For example,
i have class a,b,c,d,e,f.
and for class f, they have many variable,
so how can I call class f variable when i am in class a,b,c,d,e?
Use import? or use static variable?
Can static variable change while programming?
Thank you.
Multiple ways:
Define variable as static, final and public if that value doesn't change and is a constant.
public static final int DUMMY = 9999;
And then you could access it like:
System.out.println(F.DUMMY);
Define that variable as instance variable and provide getter and setter in f class. Inject instance of f within a,b.. classes and use getter/setter
public class F {
private int dummy;
//getter/setter
}
public class a {
..
int value = f.getDummy();
}
Can static variable change while programming?
Yes static fields can be changed later after declaring it.But only one value is maintained through out one instance of JVM.
No matter how many instance of class is created the static fileds within the class will be same for all the instance of class.
Here is good explaination in jls.
Static Fields Explaination
Related
I recently encountered this phrase:
"Class A has class member int a"
Probably obvious but this sentence just means a is an int defined in class A, right?
And another thing, for example a is defined under a method in class A. is it still
a class member?
I haven't found a clear definition of class member, I looked here:
but it wasn't very helpful.
Thank's in advance for the help
Class member is another way of calling static members.
class A {
int a; //instance variable
static int b; //class variable
public void c() {
int d; //local variable
}
}
In same docs
Fields that have the static modifier in their declaration are called static fields or class variables
Class variables are referenced by the class name itself, as in
Bicycle.numberOfBicycles
This makes it clear that they are class variables.
class member is not just a variable of the class. they can be accessed using the class name. That means they are static variable of that class.
The document mentioned it clearly.
public class Bicycle {
private int cadence;
private int gear;
private int speed;
// add an instance variable for the object ID
private int id;
// add a class variable for the
// number of Bicycle objects instantiated
private static int numberOfBicycles = 0;
...
}
in the above code numberOfBicycles is a class member. It can be accessed using
Bicycle.numberOfBicycles
And variables inside methods can't access like that. so they can't be class members. variables declared inside a method are local variables and belong to that method. So you can call them final, but not static or public or protected or private.
In the docs link you have mentiond, its clear in the first line (after heading) that
In this section, we discuss the use of the static keyword to create fields and methods that belong to the class, rather than to an instance of the class.
So it means that static keyword is used to create class fields and methods(i.e.class members).
So in your case,
class A{
int a;
public void methodA(){
int a;//inner a
}
}
What you have asked is that is int a inside methodA() still a class member?
Answer is no: since it is not preceded by static keyword.If you try to use static keyword as:
class A{
int a;
public void methodA(){
static int a;//inner a will cause compile time error
}
}
You will get compile time error.
Hope that helped!! :)
Variable in Java is a data container(memory) that stores the data values during Java program execution.There are 3 types of variables in java.
They are local variables,instance variables,static variables.
local variables - declared within the body of a method..
instance variables - declared inside the class but not inside the method,to access these variables you need to create an object
static - memory allocated only once..directly accessible and its not object specific
Static variables defined at global scope of the class and so they also refereed as class member.for example
public class TypesofVar {
int a = 10; // instance variables
static int c = 30; // static variables
public static void main(String[] args) {
int b = 20; // local variable
System.out.println(c);
System.out.println(b);
TypesofVar obj = new TypesofVar();
System.out.println(obj.a);
}
}
What you have asked is that is int a inside methodA() still a class member?
NO because it is not preceded by static keyword
This is the pac.java file
package P1;
public class pac {
int a;
public int b;
private int c;
protected int d;
public pac(){
a=1;
b=2;
c=3;
d=4;
}
public void test(){
System.out.println(a);
System.out.println(b);
System.out.println(c);
System.out.println(d);
}
}
This is other file pack1.java in different package
package P3;
class pac1 extends P1.pac{
public static void main(String args[]) {
pac1 ob=new pac1();
ob.test();
}
}
Question:
How is that possible that from the two files I have shown, the file P3\pac1.java creates an object which access the private variable of the class P1\pac.java?
In Java, a subclass object cannot access private variables (or methods) of any super class.
In your example, pac1 class only access public functionality of class pac, such as the public default constructor & the public method test(). Since the implemnetation of these methods is defined within the super class, both can access its private funvtionality - such as the private int c.
If you override these method in subclass however, the overriding methods implementation won't be able to access class pac private functionality.
HTH.
The method " test() " is inside of the class "pac". Therefore, when you create an instance of pac, the test method belongs to this instance, and can see and use all the variables.
If you had done this, it would have caused an error.
int mainC = ob.c;
the main method (where this is happening) cannot access the variable "c"
What I understand from your unalligned code is that you instantiate a public class and call a public method of it. This is perfectly fine, the test() method has access to variables. You can't access them directly from another method in pac1.
pac1.java only didn't access the private member c of pac.java. It only called the public method test of pac, and that method access the member of the same class.
If I understand you correctly, you are curious how calling ob.test () accesses the private members, despite ob being an object of the derived class. The reason is that whether a method has access is determined at compile time, not run time, and is determined by the location of the method. Your test method is declared in your pac class. So the compiler gives it access to all the private members of the pack class. Now, when it comes to executing the method, it transpires that the type of object your method is executing on is pac1. That does not matter.
This feature of object oriented languages is not a mistake. It makes it possible for the writer of a class to provide limited access to private members, by providing a method that gives only that limited access.
The statement pac1 ob=new pac1(); is creating a new object of Pac1 class and it is possible because it is public.
using ob.test() you are calling a method on the object itself. As private int c is member of the same class in which test() is defined, it can access it.
A private member variable or method is accessible throughout the class in which it is defined.
You can create the object of Pac1 because the class is public.
To clarify your doubt, you cannot access the private member outside the class like this ob.c in the main method.
Consider a very simple example. For a javabean, we make member as private and getter setters as public. We can access them using only getter and setter as they are public.
public class SomeClass{
private int someMember;
public int getSomeMember(){
return this.someMember;
}
}
Now from outside the class,suppose in your main method,
SomeClass someClass = new SomeClass();
someClass.someMember; // This is not possible as we are directly accessing private field
someClass.getSomeMember; // possible because `getSomeMember` is public and it belongs to `SomeClass` so can access private member `someMember`.
`
The following singleton class works fine,
public class Elvis
{
private static Elvis elvis = new Elvis();
private Elvis()
{
}
public static Elvis Instance()
{
return elvis;
}
}
However, when I change return elvis; to return this.elvis, I get non-static variable this cannot be referenced from a static context. Why is this?
this refers to the current object instance. A static method is not contained in an object, it is contained by the class.
The two keywords "this" and "static" are in some sense antagonic. You use "this" when you're referencing members of the current object, that is you have an object and you, presumably, are acessing an non-static member. Static members, functions and properties, are "globals" for all objects, they don't belong to any specific object but rather to the class itself.
Long history made short. "this" means the current instance of this object. Static members don't belong to any instance.
If a variable is declared as public static varName;, then I can access it from anywhere as ClassName.varName. I am also aware that static members are shared by all instances of a class and are not reallocated in each instance.
Is declaring a variable as private static varName; any different from declaring a variable private varName;?
In both cases it cannot be accessed as ClassName.varName or as ClassInstance.varName from any other class.
Does declaring the variable as static give it other special properties?
Of course it can be accessed as ClassName.var_name, but only from inside the class in which it is defined - that's because it is defined as private.
public static or private static variables are often used for constants. For example, many people don't like to "hard-code" constants in their code; they like to make a public static or private static variable with a meaningful name and use that in their code, which should make the code more readable. (You should also make such constants final).
For example:
public class Example {
private final static String JDBC_URL = "jdbc:mysql://localhost/shopdb";
private final static String JDBC_USERNAME = "username";
private final static String JDBC_PASSWORD = "password";
public static void main(String[] args) {
Connection conn = DriverManager.getConnection(JDBC_URL,
JDBC_USERNAME, JDBC_PASSWORD);
// ...
}
}
Whether you make it public or private depends on whether you want the variables to be visible outside the class or not.
Static variables have a single value for all instances of a class.
If you were to make something like:
public class Person
{
private static int numberOfEyes;
private String name;
}
and then you wanted to change your name, that is fine, my name stays the same. If, however you wanted to change it so that you had 17 eyes then everyone in the world would also have 17 eyes.
Private static variables are useful in the same way that private instance variables are useful: they store state which is accessed only by code within the same class. The accessibility (private/public/etc) and the instance/static nature of the variable are entirely orthogonal concepts.
I would avoid thinking of static variables as being shared between "all instances" of the class - that suggests there has to be at least one instance for the state to be present. No - a static variable is associated with the type itself instead of any instances of the type.
So any time you want some state which is associated with the type rather than any particular instance, and you want to keep that state private (perhaps allowing controlled access via properties, for example) it makes sense to have a private static variable.
As an aside, I would strongly recommend that the only type of variables which you make public (or even non-private) are constants - static final variables of immutable types. Everything else should be private for the sake of separating API and implementation (amongst other things).
Well you are right public static variables are used without making an instance of the class but private static variables are not. The main difference between them and where I use the private static variables is when you need to use a variable in a static function. For the static functions you can only use static variables, so you make them private to not access them from other classes. That is the only case I use private static for.
Here is an example:
Class test {
public static String name = "AA";
private static String age;
public static void setAge(String yourAge) {
//here if the age variable is not static you will get an error that you cannot access non static variables from static procedures so you have to make it static and private to not be accessed from other classes
age = yourAge;
}
}
Is declaring a variable as private static varName; any different from
declaring a variable private varName;?
Yes, both are different. And the first one is called class variable because it holds single value for that class whereas the other one is called instance variable because it can hold different value for different instances(Objects). The first one is created only once in jvm and other one is created once per instance i.e if you have 10 instances then you will have 10 different private varName; in jvm.
Does declaring the variable as static give it other special
properties?
Yes, static variables gets some different properties than normal instance variables. I've mentioned few already and let's see some here: class variables (instance variables which are declared as static) can be accessed directly by using class name like ClassName.varName. And any object of that class can access and modify its value unlike instance variables are accessed by only its respective objects. Class variables can be used in static methods.
What is the use of a private static variable in Java?
Logically, private static variable is no different from public static variable rather the first one gives you more control. IMO, you can literally replace public static variable by private static variable with help of public static getter and setter methods.
One widely used area of private static variable is in implementation of simple Singleton pattern where you will have only single instance of that class in whole world. Here static identifier plays crucial role to make that single instance is accessible by outside world(Of course public static getter method also plays main role).
public class Singleton {
private static Singleton singletonInstance = new Singleton();
private Singleton(){}
public static Singleton getInstance(){
return Singleton.singletonInstance;
}
}
Well, private static variables can be used to share data across instances of that class. While you are correct that we cannot access the private static variables using constructs like ClassName.member or ClassInstance.member but the member will always be visible from methods of that class or instances of that class. So in effect instances of that class will always be able to refer to member.
What is the use of a private static class variable?
Let's say you have a library book Class. Each time you create a new Book, you want to assign it a unique id. One way is to simply start at 0 and increment the id number. But, how do all the other books know the last created id number? Simple, save it as a static variable. Do patrons need to know that the actual internal id number is for each book? No. That information is private.
public class Book {
private static int numBooks = 0;
private int id;
public String name;
Book(String name) {
id = numBooks++;
this.name = name;
}
}
This is a contrived example, but I'm sure you can easily think of cases where you'd want all class instances to have access to common information that should be kept private from everyone else. Or even if you can't, it is good programming practice to make things as private as possible. What if you accidentally made that numBooks field public, even though Book users were not supposed to do anything with it. Then someone could change the number of Books without creating a new Book.
Very sneaky!
The private keyword will allow the use for the variable access within the class and static means we can access the variable in a static method.
You may need this cause a non-static reference variable cannot be accessible in a static method.
Another perspective :
A class and its instance are two different things at the runtime. A class info is "shared" by all the instances of that class.
The non-static class variables belong to instances and the static variable belongs to class.
Just like an instance variables can be private or public, static variables can also be private or public.
Static variables are those variables which are common for all the instances of a class..if one instance changes it.. then value of static variable would be updated for all other instances
For some people this makes more sense if they see it in a couple different languages so I wrote an example in Java, and PHP on my page where I explain some of these modifiers. You might be thinking about this incorrectly.
You should look at my examples if it doesn't make sense below. Go here http://www.siteconsortium.com/h/D0000D.php
The bottom line though is that it is pretty much exactly what it says it is. It's a static member variable that is private. For example if you wanted to create a Singleton object why would you want to make the SingletonExample.instance variable public. If you did a person who was using the class could easily overwrite the value.
That's all it is.
public class SingletonExample {
private static SingletonExample instance = null;
private static int value = 0;
private SingletonExample() {
++this.value;
}
public static SingletonExample getInstance() {
if(instance!=null)
return instance;
synchronized(SingletonExample.class) {
instance = new SingletonExample();
return instance;
}
}
public void printValue() {
System.out.print( this.value );
}
public static void main(String [] args) {
SingletonExample instance = getInstance();
instance.printValue();
instance = getInstance();
instance.printValue();
}
}
I'm new to Java, but one way I use static variables, as I'm assuming many people do, is to count the number of instances of the class. e.g.:
public Class Company {
private static int numCompanies;
public static int getNumCompanies(){
return numCompanies;
}
}
Then you can sysout:
Company.getNumCompanies();
You can also get access to numCompanies from each instance of the class (which I don't completely understand), but it won't be in a "static way". I have no idea if this is best practice or not, but it makes sense to me.
In the following example, eye is changed by PersonB, while leg stays the same. This is because a private variable makes a copy of itself to the method, so that its original value stays the same; while a private static value only has one copy for all the methods to share, so editing its value will change its original value.
public class test {
private static int eye=2;
private int leg=3;
public test (int eyes, int legs){
eye = eyes;
leg=leg;
}
public test (){
}
public void print(){
System.out.println(eye);
System.out.println(leg);
}
public static void main(String[] args){
test PersonA = new test();
test PersonB = new test(14,8);
PersonA.print();
}
}
>
14
3
When in a static method you use a variable, the variable have to be static too
as an example:
private static int a=0;
public static void testMethod() {
a=1;
}
If a variable is defined as public static it can be accessed via its class name from any class.
Usually functions are defined as public static which can be accessed just by calling the implementing class name.
A very good example of it is the sleep() method in Thread class
Thread.sleep(2500);
If a variable is defined as private static it can be accessed only within that class so no class name is needed or you can still use the class name (upto you).
The difference between private var_name and private static var_name is that private static variables can be accessed only by static methods of the class while private variables can be accessed by any method of that class(except static methods)
A very good example of it is while defining database connections or constants which require declaring variable as private static .
Another common example is
private static int numberOfCars=10;
public static int returnNumber(){
return numberOfCars;
}
*)If a variable is declared as private then it is not visible outside of the class.this is called as datahiding.
*)If a variable is declared as static then the value of the variable is same for all the instances and we no need to create an object to call that variable.we can call that variable by simply
classname.variablename;
private static variable will be shared in subclass as well. If you changed in one subclass and the other subclass will get the changed value, in which case, it may not what you expect.
public class PrivateStatic {
private static int var = 10;
public void setVar(int newVal) {
var = newVal;
}
public int getVar() {
return var;
}
public static void main(String... args) {
PrivateStatic p1 = new Sub1();
System.out.println(PrivateStatic.var);
p1.setVar(200);
PrivateStatic p2 = new Sub2();
System.out.println(p2.getVar());
}
}
class Sub1 extends PrivateStatic {
}
class Sub2 extends PrivateStatic {
}
If you use private static variables in your class, Static Inner classes in your class can reach your variables. This is perfectly good for context security.
ThreadLocal variables are typically implemented as private static.
In this way, they are not bound to the class and each thread has its own reference to its own "ThreadLocal" object.
Private static fields and private static methods can useful inside public static methods. They help to reduce the too much logic inside public static methods.
I need to reference a variable of a top level class from a method within a static class.
This method should act on unique instances of the top level class and so it feels like I shouldn't instantiate the top level class inside the static class.
Basically I want something like
public class TopLevel{
// private
int innerV
public static class Inner implements X {
for(i=0; i<innerV,i++){
doSomething
}
}
}
Is it possible to just say this.innerV or something similar in the for loop and similar places?
From a static inner class, you can't refer to (nonstatic) members of the outer class directly. If you remove the static qualifier, it will work, because instances of nonstatic inner classes are implicitly tied to an instance of the containing class, so they can refer to its members directly.
Declaring your inner class static removes this link, so you need to either pass an instance of the outer class to the inner class method (or its constructor) as a parameter, or create it inside the method.
You can't do that. Create a TopLevel instance and if you make an innerV accessor (getter/setter) or make it public, than you can.
public class TopLevel {
public int innerV
public static class Inner implements X {
for(i=0; i<innerV,i++){
TopLevel tl = new TopLevel()
tl.innerV = 12345678;
}
}
}
You can't do that because it doesn't make sense, any more than referring to a non-static member from a static function makes sense. There is no current instance of the outer class in the context of the static inner class to get the instance variable from.