substring.contains("up"))
else if (substring.contains("down"))
}
return importantPart;
}
}
I did not see the adventure tag until after I typed this, so I will start perusing through there, but will still post this, so my apologies if there is a good answer to this and I have yet to find it.
as a recap: what would be a good way to relate a few objects to create a room object (that gets its information from a file (XML being what im used to)) having exits, descriptions, and interactions. and the user interacting with these based off keywords that can be inputted freely, and not restricted to say, index values of array's holding keywords.
Im thinking when the user types something like "travel north" to first check if they typed a keyword, in this case being travel, then a direction. Then somewhree else checking if it states travel, check north with a possible northExit a room may or may not have. Then if its another keyword, say like check, to make it easy also have the exact same directions, but check for a different string.
Then if room "northExit" exists, get an option somehow, with a pointer to another roomID. though This thought process causes me issues when thinking about future possibility of requiring items for getting to the next room. Also where to store/acquire these options is causing some difficulties.
There are two things I would like to introduce to you. The first, in the enum. You can think of this as a special kind of class where all the possible options are enumerated in the class definition. This is perfect for things like, in your case, directions. Enums can be simple, where you just list all of the possible options for use in other classes:
public enum Direction {
NORTH, NORTH_EAST, EAST, SOUTH_EAST, SOUTH, SOUTH_WEST, WEST, NOTH_WEST;
}
They can be a bit more complex, if you want them to have methods and attributes of their own:
public enum Direction {
NORTH(true), NORTH_EAST(false), EAST(true), SOUTH_EAST(false), SOUTH(true), SOUTH_WEST(false), WEST(true), NOTH_WEST(false);
private final boolean isCardinal;
private Direction(boolean isCardinal){
this.isCardinal = isCardinal;
}
public boolean isCardinal(){
return isCardinal;
}
public static Collection<Direction> getCardinalDirections(){
return Arrays.asList(Direction.values()).stream().filter(Direction::isCardinal).collect(Collectors.toList());
}
public static Collection<Direction> getIncardinalDirections(){
return Arrays.asList(Direction.values()).stream().filter(x -> !x.isCardinal()).collect(Collectors.toList());
}
}
Please read more about Java enum types here.
The second thing I would like to introduce to you is the data structure known as the Map. Maps are also known as Dictionaries, and that can often help understanding how they work. A Map will take one object and map it to another object, like how a Dictionary maps a word to its definition, or a phonebook maps a person's name to their phone number. We can simplify your Room class a ton by using a Map. I am not going to reproduce all of your code, since I'm focusing on your Room exists right now:
public class Room {
private Map<Direction, Room> exits;
public Room(){
this.exits = new HashMap<>();
}
public void setExit(Direction direction, Room room){
this.exits.put(direction, room);
}
public Room getExit(Direction direction){
return this.exits.get(direction);
}
}
Please read more about the Java Map interface here.
You will, of course, need to adapt your methods which are reading from XML, etc. But, now, your Room class should be greatly simplified.
I hope this points you in a helpful direction.
I'm studying Java and am obviously very new to it. I have what appears to be a working application, however it doesn't produce any output to the console window as expected - it also doesn't give any errors which is baffling. I'm using Eclipse. I would greatly appreciate a second set of eyes to spot out what I'm doing wrong. There is some code in there from trying different approaches to get this to work - which I will delete later.
My question is: Why isn't my program producing output? As a follow up: Why isn't it producing some kind of error since it's not producing output?
I've researched for a couple of days now and the closest I found was this article which although it is similar but it doesn't really apply to this situation.
System.out.print() doesn't send any output to Eclipse console. Why?
*Edited - shortened up the code & add comments on how to fix.
I have 2 classes - Student and Roster.
Roster:
//** client class....*/
import java.util.ArrayList;
import java.util.Arrays;
public class Roster {
/* String copied from the instructions, adding info as the last item*/
static String[] students = {"1,John,Smith,John1989#gmail.com,20,88,79,59",
"2,Suzan,Erickson,Erickson_1990#gmailcom,19,91,72,85",
"3,Jack,Napoli,The_lawyer99yahoo.com,19,85,84,87"};
static ArrayList<Student> studentProfiles = new ArrayList<Student>();
public static void main(String[] args) {
//creating the arraylist - rather, converting a string of arrays to an Array list of student objects
{
for (int i = 0; i < students.length; i++) {
String component = students[i];
String[] terms = component.split(",");
String studentID = terms[0];
String firstName = terms[1];
String lastName = terms[2];
String emailAddress = terms[3];
String age = terms[4];
int grade1 = Integer.parseInt(terms[5]);
int grade2 = Integer.parseInt(terms[6]);
int grade3 = Integer.parseInt(terms[7]);
Student student = new Student(studentID, firstName, lastName, emailAddress, age, grade1, grade2, grade3);
studentProfiles.add(student);
}
}
}
//test just to generate output, didn't work. <-- Moving this inside the main generated the output I needed.
{
System.out.println(studentProfiles);
}
///
/// enter methods here
///
//
}
//other print all attempt
//-->> Put inside main
public static void print_all()
//-->> Put inside main
System.out.println(j);
`enter code here`
//-->> Put inside main
print_all();
print_invalid_emails();
print_average_grade(student);
remove("3");
remove("3");
//}
//}
}
}
You need to put your print inside main method.
The block that you are using:
{
System.out.println(studentProfiles);
}
is called Initializer block and is called when you call the constructor of Roaster.
Try putting the System.out.println(studentProfiles); inside the Main() method.
Also have a look at this:
https://docs.oracle.com/javase/tutorial/java/javaOO/initial.html
public class Pig {
private int pigss;
private Pig[] pigs;
public Pig[] pigNumber (int pigss)
{
pigs = new Pig [pigss];
return pigs;
}
Code that includes main method:
public class animals{
public static void main(String[] args){
Pig cool = new Pig();
Scanner keyboard = new Scanner (System.in);
System.out.println("How many pigs are there?");
int pigss = Integer.parseInt( keyboard.nextLine() );
cool.pigNumber(pigss);
//This is where I have trouble. I want to use the array pigs here in the main method, this is what i tried:
Pig[] pigs = cool.pigNumber(pigss);
I then tried to use a for loop and assign values (String) to the index of arrays (pigs[]). But the error that gives me is: cannot convert from String to Pig. Any tips are appreciated. THank you.
for(int j = 0; j < pigs.length; j++)
{
System.out.println("What is the pig " + (j+1) + "'s name");
pigs[j] = keyboard.nextLine();
}
Your pigs will need an attribute to contain the string values you are trying to pass:
public class Pig {
private String name;
public void setName(String n) {
name = n;
}
public String getName() {
return name;
}
Then when you want to assign this string value to your pig:
int indexOfPig = 0; // Or whatever it is supposed to be
pigs[indexOfPig].setName("I am a string");
In java you can only use ints as the indexes of arrays
It is saying 'cannot convert from String to Pig' because you can't do that!
If you want somehow convert a String to a Pig, you are going to need to write some code to do the conversion. For example, you might write a constructor that creates a new Pig from some kind of description. Or you might write a method that looks up a Pig by name or number or something.
It is hard to offer any more concrete advice because you don't tell us what is in the string values ... or how you expect the strings to become pigs. (The only suggestion I have is to try Macrame :-) )
Pig doesn't have a name member or even method that accepts a string. Also you are trying to assign a String(keyboard.nextline() to a Pig(pigs[j].
Add an attribute name to your pig.
class Pig{
public String name:
public void Pig(String name){
this.name = name;
}
}
Then assign a new instance of Pig in the loop.
pigs[j] = new Pig(keyboard.nextLine());
Also get rid of the useless class pigNumber. All you need is an ArrayList of Pigs. The array list can be dynanically sized.
List<Pig> pigs = new ArrayList<Pig>
so your loop could be something like
String name = ""
while(true){
name = keyboard.readline();
if(name== "stop"){
break;
}
pigs.add(new Pig(names);
}
Then getting the number of pigs is a simple
System.out.println(pigs.length());
The error i am having here is a infinite or near infinite loop in my method calls and class's creating other class's constructors. What my program is trying to do is semi-randomly generate survey results based off actual statistics. I would highly appreciate not only some insight in whats going wrong here. But some advice and pointers on how to prevent this from happening and ways to analyze the error messages by myself. I get how some of the work but like i stated below i am new to programming im a freshman in college so programming is new to me. Thanks in advance and sorry for my previous post, thought i would take the time to give you guys an appropriate one.
Im new to programming this is my 2nd project ive done on my own so im sorry if its not the best.
This is my Test class:
public Tester()
{
randomGenerator = new Random();
probability = new Probability();
stats = new Statistics();
double chance = randomGenerator.nextDouble();
double gender = probability.getProbabilityOfMale();
if(chance > gender)
{
male = false;
stats.incrementFemale();
}else{
male = true;
stats.incrementMale();
}
age = randomGenerator.nextInt(49)+16;
int range = stats.getNumberOfQuestion();
for(int i=0;i<range;i++)
{
probabilities = probability.probOfAnswer(i);
answers = probability.getAnswers(i);
chance = randomGenerator.nextDouble();
int size = probabilities.size();
for(int j=0;j<size;j++)
{
double qTemp = chance - probabilities.get(j);
if(qTemp <= 0.0)
{
Answer aTemp = answers.get(j);
aTemp.incrementCounter();
answers.set(j,aTemp);
}
}
}
}
Statistics class:
public ArrayList<Answer> getAnswers(int index)
{
temp = survey.getAnswers(index);
return temp;
}
public int getMale()
{
return male;
}
public int getFemale()
{
return female;
}
public int getNumberOfQuestion()
{
return numberOfQuestion;
}
public void incrementNumberOfQuestion()
{
numberOfQuestion++;
}
public void incrementMale()
{
male++;
}
public void incrementFemale()
{
female++;
}
and probability class:
public Probability()
{
stats = new Statistics();
probOfAnswer = new ArrayList<Double>(0);
}
public ArrayList<Double> probOfAnswer(int index)
{
temp = stats.getAnswers(index);
int size = temp.size();
for(int i=0;i<size;i++)
{
aTemp = temp.get(i);
for(int j=0;j<size;j++)
{
Answer aTemp = temp.get(j);
sum += (double)aTemp.getCounter();
}
double number = (double)aTemp.getCounter();
probOfAnswer.add(number/sum);
sum = 0;
}
return probOfAnswer;
}
public ArrayList<Answer> getAnswers(int index)
{
temp = stats.getAnswers(index);
return temp;
}
public ArrayList<Double> getProbofAnswer()
{
return probOfAnswer;
}
public void probabilityOfMale()
{
double male = (double)stats.getMale();
double female = (double)stats.getFemale();
probabilityOfMale = male / (male + female);
}
public double getProbabilityOfMale()
{
return probabilityOfMale;
}
These are the only real important parts where the loop exsists the rest of the code is not needed to be uploaded.
Im having difficulty uploading my error message on this site its not accepting it as code in the code insert, then it wont let me submit the message afterwards so im going to upload the code elseware and link it.
http://forum.overdosed.net/index.php/topic/56608-this-is-unimportant/
But i dont know how long that forum will let me keep that post there ><
at Question.<init>(Question.java:17)
at Survey.addQuestion(Survey.java:23)
at Statistics.<init>(Statistics.java:52)
at Question.<init>(Question.java:17)
at Survey.addQuestion(Survey.java:23)
at Statistics.<init>(Statistics.java:52)
at Probability.<init>(Probability.java:19)
You need to check why Question is creating Statistics object and again Statistics is trying to create Question object leading to infinite recursion. As the line numbers are given you can take a look at corresponding lines.
Judging by the stack trace, the problem lies in three parts which you haven't shown us - the Question and Statistics constructors and the Survey.addQuestion method:
From the stack trace:
at Survey.addQuestion(Survey.java:23)
at Statistics.<init>(Statistics.java:52)
at Question.<init>(Question.java:17)
at Survey.addQuestion(Survey.java:23)
at Statistics.<init>(Statistics.java:52)
at Question.<init>(Question.java:17)
So your Question constructor is calling the Statistics constructor. But the Statistics constructor is then calling Survey.addQuestion, which is in turn calling the Question constructor.
It feels to me like there's much more construction going on than is really useful. Why would a Statistics constructor need to add anything to a survey? I wouldn't expect a Statistics class to even know about surveys and questions.
It's entirely possible that a lot of this can be fixed by passing a reference to an existing object to the constructors - so the Probability constructor may be better taking a Statistics reference in its constructor and using that for its stats field than creating a new Statistics object itself. It's hard to say without knowing what these classes are really meant to represent though... which may be part of the problem. Do you have a firm grasp of what the responsibility of each class is? Think about that carefully before making any code changes.
We don't have the relevant source code, but the error message says what's wrong:
Tester creates a Probability
Probability constructor creates a Statistics
Statistics constructor calls Survey.addQuestion()
addQuestion() creates a Question
Question creates a Statistics (goto 3 and loop infinitely)
I think you should probably pass objects around rather than creating them each time.