The experiments will slowly converge towards one big experiment: a simple game. I have just a little interest in games (perhaps I should have more), but making one - even a simple one - should be self rewarding.
However, now to the point.
* The experiment creates an array of rectangle objects. * The rectangles are painted inside a Frame object at random x,y coords generated by a random number generator * The rectangles are stationary. * The rectangles are each assigned their own random colour. * The array of rectangle objects is created inside the constructor of the class.
The actual code contains various other variables and methods which would distract from a quick analysis, so below is code which has the same logical structure which also fails (instead of array of rectangles, I have used arrays of integers).
import java.util.Random; /** * Experiment 14 - see if it works simply - (with integer arrays) */ public class TestingArrays { // instance variables int N = 10; // the size of the array - 10 elements int[] a;
import java.io.*; import java.util.Scanner; public class asciiFile { int height; int width; Scanner input; char[][] poop; public asciiFile(File f) throws FileNotFoundException{ //constructor
[code]...
The constructor is supposed to take an ASCII file, take the numbers in the file, and populate a 2D array with the numbers in the file.
For some reason, the for loop I use to populate the array works outside of the constructor. When I put it in one of the methods, it runs normally. However, when I keep it in the constructor, I get the following error:
Exception in thread "main" java.lang.NullPointerException
I need to write a method that will consume string representation of Object type and will return one object of this type. How to set return type for the method in this case?
Here is exmaple :
public <?> identifyType(String typeString){ if (typesString.matches("String")){ return new String(""); }else if (typeString.matches("Integer")){ return new Integer(0); } //....etc..}
Is there a short term that means: "using an object reference with the datatype of a supertype to refer to an object with the datatype of a subtype"?
Saying "polymorphic reference" doesn't seem to be specific enough because ALL references other than those with the Object datatype are polymorphic. I saw one post that referred to these reference variables as a "supertype reference." In the absence of any other (more official) term, I may just use that term because calling a reference a "supertype reference" implies that there is a subtype. It sure beats what I was thinking about using: "sub-as-super." Which, when the inevitable mispronunciations occur, would lead to all kinds of off-color jokes.
Is there an "official" term that all of the dozens of Java books I have read seem to have missed? Or does everyone stumble along, spelling things out with a nearly sentence-long phrase.
It seems we have abandoned Dice/Die(s), and are now working on something completely foreign. I don't even have a code to start with because I haven't the faintest clue what is going on here (no notes given on this topic, as usual). We are given 4 half-written programs to work with.
The instructions are:
"Examine the FormLetterEntry abstract class, and create the two derived classesTextEntry and DataItemEntry. Be sure to implement all the abstract methods in each derived class."
This is the code we were given:
package homework5; import java.util.Properties; /** * Abstract class representing the entries in a form letter. **/ public abstract class FormLetterEntry { /** * Retrieve the template string for this entry * @return the value of this entry in a template **/
[code].....
I understand (and correct me if I am wrong) that a derived class is a class that is created from a base class via inheritance. What I don't see are any notes on how to write a derived class. I see some notes online on how to do so, but they don't fit with what he's written above. What he means by "Be sure to implement all the abstract methods in each derived class."
I am having an issue with a small part of my project. i am supposed to make a hash table with a file containing words. The file is being passed into my constructor, where i basically just all it "filename" of type string. Im supposed to get the contents of the file and put it into the table. the problem I'm having is making the connection between my constructor and a method called "start" which basically does all the work. Im not sure how to go by doing this, how could i use the variable "filename" from my constructer in my start method?:
import java.io.*; import java.util.*; public class WordCount { //private fields, including your HashMap variable HashMap<String,Object> hmap =new HashMap<String,Object>();//(table size,load factor) public WordCount( String infileName){ String filename =infileName;
So, this is weird for me because I don't really understand why the BorderLayout class constructor is being initialized as a parameter for the setLayout..
public Polygon polygonFrom(Point[] corners) { // method body goes here }
So from what I understand this is a constructor method for a Polygon object from the Polygon class. What I dont get is the name of the method polygonFrom()
Shouldn't a constructor for a Polygon just have the same name as the class? Because from earlier examples in the tutorial it seems to me that this is what has been done
For example:
public Bicycle(int startCadence, int startSpeed, int startGear) { gear = startGear; cadence = startCadence; speed = startSpeed; }
if i call a class that implements an interface the method inside the interface will be triggered automatically by the compiler or happens only in the observer pattern? i keep simple to be surr the message came across, a typical example would be a listener on a button, or a collection that calls comparator)
I don't understand, why when in the constructor of the superclass A the method init() is called (line 4), the overridden version of the subclass is invoked (line 26) and not the superclass version (line 7):
class A { public A () { init(); } public void init() { System.out.println("test");
[Code] ....
I would have guessed that above code prints
test 1
But instead you get a NPE (because when the constructor of B is invoked
public static void main(String[] args) { new B(); }
Then there is first the implicit call to super:
public B() { s = " "; init(); }
Which is the constructor of A:
public A () { init(); }
But here now this init() method is not the version of A ( public void init() { System.out.println("test"); }) but the overriden version of the subclass (B): public void init() { System.out.println(s+=s.length()); }...
Which throws an NPE of course, because the initialization of s has not occured yet (it happens only after the implicit call to super() has finished (see public B() { s = " "; init(); }))
1 - I don't understand why I'm getting an empty stack error when calling the removecard method in my Table class line 13?
2 - I am trying to use a constructor to populate my deck of cards instead of a method when creating my theDeck object in my Table class line 11 but I get the following error:
java.lang.StackOverflowError at java.util.Vector.<init>(Unknown Source) at java.util.Vector.<init>(Unknown Source) at java.util.Stack.<init>(Unknown Source) at Deck.<init>(Deck.java:7) at Deck.<init>(Deck.java:34)
I saw an example where an (inner)class is declared inside the main method, this is correct or not and why/when it's reasonable to use?so smth like this
public class myClass() { public static void myMethod(myInnerClass obj) { if (obj.method())
How do I create an instance of a class in a method?
I am a bit rusty whenever I think of instances. I always think of main method and objects when I see instance which gets me confused on what to do when I am not in a main method. The example:
I have a abstract class, School, and inside School I have some methods that must preform some action on an instance. For example, there is a move() method that must move the instance of School. Another method named, personOld(), which returns whether or not an instance of School surpassed some determined age.
this code won't compile because selected row must be declared as final because of it being defined outside the window listener. Is their anyway around this? If I make it final the first time that the variable is called it keeps it starting value until the GUI is closed.
butEdit.addActionListener (new ActionListener () { @Override public void actionPerformed (java.awt.event.ActionEvent evt) { int selectedRow = table.getSelectedRow (); final String [] values = custTableModel.getRowValues (selectedRow);
Created a java.sql.connection object. Refering those obj inside public void run() { } If i declare as final inside a method, i can't refer those outside method due to scope. Cannot refer to a non-final variable dbConnObj inside an inner class defined in a different method...
public Unit(String code, String name) { enrolStudent(student); this.unitCode = code; this.unitName = name; } public void enrolStudent(Student newStudent){ students = new ArrayList<Student>(); newStudent = new Student(24662496, "Kingsley", " Iwunze"); students.add(newStudent); }
how can I call this enrolStudent() method on this Unit constructor in another class when I create a new Unit. all I need is to enroll students in units when units are created. below is my create unit method.
I was practicing my java skills and came across an exercise in which a non parameter constructor calls a two parameter constructor. I tried a few searches online but they all came back unsuccessful. This is the part I am working on:
public PairOfDice(int val1, int val2) { // Constructor. Creates a pair of dice that // are initially showing the values val1 and val2. die1 = val1; // Assign specified values die2 = val2; // to the instance variables. } public PairOfDice() { // Constructor that calls two parameter constructor }
I tried calling the two constructor using the line "this(val1, val2)" but I get an error because val1 and val2 are local variables.
Then I tried to use the same signature: "this(int val1, int val2)" but that didn't work either.
I'm wonder about the issue of constructor for arrays. Let say I have a class tablica, and one component int[] tab. If I get it right until now tab is nothing more than empty reference to some unexisting array?
import java.util.Random; class tablica{ int[] tab; tablica (){ // i wish it was constructor
[code]....
Then, I'm trying to build the constructor for class tablica. What can be the parameter of such constructor? Is it fields of array? It is simple forf for basic variable
- I liken values defined in constructor with those global defined in class. But how to do it with array component tab.
If I create array object in main method then how can I use this constructor?
public class TestClass { public TestClass(String k){System.out.println(k);} public static void main(String[] args) { try { hello(); } catch(Exception e){System.out.println(e);}
[Code] ....
Explain how to catch block act as constructor with parameter?
"A constructor cannot be abstract, static, final, native, or synchronized."
I understand on why it can't be all of the above, except "final".
Why can't we have a final constructor, i understand constructors are not inherited, hence no chance/case of overriding etc. But why is it not allowed at all ?
So I am working on a school project and I have 2 classes, class FakeGravity contains all the properites and class BouncyBall is my driver class. For some reason when I try writing
FakeGravity gravity = new FakeGravity( );
I get an error. I am attaching an image of the error, and also attaching the program just in case you need more information. Also I was using blueJ to write the program
dcasarrubias, on 27 October 2014 - 02:44 PM, said:
So I am working on a school project and I have 2 classes, class FakeGravity contains all the properites and class BouncyBall is my driver class. For some reason when I try writing
FakeGravity gravity = new FakeGravity( );
I get an error. I am attaching an image of the error, and also attaching the program just in case you need more information.
The LocalStudent class inherits the Student class. The IDE states an error of "no default constructor in Student class".I understand that if a LocalStudent object is created, the default constructor of its superclass (aka Student class) will be called implicitly.there is no LocalStudent object being created, so why is the default constructor of Student being called ?
The default constructor of LocalStudent is also overloaded by the created no-arg constructor containining subjects = null; . So there is no call to the superclass default constructor from the default constructor of LocalStudent.
public class Student { private char year1; public Student(String name, char year){ year1 = year; } public char getYear(){ return year1;
I am trying to create a user defined Exception. I am using a hard-coded value in the constructor of circle class at the time of object creation.But in the constructor this value becomes 0.
import java.lang.Exception; import javax.swing.*; class InvalidRadiusException extends Exception{ private double r; public InvalidRadiusException(double radius){ r = radius;
[Code] ....
Due to this its always generating InvalidRadiusException even if i am supplying a non-zero non-negative value.
