I'm trying to create a program that reads user input and stores it and then calculates the area of a polygon. When I try and compile it it gives me one error which is the one about .toString being non static.
import java.util.Scanner;
class regularpoTest {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
boolean finished = false;
double s;
double n;
double area;
//starts loop to record data
do {
s =0;
n =0;
System.out.println("Enter the side length, or anything else to quit: ");
s = in.nextDouble();
in.nextLine();
if (in.hasNextDouble()) {
System.out.println("Enter number of sides");
n = in.nextDouble();
area = (s*s*n)/(4*Math.tan(Math.PI/n));
} else {
finished = true;
}
} while(!finished);
//This prints out the student details
System.out.println(regularpo.toString());
}
}
public class regularpo {
private double side;
private double numberOf;
private double area;
public regularpo(double side, double numberOf){
side = 0;
numberOf = 0;
area = 0;
}
public double getSide(){
return side;
}
public double getNumberOf(){
return numberOf;
}
public String toString(){
return ("area = " + area+ " side length "+side+ " number of sides "+numberOf);
}
}
You are trying to call a method of a class, when that method has been defined for (and only makes sense as) a method of an instance of that class. Maybe you mean to make an object of that class, and call its toString method, although I can't be sure from your code.
You can not access non-static methods by using classname.nonStaticMethodName. You need to instantiate your object using the new keyword. Basically, you create an instance of your object by regularpo r = new regularpo(2.0, 2.0). After that you can invoke r.toString();
Check out this SO-question for more info.
And this Oracle-tutorial explains class members well.
Suggestions:
1) Eliminate "regularpoTest". Just move "main()" into "regularpo".
2) Capitalize "RegularPo" (by convention, class names should start with a capital letter).
3) Make the RegularPo constructor actually save the initial values (not just set them to zero).
... and, most important ...
4) Your main should call RegularPo regularPo = new RegularPo (...).
Then reference object instance "regularPo".
Try to make a object of class regularpo and call toString over that object
regularpo obj=new regularpo();
obj.toString();
Also as per conventions a class name must start with Upper case,so name your class asRegularpo
toString() is a non static method in regularpro class , and we know that the non static belongs to an object so we need to create and object of same class and call it.
toString() is belongs to Object class so its non static method.
regularpo obj=new regularpo();
obj.toString();
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.
static int input;
Scanner scn = new Scanner(System.in);
public BusGenerator(Depot depot)
{
this.depot = depot;
}
public int getinput()
{
return input;
}
public void run()
{
System.out.println("Enter Number of Buses:" );
input = scn.nextInt();
}
I have a class called BusGenerator and from here i ask the user about the number of Bus and the system scans it and save it in the variable called "Input".
I have another class called Depot and i want to call the variable "Input" from the Class depot. Is there a way to do that?
As the code is, you can simply use BusGenerator.input in the Depot class to refer to it (as pointed out in the comments).
However, since you've already defined a getter for this variable, it might be more consistent to make input private and refer to it with the public getter/setter methods.
Can someone please explain why it doesn't work? The error is at obj[0][0]=1;. It says that GPA can't be converted to int, same thing for String variable assignment s.
public class GPA {
public String s;
public int n;
public GPA[][] a;
//constructor
public GPA(GPA[][] a){}
public static void main(String[] args) {
GPA[][] obj=new GPA[2][2];
obj[0][0]=1; //error here
}
}
obj is an Array of GPA objects.
obj[0] = 1 means you are assigning the first element of that array to an intvalue. It should be an object of type GPA.
You can do it like
obj[0] = new GPA("John Doe", 6);
I would also recommend using Java convention, by making variables private and set() them by public methods like setter()s.
The question is changed which makes the answer irrelevant.
It won't work and gives you compile time error because GPA is class type and you are trying to assigning int value to it.
You have two options.
Option 1:
GPA[] obj = new GPA[4];
obj[0] = new GPA();
obj[0].n = 1;
Option 2:
You can make members of GPA private and use setters to set the value. Below is example.
public class GPA {
private String s;
private int n;
private GPA[] a;
public GPA() {}
public GPA(GPA[] a) {}
public String getS() {
return s;
}
public void setS(String s) {
this.s = s;
}
public int getN() {
return n;
}
public void setN(int n) {
this.n = n;
}
public GPA[] getA() {
return a;
}
public void setA(GPA[] a) {
this.a = a;
}
}
and then set using setter.
obj[0].setN(1);
It's not good programming practice to make your members public. It is always advised to use setters.
What you're actualy doing is trying to assign int and/or string to variable that is expecting object of GPA class.
Didn't you want to do
obj[0].n=1;
obj[0].s="text;"
For array of object you always have to create on object at that position first. otherwise you alway get a NullPointerException.
So what you need goes something like this
GPA[][] obj = new GPA[2][2];
obj[0][0] = new GPA();
obj[0][0].s="text";
obj[0][0].n=1;
...
and so on for every position there is.
Java Arrays are homogeneous(Javascript arrays are heterogeneous). That means you can only store the type of elements which you used while creating an Array.
ex: `int intArray[];` //We can store only int type elements(it also accepts Integer etc.. types but java converts to int then store it)
Now, apply the same rule to public GPA[] a; here a is an array of type GPA. So it accept only GPA type object.
That mean, you can store values like as below
a[0] = new GPA("nameHere", 6);
If I want to store either a string or an int, one at a time( I have
to make table of Student Name vs GPA) ,how do I do it?
One solution to this requirement is, assign a variable using constructor or setter method.
GPA[] obj = new GPA[2];
obj[0] = new GPA("first", 6); // here you need to create a new constructor
or
obj[1] = new GPA(); // Here default constructor will work and you need to have setter methods
obj[1].setName("second");
Hope this help...
I am doing an assignment for a beginning Java class. We have to write a commission calculator and use two classes in it. I am stuck on how to use variables from one class in a second class. The methods that i have found so far are not working for me. Any advice would be greatly helpful. Below is the code i have so far.
package calc;
/**
*
* #author Ethan
*/
import java.util.Scanner;
public class Calc {
public static void main(String[] args) {
Scanner calcu = new Scanner(System.in); //Creats the scanner used for Input
//Sets all of the variables to be used
double sal; //salary
double com1; //Comission percentage
double com2; // Find the ammount of comisssion from yearly sales
double comp; //Yearly Sales
double frac; //Decimal Comission form
double total; // Total of Compensation of Yearly salary + Comission
System.out.println("Enter the annual Salary here: ");
sal = calcu.nextDouble();
System.out.println("Enter the total Sales here: ");
comp = calcu.nextDouble();
Rate();
frac = com1/100; //converts the Sales Comission to a decimal
com2 = frac * comp; // Find the total Comission based on the
total = sal + com2; //find the total compensation
System.out.print("Your total compensation is " + total);
}
private static void Rate(){
// Rate of commission is determined below.
if (comp < 10000)
com1 = 20; //20% commission rate
else if (comp < 30000)
com1 = 22; // 22% commission rate
else if (comp < 50000)
com1 = 23; // 23% commission rate
else if (comp < 100000)
com1 = 24; // 24% commission rate
else
com1 = 25; // 25% commission rate
}
}
The problem i have is the i can't pull the rates so they can be used in the calculations.
A few major issues on your code. Your code is encapsulated in the Calc class, but no constructor is ever called. (eg. new Calc()). Rate() is a method in Calc, so it would be called by Calc.Rate(). Check out a few more examples of how to construct a java class, and you will understand encapsulation of your variables.
You only have posted one class so far. Where is the second? You will need to make sure both classes are in separate files as well.
public class Class1(){
//Class variables
public int myPublicInt; //public variables are accessable to any class that calls it
private int myPrivateInt; //private variables can not be modified outside of the class
//Class constructor, this is used to create objects of a class
public Class1(){
myPublicInt = 0;
myPrivateInt = 1;
}
//Now we have setter and getter methods to handle private variables
public void setMyPrivateInt(int newValue){
myPrivateInt = newValue;
}
public void getMyPrivateInt(){
return myPrivateInt;
}
//lastly this is a private method, only Class1 objects can call this method
//internally, so we can call it in our constructor but not from Class2
private void somePrivateMethod(){
//does something
}
}
So Now you will have some first class, it has some data and some methods and a constructor. Making variables public is not a good idea for the security of your code, you will want to make your second program create an object of the first one to access its variables and method calls. See the below program.
public class Class2{
//Class Variables, declares an object of the first class
private Class1 myClassObject;
//Constructor for this class, instantiates the first class object
public Class2(){
myClassObject = new Class1();
}
//Main method and runtime of our program
public static void main(String args[]){
Class2 thisClass = new Class2();
thisClass.printMyPrivateIntFromClass1();
}
//Method from this class, Class2
public void printMyPrivateIntFromClass1(){
System.out.println(myClassObject.getMyPrivateInt());
}
}
So in the second class we make an object of the first one, this way we can manipulate and use that object, its data and its methods. Notice that I used constructors to make objects of each class. When you call that object you can also call its public methods and variables. Again I would not recommend making variables public because then they are too easy to manipulate. If you have public methods that lead to your variables you can use those as a safer way to change or access the data stored in them. Hope this helps explain using a class inside another.
A couple of other notes, work on indentation, you should use the same indentation for code nested on the same level, and anything nested underneath something else by use of {} should be indented in once more so that code is more readable for you and others.
I'm not sure if the title is right but basically I have this piece of code:
for(int i = 0; i < ArrEmployee.size(); ++i){
ArrEmployee.get(i);
double provTax = ProvincialTax.calculateTax(ArrEmployee.get(i));
And this arrayList:
List<Employee> ArrEmployee = new ArrayList<>(); // array for employee objects
And the ProvincialTax class is like this:
public class ProvincialTax extends Deductions { // deductions is an abstract class with the abstract method calculateTax()
public double calculateTax(Employee employee) {
if (employee.getAnnualGrossSalary() < 41495){
return employee.getAnnualGrossSalary()*0.16;
}
else if (employee.getAnnualGrossSalary() < 82985){
return employee.getAnnualGrossSalary()*0.20;
}
else if(employee.getAnnualGrossSalary() < 100970){
return employee.getAnnualGrossSalary()*0.24;
}
else
return employee.getAnnualGrossSalary()*0.2575;
}
}
So basically my arrayList ArrEmployee stores Employee objects that have the variables:
public long EmployeeNumber;
public String EmployeeName;
public String LastName;
public double HoursWorked;
public double HourlyWage;
And the HoursWorked and HourlyWage are used to calculate AnnualGrossSalary used in my ProvincialTax class. With my for loop, I'm trying to calculate the provincial tax for each object inside the ArrEmployee, but it keeps asking me to make the method calculateTax() static, but it's supposed to override an abstract method?
How do I bypass this?
The problem isn't that you should make your method static, the problem is that you're invoking the method as if it was static, which you shouldn't do. The problem is in the line:
double provTax = ProvincialTax.calculateTax(ArrEmployee.get(i));
Which instance of ProvincialTax do you want to invoke it on? Invoke it on that instance.
ProvincialTax taxInstance = ...; // Get an instanceof of ProvincialTax
double provTax = taxInstance.calculateTax(ArrEmployee.get(i));
Without further evidence and the fact that you're not using preferred naming conventions, ProvincialTax.calculateTax appears to be trying to call the calculateTax via the ProvincialTax Class and not a instance of it...
ProvincialTax provincialTax = new ProvincialTax();
for(int i = 0; i < ArrEmployee.size(); ++i){
double provTax = provincialTax.calculateTax(ArrEmployee.get(i));
}
You might like to have a read through Code Conventions for the Java TM Programming Language, it will make it easier for people to read your code and for you to read others