Why would you declare a static final variable as private for an immutable type?
Could it possibly do any harm to declare them public on any case?
So that nobody can access it from outside and rely on that value, giving you the freedom to change it without risk of side-effect (at least outside of the class where it's declared).
There are serveral reasons...
Privacy
Keep implementation details hidden from the clients, for example constants for internal use only and with no use for clients
Security
Protect your code from maliscious client codes for example:
static class A
{
public final static List<String> list = buildList();
public static List<String> buildList()
{
ArrayList<String> list = new ArrayList<>();
list.addAll(Arrays.asList("A", "B", "C"));
return list;
}
}
static class B
{
public static void main(String[] args)
{
A.list.clear();
System.out.println(A.list);
}
}
You dont want any one to manipulate you internal data.
It's just best OOP practice to keep variables within the proper scope. An example would be a global static final integer for performing a number of calculations within the class. Having it exposed (public) not only could cause problems, but confusion as well.
Say you had a class where you needed to know the value of pi:
public class MyClass {
private static final double PI = 3.14159;
//functions performing calculations with value of PI
}
The value of pi is only needed within MyClass, so leaving it to be exposed makes no sense - all calculations with the variable are done within MyClass. If you need it again to perform a calculation, it should be done within the class holding the value to keep your code organized.
public class GUICommon {
private static final ExecutorService executorServices = Executors.newWorkStealingPool();
public static void useExecutors(Runnable run)
{
executorServices.execute(run);
}
}
I used it on this way.
I have seen this from a recognised sample book so its hard to question then there is something I dont understand.
A class called DataflightsService contains a private static variable called FlightFileAccess that appears to be instantiated everytime we create a new object for DataflightsService as FlightFileAccess's initiation its in the constructor
ie
public class DataflightsService{
private static FlightFileAccess fileAccess=null;
public DataflightsService(String path){
fileAccess=new flightFileAccess(path);
}
public boolean removeflight(String code){
//We use this static instance that wraps functionality to remove a flight
fileAccess.remove(code);
}
}
For me that means that every time we create an instance of DataflightsService, in the constructor are using a different object all the time for the static variable FlightFileAccess
In the original FlightFileAccess Class: we have the remove method that synchronizes a RandomAccessFile
Class FlightFileAccess{
private RandomAccessFile database = null;
private boolean remove(String code){
// Other code goes here and there
synchronized (database) {
//Perform deletion code
}
}
So because we are using a different reference of FlightFileAccess we are also using a different reference of RandomAccessFile?
That means that having FlightFileAccess as static in service does not serve here to synchronize on the RandomAccessFile because it is a new one every time so each DataflightsService instance will do their thing on the random access file ignoring the synchronization.
As opposed to instantiating FlightFileAccess in a static initiator. Am I right?
I would appreciate as many explanations as possible to provide the best way to be able to instantiate DataflightsService as many times as we want (as lets say imagining each client has their own instance of DataflightsService) and after that being able to synchronize on a file for removals for example so that there is no mess of several clients accessing the file. Sorry I need to include a DataflightsService per client bc there are no cookies.
Your example won't compile because the name of the constructor doesn't match the class. But if you mean to name the constructor public DataflightsService(), then part of the issue is that you are overwriting the static variable each time a new object is created.
It sounds like you want this static variable to be initialized only once. Normally you would just assign the variable directly with private static final FlightFileAccess fileAccess = new FlightFileAccess(); or if you wanted to add more logic as if you had a constructor, you could use a static initializer block as follows:
public class Dataflights {
private static final FlightFileAccess fileAccess;
static {
// Static initializer block gets run once when the class is first referenced.
// Not usually used unless you want to add more logic besides just initializing variables.
fileAccess = new FlightFileAccess();
}
private final String path;
public final int id;
public Dataflights(String path) {
this.path = path;
this.id = fileAccess.generateId();
}
static class FlightFileAccess {
private volatile int nextId = 0;
synchronized public int generateId() {
return nextId++;
}
}
public static void main(String[] args) {
Dataflights d = new Dataflights("my/path");
System.out.println("Id is: " + d.id);
}
}
There are many ways to handle contention. I recommend Java Concurrency in Practice if you aren't familiar with Java concurrency.
You are on the right track in your FlightFileAccess class. I can't see the details, but you might also want to use the synchronized keyword in the signature of the remove() method to protect the entire function first. Then, once you have things working, use more tightly targeted synchronize {...} blocks to reduce the amount of code that has to be singly threaded.
Edit: I am trying to create a shared database connection pool for all sessions of a web application. A different post said the best way to create a servlet context object was by having the init listener create it. I am however unclear on how to make this object available for use by my servlet.
Another way you could do this is use static initialization:
public class SomeClass {
private static final Object[] CONTENT;
static {
CONTENT = new Object[SomeOtherClass.getContentSize()]; // To show you can access runtime variables
}
}
This will initialize the CONTENT array once the class is loaded using the ClassLoader.
One solution is using a private holder class:
public class SomeClass {
private static class ResourceHolder {
private static final Resource INSTANCE = new Resource();
}
public static Resource getInstance() {
return ResourceHolder.INSTANCE;
}
}
the instance will be initialized when SomeClass.getInstance() is called the first time.
The simplest lazy initialisation is to use an enum with one instance.
enum Singleton {
INSTANCE; // lazy initialised
}
The added problem is you want initialisation values. To handle this you can nest the class.
enum Utility {;
static MyType val;
static OtherType val2;
enum Holder {
INSTANCE;
Holder() {
// uses val and val2
}
}
public static Holder getInstance(MyType val, OtherType val2) {
Utility.val = val;
Utility.val2 = val2;
return Holder.INSTANCE; // only created the first time.
}
}
Note: this is thread safe as static block initialisation is safe.
Something like:
public static abstract class Lazy<T> {
private T t = null;
public synchronized T get() {
if (t == null) {
t = create();
}
return t;
}
protected abstract T create();
}
public static final Lazy<List<String>> lazyList = new Lazy<List<String>>(){
#Override
protected List<String> create() {
return new ArrayList<String>();
}
};
I'll caution you up front, what you're describing has a bit of code smell, and I suspect you'll do better to avoid this pattern entirely. A static resource that depends on external runtime state breaks all sorts of best practices about variable scope.
What you're describing, however, would best be implemented by either a Supplier or a Future, depending on the work involved in successfully constructing the object you need. The difference is somewhat pedantic, but you'd generally use a Future to hold a reference that will take a long time to compute, while a Supplier generally returns quickly. Future also has some nice hook-ins with Java's concurrency utilities, but by the sound of it you don't need that.
You'd use a Supplier like so:
public class GlobalState {
public static final Supplier<LazyData> MY_DATA = Suppliers.memoize(
new Supplier<LazyData>() {
public LazyData get() {
// do whatever you need to construct your object, only gets executed once needed
}
});
...
}
Suppliers.memoize() will cache the result of the first call to the underlying Supplier in a threadsafe way, so simply wrapping the Supplier you define with this call prevents duplicate processing.
I'm in a position where I have referenced a static field variable to a custom class. I need to make alterations to the variable with methods from the class it references. I can not instantiate the class under construction. Simplified example:
public class House {
private static MaterialsRequired matsReq = new MaterialsRequired();
private String size;
private House(String size) {
this.size = size;
}
public static MaterialsRequired getMaterialsRequired() {
}
public static void Build(String size) {
new House(size); //Do I need to put 'this(size)' here?
//some code here to expend the materials required factored based on size.
}
To construct the house I need to know the materials required (for standard size). To know the materials required I need to call the addMaterial() method of MaterialsRequired. What do I do?
EDIT: I need to call the addMaterial() method repeatedly on matsReq. Maybe ten times. When I call House.build() I want the matsReq to have been altered by method calls (if my question was unspecific). Also it doesn't satisfy me to just set the materials required every time the build() or getMaterialsRequired() methods are called.
The question have been answered! Solution:
private static MaterialsRequired matsReq = new MaterialsRequired();
static {
matsReq.addMaterial(mat1);
matsReq.addMaterial(mat2);
matsReq.addMaterial(mat3);
}
private String size;
Having a build method doesn't work, because it is just another constructor. It's what happens when you make an instance. I'm guessing, if anything, you build to be outside of the class. It would be something that called the constructor and did a couple other things.
What you probably want is, outside of the class, a method that creates an instance of a house (using the constructor) and then some code to "use up" the materials needed to make the house from a structure that holds the materials (again, outside of the class).
Since you seem to want to make houses with different material requirements, why don't you make subclasses of the most basic house (or better yet, an abstract class, maybe called "dwelling") that have a different required materials list.
Doing some guesswork...
public class StandardHouse{
public static MaterialsList MATERIALS_REQUIRED = new MaterialsList();
MATERIALS_REQUIRED.add("material", numMats);
...
public StandardHouse() {
//probably don't need anything here
}
}
Then later
MaterialsList matsYouHave = ...;
StandardHouse house1 = new StandardHouse();
matsYouHave.remove(StandardHouse.MATERIALS_REQUIRED);
Try making them in two classes
public class House {
private List<String> materialList = new ArrayList<String>();
private House(List<String> materialList ) {
this.materialList = materialList ;
}
public void Build() {
MaterialsRequired matsReq = new MaterialsRequired();
materialList .add(matsReq.setMaterialsRequired("material1"));
materialList .add(matsReq.setMaterialsRequired("material2"));
materialList .add(matsReq.setMaterialsRequired("material3"));
//some code here to expend the materials required factored based on size.
}
And then
class MaterialRequired
{
private String materialsRequired;
public String setMaterialsRequired(String material) {
materialRequired = material;
return materialRequired;
}
How do you define Global variables in Java ?
To define Global Variable you can make use of static Keyword
public class Example {
public static int a;
public static int b;
}
now you can access a and b from anywhere
by calling
Example.a;
Example.b;
You don't. That's by design. You shouldn't do it even if you could.
That being said you could create a set of public static members in a class named Globals.
public class Globals {
public static int globalInt = 0;
///
}
but you really shouldn't :). Seriously .. don't do it.
Another way is to create an interface like this:
public interface GlobalConstants
{
String name = "Chilly Billy";
String address = "10 Chicken head Lane";
}
Any class that needs to use them only has to implement the interface:
public class GlobalImpl implements GlobalConstants
{
public GlobalImpl()
{
System.out.println(name);
}
}
You are better off using dependency injection:
public class Globals {
public int a;
public int b;
}
public class UsesGlobals {
private final Globals globals;
public UsesGlobals(Globals globals) {
this.globals = globals;
}
}
Lots of good answers, but I want to give this example as it's considered the more proper way to access variables of a class by another class: using getters and setters.
The reason why you use getters and setters this way instead of just making the variable public is as follows. Lets say your var is going to be a global parameter that you NEVER want someone to change during the execution of your program (in the case when you are developing code with a team), something like maybe the URL for a website. In theory this could change and may be used many times in your program, so you want to use a global var to be able to update it all at once. But you do not want someone else to go in and change this var (possibly without realizing how important it is). In that case you simply do not include a setter method, and only include the getter method.
public class Global{
private static int var = 5;
public static int getVar(){
return Global.var;
}
//If you do not want to change the var ever then do not include this
public static void setVar(int var){
Global.var = var;
}
}
Truly speaking there is not a concept of "GLOBAL" in a java OO program
Nevertheless there is some truth behind your question because there will be some cases where you want to run a method at any part of the program.
For example---random() method in Phrase-O-Matic app;it is a method should be callable from anywhere of a program.
So in order to satisfy the things like Above "We need to have Global-like variables and methods"
TO DECLARE A VARIABLE AS GLOBAL.
1.Mark the variable as public static final While declaring.
TO DECLARE A METHOD AS GLOBAL.
1. Mark the method as public static While declaring.
Because I declared global variables and method as static you can call them anywhere you wish by simply with the help of following code
ClassName.X
NOTE: X can be either method name or variable name as per the requirement and ClassName is the name of the class in which you declared them.
There is no global variable in Java
Nevertheless, what we do have is a static keyword and that is all we need.
Nothing exists outside of class in Java. The static keyword represents a class variable that, contrary to instance variable, only has one copy and that transcends across all the instances of that class created, which means that its value can be changed and accessed across all instances at any point.
If you need a global variable which can be accessed beyond scopes, then this is the variable that you need, but its scope exists only where the class is, and that will be all.
Nothing should be global, except for constants.
public class MyMainClass {
public final static boolean DEBUGMODE=true;
}
Put this within your main class. In other .java files, use it through:
if(MyMainClass.DEBUGMODE) System.out.println("Some debugging info");
Make sure when you move your code off the cutting room floor and into release you remove or comment out this functionality.
If you have a workhorse method, like a randomizer, I suggest creating a "Toolbox" package! All coders should have one, then whenever you want to use it in a .java, just import it!
There is no such thing as a truly global variable in Java. Every static variable must belong to some class (like System.out), but when you have decided which class it will go in, you can refer to it from everywhere loaded by the same classloader.
Note that static variables should always be protected when updating to avoid race conditions.
Understanding the problem
I consider the qualification of global variable as a variable that could be accessed and changed anywhere in the code without caring about static/instance call or passing any reference from one class to another.
Usually if you have class A
public class A {
private int myVar;
public A(int myVar) {
this.myVar = myVar;
}
public int getMyVar() {
return myVar;
}
public void setMyVar(int mewVar) {
this.myVar = newVar;
}
}
and want to access and update myvar in a class B,
public class B{
private A a;
public void passA(A a){
this.a = a;
}
public void changeMyVar(int newVar){
a.setMyvar(newVar);
}
}
you will need to have a reference of an instance of the class A and update the value in the class B like this:
int initialValue = 2;
int newValue = 3;
A a = new A(initialValue);
B b = new B();
b.passA(a);
b.changeMyVar(newValue);
assertEquals(a.getMyVar(),newValue); // true
Solution
So my solution to this, (even if i'm not sure if it's a good practice), is to use a singleton:
public class Globals {
private static Globals globalsInstance = new Globals();
public static Globals getInstance() {
return globalsInstance;
}
private int myVar = 2;
private Globals() {
}
public int getMyVar() {
return myVar;
}
public void setMyVar(int myVar) {
this.myVar = myVar;
}
}
Now you can get the Global unique instance anywhere with:
Globals globals = Globals.getInstance();
// and read and write to myVar with the getter and setter like
int myVar = globals.getMyVar();
global.setMyVar(3);
public class GlobalClass {
public static int x = 37;
public static String s = "aaa";
}
This way you can access them with GlobalClass.x and GlobalClass.s
If you need to update global property, a simple getter/setter wrapper class can be used as global variable. A typical example is shown below.
public class GlobalHolder {
private static final GlobalHolder INSTANCE = new GlobalHolder();
private volatile int globalProperty;
public static GlobalHolder getInstance() {
return INSTANCE;
}
public int getGlobalProperty() {
return globalProperty;
}
public void setGlobalProperty(int globalProperty) {
this.globalProperty = globalProperty;
}
public static void main(String[] args) {
GlobalHolder.getInstance().setGlobalProperty(10);
System.out.println(GlobalHolder.getInstance().getGlobalProperty());
}
}
public class GlobalImpl {
public static int global = 5;
}
you can call anywhere you want:
GlobalImpl.global // 5
Creating an independent file, eg. Example.java to use the 1st solution, is just fine. You can do that also within the app, if e.g. the global variables are special to your current app, etc.:
Create a class at the beginning and declare your variables in there:
class Globals {
static int month_number;
static String month_name;
}
You can then access these variables -- using them as 'Globals.month_number', etc. -- from averywhere in your app.
very simple:
class UseOfGlobal
{
private static int a;
private static int b;
}
but it is always good to have local variables defined inside method blocks where ever possible.
As you probably guess from the answer there is no global variables in Java and the only thing you can do is to create a class with static members:
public class Global {
public static int a;
}
You can use it with Global.a elsewhere. However if you use Java 1.5 or better you can use the import static magic to make it look even more as a real global variable:
import static test.Global.*;
public class UseGlobal {
public void foo() {
int i = a;
}
}
And voilà!
Now this is far from a best practice so as you can see in the commercials: don't do this at home
There are no global variables in Java, but there are global classes with public fields. You can use static import feature of java 5 to make it look almost like global variables.
Generally Global variable (I assume you are comparing it with C,Cpp) define as public static final
like
class GlobalConstant{
public static final String CODE = "cd";
}
ENUMs are also useful in such scenario :
For Example Calendar.JANUARY)
To allow an unqualified access to static members of another class, you can also do a static import:
import static my.package.GlobalConstants;
Now, instead of print(GlobalConstants.MY_PASSWORD);
you can use the Constant directly: print(MY_PASSWORD);
See What does the "static" modifier after "import" mean? to decide about.
And consider the answer of Evan Lévesque about interfaces to carry the Constants.
// Get the access of global while retaining priveleges.
// You can access variables in one class from another, with provisions.
// The primitive must be protected or no modifier (seen in example).
// the first class
public class farm{
int eggs; // an integer to be set by constructor
fox afox; // declaration of a fox object
// the constructor inits
farm(){
eggs = 4;
afox = new fox(); // an instance of a fox object
// show count of eggs before the fox arrives
System.out.println("Count of eggs before: " + eggs);
// call class fox, afox method, pass myFarm as a reference
afox.stealEgg(this);
// show the farm class, myFarm, primitive value
System.out.println("Count of eggs after : " + eggs);
} // end constructor
public static void main(String[] args){
// instance of a farm class object
farm myFarm = new farm();
}; // end main
} // end class
// the second class
public class fox{
// theFarm is the myFarm object instance
// any public, protected, or "no modifier" variable is accessible
void stealEgg(farm theFarm){ --theFarm.eggs; }
} // end class
Going by the concept, global variables, also known as instance variable are the class level variables,i.e., they are defined inside a class but outside methods. In order to make them available completely and use them directly provide the static keyword.
So if i am writing a program for simple arithmetical operation and it requires a number pair then two instance variables are defined as such:
public class Add {
static int a;
static int b;
static int c;
public static void main(String arg[]) {
c=sum();
System.out.println("Sum is: "+c);
}
static int sum() {
a=20;
b=30;
return a+b;
}
}
Output: Sum is: 50
Moreover using static keyword prior to the instance variables enable us not to specify datatypes for same variables again and again. Just write the variable directly.
In general, Java doesn't have any global variables. Other than local variables, all variables comes under the scope of any class defined in the program.
We can have static variables to have the scope of global variables.
without static this is possible too:
class Main {
String globalVar = "Global Value";
class Class1 {
Class1() {
System.out.println("Class1: "+globalVar);
globalVar += " - changed";
} }
class Class2 {
Class2() {
System.out.println("Class2: "+globalVar);
} }
public static void main(String[] args) {
Main m = new Main();
m.mainCode();
}
void mainCode() {
Class1 o1 = new Class1();
Class2 o2 = new Class2();
}
}
/*
Output:
Class1: Global Value
Class2: Global Value - changed
*/
Object-Oriented Programming is built with the understanding that the scope of variables is closely exclusive to the class object that encapsulates those variables.
The problem with creating "global variables" is that it's not industry standard for Java. It's not industry standard because it allows multiple classes to manipulate data asyncronized, if you're running a multi-threaded application, this gets a little more complicated and dangerous in terms of thread-safety. There are various other reasons why using global variables are ineffective, but if you want to avoid that, I suggest you resort to Aspect-Oriented Programming.
Aspect-Oriented Programming negates this problem by putting the parent class in charge of the scope through something called "advices", which adds additional behavior to the code without actually modifying it. It offers solutions to cross-cutting concerns, or global variable usage.
Spring is a Java framework that utilizes AOP, and while it is traditionally used for web-applications, the core application can be used universally throughout the Java framework (8.0 included). This might be a direction you want to explore more.
To define Global Variable you can make use of static Keyword
public final class Tools {
public static int a;
public static int b;
}
now you can access a and b from anywhere by calling
Tools.a;
Tools.b;
Yoy are right...specially in J2ME...
You can avoid NullPointerException by putting inside your MidLet constructor
(proggy initialization) this line of code:
new Tools();
This ensures that Tools will be allocated before any instruction
that uses it.
That's it!