I have a task to operate on complex number. Each number consists of double r = real part, double i = imaginary part and String name. Name must be set within constructor, so I've created int counter, then I'm sending its value to setNextName function and get name letter back. Unfortunately incrementing this 'counter' value works only within costructor and then it is once again set to 0. How to deal with that?Some constant value? And second problem is that I also need to provide setNextNames(char c) function that will change the counter current value.
The code :
public class Imaginary {
private double re;
private double im;
private String real;
private String imaginary;
private String name;
private int counter=0;
public Imaginary(double r, double u){
re = r;
im = u;
name = this.setNextName(counter);
counter++;
}
public static String setNextName(int c){
String nameTab[] = {"A","B","C","D","E","F","G","H","I","J","K","L","M","N",
"O","P","Q","R","S","T","U","W","V","X","Y","Z"};
String setName = nameTab[c];
System.out.println("c: "+c);
return setName;
}
public static String setNextName(char c){
//
//don't know how to deal with this part
//
}
It's hard to tell what you're doing, but I suspect this will solve your immediate problem:
private static int counter = 0;
You should make counter static.
You should also make nameTab a private static field, then in setNextName(), you can iterate through it to find the name corresponding to the given character, and get its index. (in the plain ASCII world, of course one could simply calculate the index by subtracting the numeric value of 'A' from the given character, but I am not quite sure how it would work out with Java, in Unicode, with crazy inputs - iteration is on the safe side.)
In OO languages there are typically two types of variables that go into a class:
instance variables that are unique to each instance
class variables that are shared by all instances of the class
Given a class like:
public class Person
{
// class variable
private static int numberOfEyes;
// instance variable
private String name;
// other code goes here
}
If you were to do something like:
Person a = new Person("Jane Doe");
Person b = new Person("John Doe");
and then do something like:
a.setName("Jane Foe");
the name for Person "a" would change, but the one for Person "b" would stay the same.
If you woke up one morning and decided you wanted 3 eyes:
Person.setNumberOfEyes(3);
then Person "a" and Person "b" and every other Person instance out there would suddenly have 3 eyes as well.
You want to put "static" in your counter declaration.
is your code being used by multiple threads than i would suggest that making counter static won't solve ur problem.
you need to take extra care by implementing thread synchronization use lock keyword as shown below.
private static readonly obj = new Object();
private static int counter =0;
public Imaginary(double r, double u)
{
re = r;
im = u;
lock(obj)
{
name = this.setNextName(counter);
counter++;
}
}
this will ensure thread safety also while incrementing your counter (there are another ways also to provide thread security but this one is having least code).
Because the field counter is not static, every object has its own counter.
Related
I am learning Java, so I understand this is a very simple question, but I still want to understand it.
I want to let my code automatically generate soldiers, and the number automatically increases, but I failed.
the Soldier.class:
package com.mayer;
import java.util.Random;
public class Soldier {
private int number=0;
private int ATK;
private int HP;
Random ra = new Random();
public Soldier(){
this.number++;
this.ATK = ra.nextInt(10)+90;
this.HP = ra.nextInt(20)+180;
}
public void report(){
System.out.println("number:"+this.number+"\t"+
"ATK:"+this.ATK+"\t"+
"HP:"+this.HP);
}
}
the main.class
package com.mayer;
public class Main {
public static void main(String[] args) {
Soldier[] soldiers = new Soldier[5];
int i = 0;
while(i<5){
soldiers[i] = new Soldier();
i++;
}
for(Soldier sol:soldiers){
sol.report();
}
}
}
That's what I get:
number:1 ATK:94 HP:187
number:1 ATK:94 HP:181
number:1 ATK:96 HP:193
number:1 ATK:90 HP:183
number:1 ATK:95 HP:193
So you see,each of this number is 1.
You have added number field which is instance field. It will initialize per instance. You are looking for static type variable. Please check static into java.
Instance Variables (Non-Static Fields) Technically speaking, objects
store their individual states in "non-static fields", that is, fields
declared without the static keyword. Non-static fields are also known
as instance variables because their values are unique to each instance
of a class (to each object, in other words); the currentSpeed of one
bicycle is independent from the currentSpeed of another.
Class Variables (Static Fields) A class variable is any field declared with the static modifier; this tells the compiler that there
is exactly one copy of this variable in existence, regardless of how
many times the class has been instantiated. A field defining the
number of gears for a particular kind of bicycle could be marked as
static since conceptually the same number of gears will apply to all
instances. The code static int numGears = 6; would create such a
static field. Additionally, the keyword final could be added to
indicate that the number of gears will never change.
The constructor is changed to：
public Soldier(int number){
this.number = number;
this.ATK = ra.nextInt(10)+90;
this.HP = ra.nextInt(20)+180;
}
As others have said, each Soldier instance has its own separate number field which starts with 0. You can use a static field to count the instances:
public class Soldier {
private static int counter = 0;
private int number;
// other fields left out for clarity
public Soldier(){
Soldier.counter++; // field shared among all Soldier instances
this.number = counter; // number belongs to this instance only
// ...
}
// ...
}
However, I wouldn't recommend doing it this way. When you get more advanced, you'll learn that using a static field like this can cause problems in a multi-threaded application. I would instead advise passing the number to the Soldier constructor:
public class Soldier {
private int number;
// ...
public Soldier(int number){
this.number = number;
// ...
}
// ...
}
And then:
public static void main(String[] args) {
Soldier[] soldiers = new Soldier[5];
int i = 0;
while(i<5){
soldiers[i] = new Soldier(i);
i++;
}
Soldier.class
all-uppercase field names tend to be used for constants.. basic fields use headless camel-case.. They should also be descriptive, i.e. you should look at them an it should be apparent what they represent - for example a variable "number" is not a good idea, because it's ambiguous
Random can be converted to a local variable, no need to keep it on the class level
The mechanism by which soldiers are assigned IDs should be on a higher level - it can't be managed by the soldier object itself, hence the constructor with an argument
overriding the toString method is the traditional way of transforming the object to string for debugging purposes.. also most IDEs can generate it with a press of a button so no space for human error
You will obviously need getters and setters for your variables, if you wish to read or change them from elsewhere, but I don't think that's necessary to post here.
private int soldierID;
private int attack;
private int health;
public Soldier(int id){
this.soldierID = id;
Random random = new Random();
this.attack = random.nextInt(10) + 90;
this.health = random.nextInt(20) + 180;
}
#Override
public String toString() {
return "Soldier{" +
"soldierID=" + soldierID +
", attack=" + attack +
", health=" + health +
'}';
}
Main.class
it's perfectly fine and actually preferred to use a List instead of an array, because it's more comfortable to work with
this way it's even much easier to add them dynamically and use the iterator for ID
you can "report" in the creation cycle
This even shortens the method a bit, not that it's that important here.
public static void main(String[] args){
List<Soldier> soldiers = new ArrayList<>();
for(int i=0; i<5; i++){
Soldier newSoldier = new Soldier(i);
soldiers.add(newSoldier);
System.out.println(newSoldier.toString());
}
}
This way when you define the soldier IDs it's not from within the Soldier class but rather from something that is "observing" all the soldier classes and knows which is which.
I am really new to java and just trying to get my head around how everything works. I have a method like this:
public assignmentmarks(String name, int mark1, int mark2, int mark3)
{
}
and the question asks to create the constructor that uses all the fields (courseName, assignment1, assignment2, assignment3)
This is what I have tried
import java.util.Scanner;
public class assignmentmarks {
private String courseName;
private int assignment1;
private int assignment2;
private int assignment3;
int average;
int mark;
Scanner scanner = new Scanner(System.in);
public void AssignmentMarks(String name, int mark1, int mark2, int mark3)
{
assignment1 = mark1;
assignment2 = mark2;
assignment3 = mark3;
courseName = name;
AssignmentMarks assignmentMarks = new AssignmentMarks(mark1, mark2, mark3, name);
}
I have a method like this:
public assignmentmarks(String name, int mark1, int mark2, int mark3)
{
}
That is not a method. It is a constructor!!
A constructor is a "method-like thing" that has no return type, and the same name as the enclosing class.
All you need to do is add some statements that will assign the parameters to the fields of your class.
Having said that, assignmentmarks is a bad choice for a class name. The Java style rules say that a class name should:
Start with a capital letter
Use camel case; i.e. each embedded word should start with a capital letter.
Thus ... AssignmentMarks would be a better name.
(Yes ... this kind of stuff really does matter. Conforming to standard style makes your code readable, which makes it more maintainable, which will save you and your future colleagues time and hair-tearing.)
Also note the names (identifiers) in Java are case sensitive. So you need to be consistent. Don't use assignmentmarks in one place and AssignmentMarks in another. That is likely to lead to compilation errors ... or worse.
There are few ways to go around that.
Firstly the thing you aimed to create is called an "All-arguments-constructor" meaning you want to have a declared way to create an instance(entity) of a class and while doing so you want to have all properties(fields\parameters) of it filled with values specified on the call of the said constructor.
There is a hack way to do so using lombok and just annotating your class with #AllArgsConstructor, but I recommend you continue learning how those things are made by hand and then revisit mentioned syntaxis sugar later.
With that being said you want to create something like a method that neither has return type nor "void" written in its signature, then refers to every property(field\parameter) of an instance trough this (which literally means "I want to work with this particular entity") and then assigns them values that you passed through constructor.
In your case, it would look like that:
// We have passed all the values that we need trough constructor.
public AssignmentMarks(String name, int ass1, int ass2, int ass3, int mark, int average) {
// Now we assign them to the properties of an instance we creating.
// "courseName" of the created instance becomes "name" we passed.
this.courseName = name;
// "assignment1" of the created instance becomes "ass1" we passed
this.assignment1 = ass1;
// I bet you are getting the hang of it now.
this.assignment2 = ass2;
// And so on.
this.assignment3 = ass3;
// And so forth.
this.mark = mark;
// Until you have assigned values to all properties you want to assign in the constructor.
this.average = average;
}
Now that you have this constructor you could just simply create a new instance like so:
AssignmentMarks instance = new AssignmentMarks("programming", 1, 2, 3, 17, 20);
Where we also declared all the values we want to be assigned.
This how you could have done it with lombok:
//This is an entire class
#AllArgsConstructor
public class AssignmentMarks {
private String courseName;
private int assignment1;
private int assignment2;
private int assignment3;
int average;
int mark;
}
Now it already has "All argument constructor" because of #AllArgsConstructor annotation.
P.S. I double the previous writer on naming your class in CamelCase it is important.
I have a main class "number guess game" but now I need to create an object to get from. I'm trying to add the constructor to the Object class that takes an integer value for the upper limit and uses it to set the upper limit instance variable. Then, generate the number that the user should try to guess and set that instance variable. Finally, initialize the instance variable for the number of guesses to 1.
here is my code
public class NumberGame {
//instance variable
private int upperLimit;
private int number;
private int guess;
//constructor
public NumberGame(int upperLimit){
this.upperLimit = upperLimit;
this.number = number;
this.guess = guess;
}
}
unsure if I am on the right path or need to change my variables or add to the constructor. Still learning about instance variables and constructors any help would be greatly appreciated! thank you!
When you are saying "this.xx" you are referring to the instance variable, when you don't use "this" you are referring to local variables that are getting passed to the constructor. Because you are only passing the upper limit, what you want to do is only set that in the constructor. If the number of guesses always needs to start at "1", you can just initialize it to one in the object class. Lastly,if I'm understanding the question correctly, if you have a method to generate the random number they need to guess, you can call it in your constructor, so you'd end up with something like this:
public class NumberGame {
//instance variable
private int upperLimit;
private int number;
private int guess = 1; //number of guesses to 1
//constructor
public NumberGame(int upperLimit){
this.upperLimit = upperLimit;
this.number = generateNumber() //or some method you are using to generate the number for the user to guess
}
}
General part first:
This is for my Java 1 final: I need to make an NFL playoff simulator. So, I would make a "Team" object that holds values for "Offense" and whatnot. I would need a method that holds an algorithm to calculate who wins each matchup. However, I don't know how to go about applying the method to the appropriate teams..
I was going to make another object called "Matchup" which would hold 2 "Team" objects, then comparing the 2 teams' values with the algorithm method. But, I don't think you can use objects inside another object, to my knowledge... Therefore, I've just been trying to hard-code everything in an ugly way.
Would anyone point me in the right direction of forming such a program?
Anyway, on to my immediate issue:
Here is my small bit of code thus far..
public class Final_Larrabee {
static Team pit = new Team("Steelers", "AFC", 3, 75, 70);
static Team bal = new Team("Ravens", "AFC", 6, 70, 80);
static String teamA1 = "bal";
public static void main(String[] args) {
System.out.println(+teamA1.seed);
}
}
And my other class:
package final_larrabee;
public class Team {
String name;
String conference;
int seed;
int offense;
int defense;
boolean wcWin;
boolean divWin;
boolean conWin;
boolean alive;
public Team(String n, String c, int s, int o, int d) {
name = n;
conference = c;
seed = s;
offense = o;
defense = d;
wcWin = false; // these wins will be used in the betting part
divWin = false;
conWin = false;
alive = true; // determines if team is out of playoffs or not
}
public String getName(){
return this.name;
}
public void setName(String n){
name = n;
}
public int getConference(){
return this.seed;
}
public void setConference(String c){
conference = c;
}
public int getSeed(){
return this.seed;
}
public void setSeed(int s){
seed = s;
}
public int getOffense(){
return this.offense;
}
public void setOffense(int o){
offense = o;
}
public int getDefense(){
return this.defense;
}
public void setDefense(int d){
defense = d;
}
}
I realize this won't work, because it is trying to access teanA1's seed value (which doesn't exist of course). But, what I wanted was to simply call the bal.seed value. I wanted to do it this way so that later, I can use teamA1 and teamA2 variables for the 2 teams in a given matchup. Is there a way to do it this way?
If not, perhaps ideally, it would be best to create a Team object called teamA1 which holds the values of whatever team I need at the time. It would need to change like a variable instead of make a million objects every time I need it (ideally..) I found the method 'clone' but I'm not sure this is what I want?
Any help, be it to my immediate question, or the general layout of my program, would be much appreciated. Thanks!
make team implements comparable this will give you the ability to write compareTo method and then you can put you win condition based
Just as your class Final_Larrabee holds 2 team objects in your code, your matchup class can also hold other objects.
Yes, you're on the right track with the idea of creating a matchup class which holds instances of team objects.
Inside your main class you can initialize 2 team objects. I would then create a matchup object, which gets passed the 2 team objects you have previously created and do some functionality.
Inside your team class you could implement a method, which returns a score based on the teams stats. For example it could calculate the offense * defence * seed or something like and return it that and then inside your match class, you can then implement a method such as playGame() which calls the getScore() method of each object and compares the values and then returns the winner (or prints them out for simplicity).
As regards your code, you're using the keyword static. Static is a keyword which changes a variable/object/method to a class variable/object/method. This means that every instance of the class shares this variable. I would not use it for creating the teams. An example of where to use static (a very basic example), would be if for some reason you needed to keep track of the amount of team objects you created. You could implement a static COUNT variable inside the team object and increase it by one every time the constructor is called. This could also be used for assigning IDs to the teams.
static int TEAM_COUNT = 0;
public Team()
{
this.id = ++TEAM_COUNT;
}
This would make the TEAM_COUNT be a value which shared by all instances of the class team. If you were to call
teamA.setTeamCount(0);
Then every other instance of Team would also be reset to 0.
Edit: With regards to your playoffs
public static void main(String args[])
{
Team team1 = new Team("parameters required here");
Team team2 = new Team("parameters required here");
Playoff p = new Playoff();
p.add(team1);
p.add(team2);
p.startPlayoffs();
}
If you create all of the objects inside the main class, then they are available to that method as they are local. When you call p.add(), you can add them to an array or collection, which is inside the Playoff class. Your startPlayoffs() method can then process the array of objects already stored inside the class.
I've got this problem, I have a java file that obtains 2 variables from another file and is supposed to add them together and return the summed value. So far it works on obtaining the values aFirst and aSecond but I'm not sure why value one and two is lost (is back at 0) when it gets to the sum method. This is for an assignment I have for homework.
public class Pair
{
private double one, two ;
public Pair(double aFirst, double aSecond)
{
double one = aFirst;
double two = aSecond;
}
public double sum()
{
double xys = one + two;
return(xys);
}
}
You're declaring one and two as local variables, shadowing the instance variables.
the problem is the constructor. you are creating local variables and not using the class fields
private double one, two ;
public Pair(double aFirst, double aSecond)
{
this.one = aFirst;
this.two = aSecond;
}
you can do it without the "this." but dont put type ahead
Use this to access the class member variables in override cases when you have the same variable name in local scope and in the class members.