I have global variable private int temp=0;. In the class it is incrementing (At some what stage say it is temp=10).When again loading the class temp is still 10. But I need it 0. How can I do that?
code:
public class MyClass
{
private int temp = 0;
public void method1() // while calling this method temp increments say temp =1;
{
temp++;
}
public void method2()
{
if(temp == 0)
System.out.println("temp = "+temp):
}
}
After this suppose temp = 10, and when loading MyClass still temp=10, but I need temp=0 again. Since I'm new to programming I don't know whether it make sense.
temp is always going to be 0 unless it is declared as static.
MyClass mc = new MyClass();
mc.method1() // 'temp' of mc object is now 1
MyClass mc2 = new MyClass();
mc2.method2() //'temp' of mc2 object is still 0!
I'm not sure what you meant by loading the class, calling the class etc
Note that each new instance of the class will give you temp = 0 and If you mean within the same instance,See this example, I added a new method, method0()
public class MyClass
{
private int temp = 0;
public void method0()
{
temp = 0;
}
public void method1()
{
temp++;
}
public void method2()
{
if(temp == 0)
System.out.println("temp = "+temp):
}
}
In this case,
MyClass mc = new MyClass();
mc.method2();
mc.method1();
mc.method2();
mc.method0();
mc.method2();
Will give you,
temp = 0
//Incremented value of temp
//condition if(temp==0) fails
//reset value of temp
temp = 0
Hope this is what you meant.cheers.
If i understand your question correctly, you want to re-initialize temp to 0 every time you create a new object of the class - MyClass.
If this is what you want, then make use of a constructor. And initialise temp to 0 in the constructor.
public MyClass
{
temp = 0;
}
This way, every time you create a new object of MyClass, temp will be set back to 0.
Related
I have created a class using linked list to display 20 Fibonacci numbers. Here is my code:
import java.util.LinkedList;
public class FibonacciLinkList {
private LinkedList<Integer> fibonacciList;
public FibonacciLinkList(LinkedList<Integer> FibonacciLinkList) {
this.fibonacciList = FibonacciLinkList;
}
public LinkedList<Integer> sum()
{
int n, a = 0, b = 0, c = 1;
for(int i = 1; i <= 20; i++)
{
a = b;
b = c;
c = a + b;
}
return fibonacciList;
}
public void display() {
System.out.println(fibonacciList);
}
public static void main(String[] args) {
LinkedList fibonacciList = new LinkedList();
fibonacciList.display(); //This is where the error is
}
}
The problem I am having is displaying the Fibonacci numbers on the console.
I have tried to do this by using a display method but it hasn't really worked for me. I have done a lot of searching online and on SO and have tried them but they have not worked for me. It would be appreciated if you could fix my code so that it does work.
I am new to linked list and this is the first time I am coding a linked list myself and I feel that a solution to this problem will help me understand linked lists better.
As I mentioned, LinkedList is not an instance of FibonacciLinkedList, and it does not possess the display() method. Attempting to invoke it on the LinkedList object will lead to failure to compile.
The sum() method is not invoked nor does it actually do anything. That is, it does not assign anything to the fibonacciList you have.
I would recommend that you extend the LinkedList class and generate the items on instantiation. Then, using the default toString() you can display to console. After all, the class is simply an extension of the LinkedList data structure to store Fibonacci numbers up to 20.
As you extend LinkedList, you inherit the AbstractCollection.toString() method for which the "string representation consists of a list of the collection's elements in the order they are returned by its iterator, enclosed in square brackets ("[]")."
public class FibonacciLinkedList extends LinkedList<Integer> {
public FibonacciLinkedList(int n){
int a = 0, b = 0, c = 1;
for(int i = 1; i <= n; i++) {
a = b;
b = c;
c = a + b;
this.add(c);
}
}
public void display() {
System.out.println(this.toString());
}
public static void main(String[] args) {
FibonacciLinkedList list = new FibonacciLinkedList(20);
list.display();
}
}
I fixed your code:
import java.util.LinkedList;
public class FibonacciLinkList {
private LinkedList<Integer> fibonacciList;
public FibonacciLinkList() {
this.fibonacciList = new LinkedList<Integer>();
}
public LinkedList<Integer> sum()
{
int n, a = 0, b = 0, c = 1;
for(int i = 1; i <= 20; i++)
{
fibonacciList.add(a);
a = b;
b = c;
c = a + b;
}
return fibonacciList;
}
public void display() {
System.out.println(fibonacciList);
}
public static void main(String[] args) {
FibonacciLinkList fibonacciList = new FibonacciLinkList();
fibonacciList.sum();
fibonacciList.display();
}
}
Try this.
There is several points that you need to take care :
sum() is never called.
the look in sum() does not change fibonacciList, it only uses local variables and does nothing else with it.
display() is NOT a LinkedList function, so it will likely not work. And even if it were working, it will likely not display what you expect : you need to loop through the list and print each value.
an other fibonacciList is created in the main function, so the display (if it was working) would show the content of this local list and not the global one.
Note: This is a troubling problem, possibly a bug, although I might be incorrect and missing something small
Problem:
Issue is the separately instantiated objects are referring to the same data structure.
Calling a.add() adds an object to data[NEXT], where is instantiated to NEXT = 0, followed by NEXT++ for increment purposes.
Thereafter, b.add() is called, and following the logic of the add() method, the array is extended,
BUT no initial value has been inserted into b i.e. b.data[0] = null
TL;DR
a.add() adds value to a.
b.add() extends a's array. This should not happen as a and b are 2 separate objects of the same type
main class code:
//...
SimpleSet<Integer> a = new SimpleSet<>();
SimpleSet<Integer> b = new SimpleSet<>();
// add a maximum of 20 unique random numbers from 0..99
Random rand = new Random();
for (int i = 0; i < 20; i++) {
a.add(rand.nextInt(100)); //i=0 - adds to data[0] with no issue
b.add(rand.nextInt(100)); //i=0 - extends a's array? why?
}
//...
class SimpleSet
public class SimpleSet<E> {
private static int MIN_SIZE = 1;
private static int NEXT = 0;
private Object[] data;
/**
* constructor of SimpleSet
*/
public SimpleSet() {
data = new Object[MIN_SIZE];
}
public void add(E e) {
if(NEXT > 0.75*MIN_SIZE){
extendArray();
}
if (data != null) {
data[NEXT] = e;
NEXT++;
}
}
private void extendArray() {
MIN_SIZE = MIN_SIZE*2;
Object[] newData = new Object[MIN_SIZE];
for (int i = 0; i < data.length; i++) {
newData[i] = data[i];
}
data = newData;
return;
}
//...
}
Am I missing something small or is this a bug?
IDE = IntelliJ 2016.3
I've looked all over the internet, but I can't figure out what I'm doing wrong. I'm trying my hand at using private variables from another class using get/set methods. Something's going wrong, but I can't figure it out.
public class Character
{
private int atk = 0;
private int def = 0;
private int spd = 0;
public void setStat(String stat, int n)
{
stat = stat.toLowerCase();
if(stat.equals("def") || stat.equals("defence") || stat.equals("defense"))
{
def = n;
}
if(stat.equals("atk") || stat.equals("attack"))
{
atk = n;
}
if(stat.equals("spd") || stat.equals("speed"))
{
spd = n;
}
}
public int getStat(String stat)
{
stat = stat.toLowerCase();
int n = -1;
if(stat.equals("def") || stat.equals("defence") || stat.equals("defense"))
{
n = def;
}
if(stat.equals("atk") || stat.equals("attack"))
{
n = atk;
}
if(stat.equals("spd") || stat.equals("speed"))
{
n = spd;
}
return n;
}
public Character(int a, int d, int c)
{
atk = a;
def = d;
spd = c;
}
}
This is my first class, Character which will be used as the template for the object, complete with get/set methods.
public class newCharacters
{
Character person1 = new Character(2, 4, 3);
person1.getStat("atk");
}
This is my second class, which constructs a character object and then tries to get a variable. Problem is, whenever I compile, it says that the object method needs an identifier. Exact quote: <identifier> expected
I can't figure out what it means, or what I'm doing wrong? I made get/set methods for each class, created the object in both classes, even constructed and called the object method within the Character class. Same problem every time. Can someone help?
public class newCharacters
{
Character person1 = new Character(2, 4, 3);
person1.getStat("atk");
}
This should not be in a class. This does not mean anything. A class can have bunch of instance variables and methods.
Please study the basics well ;)
Put it in a main method inside the Character class
public static void main(String [] args) {
Character person1 = new Character(2, 4, 3);
person1.getStat("atk");
}
public class NewCharacters
{
public static void main(String[] args) {
Character person1 = new Character(2, 4, 3);
person1.getStat("atk");
}
}
A program starts a main method like above.
Inside a class, at the top level only fields and methods may be declared (and constructors and initializer blocks, and other classes).
I just started with java and I create a class Range() inside my superclass with a method inside makeRange but when I tried to access to that method throws an error. Whats wrong here?
Here is my code...
public class iAmRichard {
class Range{
int[] makeRange(int upper, int lower){
int[] ary = new int[(upper - lower)+1];
for(int i = 0; i > ary.length; i++ ){
ary[i] = lower++;
}
return ary;
}
}
public static void main(String[] args) {
int foo[];
Range fui = new Range();
foo = Range.(here do not apear makeRange method)
You're creating an inner class here called Range. I don't believe that's what you intended to do, but I'll answer it as stated.
You're referring to this class in a static context, and the inner class can't be referenced with a static context. To address that, you need to make the change to Range: make it static.
public class iAmRichard {
static class Range {
}
}
Further, you're already getting an instance of Range, so all you need to do is use it.
foo = fui.makeRange(1, 10);
If you elected to only create a class called Range, you wouldn't have to deal with any inner classes at all, which I think would be the cleaner approach here.
public class Range {
int[] makeRange(int upper, int lower) {
int[] ary = new int[(upper - lower) + 1];
for (int i = 0; i > ary.length; i++) {
ary[i] = lower++;
}
return ary;
}
public static void main(String[] args) {
int foo[];
Range fui = new Range();
foo = fui.makeRange(1, 10);
}
}
To access a method without creating an instance you have to declare it static. In your case you have also to declare the class Range as static.
Or you can just use the instance you already have with a few changes:
iAmRichard richard=new iAmRichard();
Range fui=richard.new Range();
foo = fui.makeRange(...);
Note tha you need an instance of iAmRichard to create a Range.
Since the call is made from a static block in a static way(No instance is used for calling makeRange method) we need to have the called method to be either static or we need the object of the class to call instance methods.
statically you can use this example to access your method. Here is a link for more information on static methods.
public class IAmRichard {
public static void main(String[] args) {
int foo[];
foo = Range.makeRange(10,1);
}
static class Range{
static int[] makeRange(int upper, int lower){
int[] ary = new int[(upper - lower)+1];
for(int i = 0; i > ary.length; i++ ){
ary[i] = lower++;
}
return ary;
}
}
}
I need an array to be public (accessible to other methods in the class) but the array needs an input value "T" to create it. How do I instantiate a "global" variable that requires user input?
My code is as follows:
public class PercolationStats {
**private double myarray[];**
public PercolationStats(int N, int T) {
**double myarray = new double[T];**
for (i=0;i<T;i++) {
Percolation percExperiment as new Percolation(N);
//do more stuff, make calls to percExperiment.publicmethods
myarray[i] = percExperiment.returnvalue;
}
}
public static void main(String[] args) {
int N = StdIn.readInt();
int T = StdIn.readInt();
PercolationStats percstats = new PercolationStats(N, T);
//do more stuff, including finding mean and stddev of myarray[]
StdOut.println(output);
}
Another example in pseudocode:
class PercolationStats {
Constructor(N, T) {
new Percolation(N) //x"T" times
}
Main {
new PercolationStats(N, T) //call constructor
}
}
class Percolation {
Constructor(N) {
**new WQF(N)** //another class that creates an array with size dependent on N
}
Main {
**make calls to WQF.publicmethods**
}
}
In the second example, it seems to me that I need to have the new instance of class WQF made in the constructor of the Percolation in order to accept the parameter N. However, WQF would not be accessible to the Main method of Percolation.
Help!
Don't include the type declaration in your constructor. You are creating a local variable that masks the field. It should look like this:
public class PercolationStats {
public double myarray[];
public PercolationStats(int n, int y) {
myarray = new double[t];
for (i=0; i<t; i++) {
Percolation percExperiment = new Percolation(n);
//do more stuff, make calls to percExperiment.publicmethods
myarray[i] = percExperiment.returnvalue;
}
}
public static void main(String[] args) {
int n = StdIn.readInt();
int t = StdIn.readInt();
PercolationStats percstats = new PercolationStats(n, t);
//do more stuff, including finding mean and stddev of myarray[]
StdOut.println(output);
}
}
There's certainly no problem using a variable as the length when creating a new array.
Tedd Hopp's answer corrects the bug in your code.
I'd just like to point out that myarray is NOT a global variable.
Java doesn't have global variables,
the closest it has is static variables, and
myarray isn't one of those either. It is an instance variable, as you have declared it.
(And an instance variable is the right way to implement this ... IMO)