Servlets :: Thread Safety Should Not Use Any Variables Or Objects At Instance / Class Level
Jun 3, 2014
As web server has multiple threads to serve client requests in Thread Pool & to ensure Thread Safety we should not use any variables or Objects at Instance/Class level.But in case of Session Variable which one is the Best Practice as the Session object is used by all the requests to have the same Session ID.
My Code :
public class MyServlet extends HttpServlet {
private static Logger log = Logger.getLogger(ClientRegistrationServlet.class);
private HttpSession session; /* This is used at Instance Level*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
I am having some problem accessing variables from an array instance of a class. Heres what i have done;
In the main class:
Example obj[]= new Example[4];
In the main class constructor:
obj[0] = new Example(0); obj[1] = new Example(1); obj[2] = new Example(2); obj[3] = new Example(3);
In the main update() method:
if(condition) //update
In the Example class constructor:
private boolean change = false;
In the Example class update() method:
if(x >20) change= true;
Now, i want to access the variable change from the main class, how do i do it? The 'condition' in the if statement is the condition of wether the change variable ia true or false. How do i access it?
I have to create an application that deals with maps.
I first have to create the instance variables for the class.
So very simply if my hashmap is going to consist of football clubs and players. Football clubs being a string value for the key and players being a set of strings for the values. How would I go about creating the instance variable in a class for this?
I can't seem to find anything that specifically deals with instance variables and constructors for maps.
Create a class called Employee that includes three pieces of information as instance variables:
-Employee ID (string type) -first name (string type) (default value 'John') -last name (string type) (default value 'Smith') and -monthly salary (type double). -No argument constructor that initializes the three instance variables. The employee id should be generated using the following process:
The employee id should be a combination of first initial, last initial and a number starting from 10001 for the first employee and increasing by one for each employee. e.g. if John Smith is the first employee then its id will be JS10001 and if George Brown is the second employee then its id will be GB10002
-Provide get and set methods for each instance variable. The set method for monthly salary should ensure that its value remains positive - if an attempt is made to assign a negative value, leave the original value.
package com.emp; public class salarybean { private String name; private Double days; private Double id; public String getName() { return name;
[code]...
now i want to retrieve all these values in another servlet where i want to do some calculation but not able to retrieve it is showing null and indicating for this value in my eclispe IDE " Iterator<salarybean> itr=list.iterator(); "
public class Time extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, java.io.IOException
I have a school assignment that involves me sorting an array of objects based on one of the class String variables. I am using as the title says a simple selection sort method. The problem I'm having is that when I run the program in debug mode, it never seems to enter the if statement in the inner loop. I would like to say I've tried a number of things to figure it out, but honestly I'm just stumped as to why it's not working.
Here is the code:
public static void sortTransactions(Transaction[] oTransaction){// This is the sorting method, obviously it's not done so it currently just prints to screen. System.out.println("Successful call to sortTransaction()"); String min = ""; int curInd = 0; Transaction[] temp = new Transaction[1];
[Code] ....
The output when I check to see if the array is sorted verifies that the array never does get sorted.
So far in my assignment I have successfully opened a text file. However I am required to do more:
1) As each line of text (containing names and ages) is read a new Runner object is created with its instance variables set thus: ! (Runner class already created )!
- name : set directly set from the value in the file - agaGroup : can be worked out from the given ages: < 18 should be 'junior' > 55 should be 'senior' the rest should be 'standard'
2) the instance of Runner should be added to the list referenced by the instance variable runners.
I have used if statements to create the junior list, however I do not see the full list of names and ages in the variable runners as I am requested to.
I am sure there is a for loop involved somewhere but I do not know how to:
a) use the for loop in my method add a new Runner object with the variable mentioned.
I include the code I have done so far as a file - p.s I use Bluej.
public class MarathonAdmin { // instance variables private String runners; private String age;
Do inherited methods use their instance variables or do they use the ones in the method that inherits them?
For example, Class B extends Class A. Class A and B both have the instance variable "potato". A client program tries to use method "cut" using an object of Class B, but class B has no cut method. So, class B uses the "cut" method inherited from class A. What I want to know is will that cut class A's potato or class B's?
so, i was reading my java book and learning about objects and methods and it starts talking about Encapsulation and mentions that it's good practice to set instance variables as private and instead of accessing the instance variables directly, we should create a set method and get method to get and set the stuff we want to pass to the class containing the object...
for example, in this class, we're passing the integer 70 for object dog one and integer 8 for object dog two for the dog class... and these these 2 integers are sent to the setsize method so we're not accessing instance variable size directly.
i dont quite get it though....if we the programmer are the one deciding what size the integer is for the dog, and the setsize method takes the one.setSize(70) or (8) and puts them in setsize(int s) as s... but only to copy that integer stored in s back to private int size.... why do we even need to bother with making these two extra methods such as setSize, getSize?
in the book it says that... well what if the code gets into the wrong hand and someone writes something like one.setSize(0) then you would get a dog with size 0 which is essentially illogical. but then again, i'm the programmer, and i am the person who writes the code and passing the right integer.The reason for public and private... that part i understand... i can see why if a variable's data can get changed amidst the code during calculations and you dont want it to directly change the original variable and have it mess up the code, but this code from the book just a bad example of demonstrating the reason? since we manually pass the information ourselves and passing it to method setSize... and all setSize does is stores it in another integer, only to copy it right away to size (which is the original private variable we were tryign to protect?
Any simple code to demonstrate how the code might end up changing an instance variable and why we would want to protect it by using private?
class GoodDog { private int size; public int getSize() { return size; } public void setSize(int s) { size = s;
why overridden doesn't apply to variables. However, instance variables are stored inside the object.I ran below program and expected to print "two" but it gets printed "one".
class SupCont { String s = "one"; } class Cont extends SupCont { public static void main(String a[]) { String s = "two"; SupCont c = new Cont(); System.out.println(c.s); } }
I am suppose to create a rectangle and I have created two classes; Rectangle.java and RectangleTester.Java.
So far my code for the class Rentangle.java is:
package edu.sbcc.hw2; public class Rectangle { private int width = 25; private int height = 25; public rectangle(int xcoord, int ycoord, int thewidth, int theheight) { this.width = width; this.height = height; } public int getWidth() { return width;
So for my assignment I need two instance variables for height and width for which I have, but it says in the assignment I need methods (settings and getters /mutators and accessors that allow manipulation of my instance variables which is a little confusing. Do I put these methods on Rectangle.java or RectangleTester.java.
The same goes for the calculateArea, where am I suppose to put this?
Can we set the class path for one level down the current directory? My structure is like
binlibresourceshexicon.cmdinside the folder dist.
In this case I could run the .cmd file which has java -cp .;lib;resources client.Test .
c: rycpdist>java hexicon.cmdBut if i put the .cmd file inside bin as like this and run as c: rycpdistin>java hexicon.cmdI am getting this error Error: Could not find or load main class
Is there any possibility to set the class path in this case
I thought static methods could never use instance variables, because they wouldn't know which instance to look at.
From Head First Java, p. 284: "A static method is not associated with a particular instance - only the class - so it cannot access any instance variable values of its class. It wouldn't know which instance's values to use."
Now I was answering some mock exam questions from Cameron McKenzie's SCJA book, and I don't understand one of the options. On page 205, the last question has an option that says: "Instance variables ... are not visible in static methods, unless passed in as arguments." This option is supposed to be correct. Now... how does that work?
I am working on a project and it asks me to "Provide appropriate names and data types for each of the instance variables. Maintain two GVdie objects" under class fields. I am unsure as to what is being asked when asking for two objects as instance variables and how I would write that...
Let's say I have a loop that loops through objects in an ArrayList then does stuff with them. Is it better for me to store the object in a temporary local variable and do stuff with it, or always use the ".get(arrayindex)" thing?
I'm new to programming and I would just like to know what happens inside the memory once you run a java application. How do the memory allocate space for your objects or variables? Where does it start?does it start in memory location 1 first when I create my first variable or object? What is the use of variables and how is it used by the memory?Also if I create a variable named String s1 then I created another variable again named s1= "myString" Did I create a new object or I just edited the value of the s1 variable??
And if I create a variable int num1 = 10 then I created another variable int num1 = 12 did I create another one or I just edited the num1 variable?? Also how much memory would an object have? Does it depend on how many primitives objects like arrays or strings it have?Also I want to develop 3D games also, in the future is Java going to be great as I know machines in the future will have massive amounts of RAM and the JVM will be better...
I am getting "Type safety: Unchecked cast from Object to LinkedList<EventData>" in eclipse for a piece of code stated below
public LinkedList<EventData> loadFromFile(File file) { queue=new LinkedList<EventData>(); //Some piece of code return (LinkedList<EventData>)queue.clone(); //--->getting warning here }
I know that because clone() method is returning Object, hence compiler doesn't have type information that's why showing warning. I don't want to suppress this warning instead i want to fix it.
How do you declare methods for a class within the class whilst objects of the class are declared else where?
Say for instance, I have a main class Wall, and another class called Clock, and because they are both GUI based, I want to put a Clock on the Wall, so I have declared an instance object of Clock in the Wall class (Wall extends JFrame, and Clock extends JPanel).
I now want to have methods such as setClock, resetClock in the Clock class, but im having trouble in being able to refer to the Clock object thats been declared in the Wall class.
Is this possible? Or am I trying to do something thats not possible? Or maybe I've missed something really obvious?
