I am developing a plugin for an RCP application.
Within the plugin.xml, I need to register certain classes at a given extension point.
One of these classes is an anonymous (?) class defined like this:
package de.me.mypackage;
import org.something.AnotherClass;
public class ClassOne {
...
public static AnotherClass<ClassOne> getThat() {
return new AnotherClass<ClassOne>() {
...
};
}
}
Is there any way to reference AnotherClass<ClassOne> within the plugin.xml?
I already tried something like de.me.mypackage.ClassOne$AnotherClass but that does not work. Do I have to declare that class within its own file to be able to reference it?
As far as I know, it would have a numeric index:
class Bla {
public static void main(String[] args) {
(new Runnable() {
public void run() {
System.out.println(getClass().getName()); // prints Bla$1
}
}).run();
}
}
After compiling, you get:
$ ls *.class
Bla$1.class Bla.class
That said, you can't rely on the numbering in case the source file is modified.
Can you instead define a static inner class, like:
public class ClassOne {
public static class MyClass extends AnotherClass<ClassOne> {
public MyClass(/* arguments you would pass in getThat()? */) {
...
}
...
}
public static AnotherClass<ClassOne> getThat() {
return new MyClass(...);
}
}
I need to say the obvious here - you should make it a named class if you want to refer to it. Whether you can access it otherwise is a technical curiosity (that I don't happen to know the answer to), not something you should actually do in production.
The dollar sign only comes into play in the class's binary name; in Java source, just use de.me.mypackage.ClassOne.AnotherClass.class.
I face a strange error. During the startup of the server, i initialize a set of variables in the init() method of a java class. I could see this value is persisted during the server startup. However, when i try to login through the WebUI, these local variables goes missing.
However, if i assign the variable in a static block, it stays on.
I dont know how this could happen. Any pointers would help.
Java version : JRE1.7.0_40
My piece of code looks like
ClassA.java
public class ClassA { public static String testString; public static
void init() throws Exception {
testString = "testSTring222"; } }
ClassB.java
ClassA.init(); System.out.println(ClassA.testString)
Please help me get out of this.
It certainly depends on what does your server.
Use some static variable.
This could happen due to the usage of multiple class loaders, since static variables are not global across multiple class loaders. i.e. if the same class is loaded in 2 different class loaders, then you will have 2 copies of the static variable.
Class A
public class ClassA
{
public static String testString = "testSTring222";
public static void main(String[] args)
{
}
}
Class B
public class ClassB
{
static ClassA cA = new ClassA();
public static void main(String[] args)
{
System.out.println(cA.testString);
}
}
So you should reference Class A as a static class and make the and set the static string when you first declare it as a variable
I understand how to use and import outside packages, but I've never packaged my own classes before. I read the Oracle Tutorial on Creating a Package, and looked at In Java, what's the difference between public, default, protected and private in addition to several sites/SO threads on packages. For the life of me, I can't figure out why this extraordinary simple example doesn't work:
package PTest;
public class A
{
protected final int SIZE = 10;
public void printSize()
{
System.out.println(SIZE);
}
}
package PTest;
public class B
{
public static void main(String[] args)
{
System.out.println(SIZE);
hello();
}
}
I used eclipse's autopackaging feature, so I assume that the actual packing is correct. Here's an image to show that they are indeed packaged correctly:
As you can see, neither the protected SIZE or the public hello() are recognized. I've tried this outside of eclipse, also to no avail. Any help would be greatly appreciated.
SIZE is an instance field of A objects. You need to make it a static field. Even then, it'll be a member of the A class, so you have to specify A.SIZE to use it in B.
Class methods cannot access instance variables or instance methods directly—they must use an object reference.
Errors you getting are fixed here
package PTest;
public class B
{
public static void main(String[] args)
{
A MyClassA = new A();
System.out.println(MyClassA.SIZE);
MyClassA.printSize();
}
}
You can not directly access methods or fields which are not static (instance members) being in a static scope(main) other than using an object and then access or making those instance members as staic.(class members)
In Java, how do you set variables in a calling object from the object that is being called? I guess I could set up some kind of struct class, but can someone show me if there is a simpler way to do it, such as some modification of the pseudocode below:
public class Example(){
int thisInt;
int thatInt;
public static void main(String[] args){
Another myAnother = new Another();
}
setThisInt(int input){thisInt=input;}
setThatInt(int input2){thatInt=input2;}
}
public class Another(){
void someFunc(){
this.Example.setThisInt(5);//I know this syntax is wrong
this.Example.setThatInt(2);//I know this syntax is wrong
}
}
Pass in a reference to the object.
public class Another{
void someFunc(Example ob){
ob.setThisInt(5);
ob.setThatInt(2);
}
}
If you're using nested classes(one class inside the other with an implied parent-child relationship), use:
OuterClass.this.setThisInt(5);
and so on.
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!