We can access a static property of a class by writing className.propertyName, but if the property (method/variable) is private then is it possible to access that property?
For example,
class A{
static int a = 50;
}
public class HelloWorld{
public static void main(String []args){
System.out.print("A.a = ");
A obj = new A();
System.out.println(A.a);
}
}
This will print A.a = 50
But if I change static int a = 50; to private static int a = 50; then can I access that variable any how?
The private keyword means that it'll only be visible within the class. So in your example it means that you cannot access it like A.a. What you can do though is to create a public method that returns a.
private static int a = 5;
public static int getA () {
return a;
}
You can then statically call this method and retrieve the private static field.
// ...
System.out.println(A.getA());
Usually private static fields are rarely used though.
One more thing I'd like to add is the general use of static here.
As you actually create an instance of the class A the static modifier is redundant.
You cannot access the private in outside class or outside the package .Because private making them only accessible within the declared class.if you want to access the variables in the class means public,default and protected are only accessible .outside the package
means default is not possible only public and protected is possible ,protected also have different package and non sub class means not possible only sub class is possible(need to extend the class).public only accessible for all inside and outside the packages.
I'm a begginer programmer for Android and I found some code over the internet and I couldn't get what this "Class not meant to be instantiated" means?! Also what's the use of it. I would be very happy if somebody could help here.
public class Settings
{
//some code
private Settings() {} // Class not meant to be instantiated
//some code
}
The constructor is private so only the class itself can create instances. There are several reasons for doing this. A couple off the top of my head...
The class is a "utility" class that only contains static methods and so instantiating it would make no sense. As the class is commented "Class not meant to be instantiated" I guess this is the most likely reason.
The class itself controls its own lifecycle and provides methods for creating instances. For example if the class is a lazy singleton it might provide a method that creates an instance when first called and return this instance on subsequent calls.
It is a private constructor. This means that outside classes cannot create new instances using the default constructor.
A little more info
All Objects in Java have a default constructor:
public MyObject() {}
That is how you can have this class:
public class MyObject{}
and still be able to call:
MyObject mObj = new MyObject();
Private Constructors
Sometimes a developer may not want this default constructor to be visible. Adding any other constructor will nullify this constructor. This can either be a declared constructor with empty parameters (with any of the visibility modifiers) or it can be a different constructor all together.
In the case above, it is likely that one of the following models is followed:
The Settings object is instantiated within the Settings class, and is where all the code is run (a common model for Java - where such a class would also contain a static main(String[] args) method).
The Settings object has other, public constructors.
The Settings object is a Singleton, whereby one static instance of the Settings Object is provided to Objects through an accessor method. For example:
public class MyObject {
private static MyObject instance;
private MyObject(){}//overrides the default constructor
public static MyObject sharedMyObject() {
if (instance == null)
instance = new MyObject();//calls the private constructor
return instance;
}
}
This inner construct
private Settings() {}
is a constructor for Settings instances. Since it is private, nobody can access it (outside of the class itself) and therefore no instances can be created.
The constructor is private so its not meant to be called by anything outside of the class
It's not a nested class, it's a constructor. A private constructor means that you can't construct instances of this class from outside, like this:
Settings s = new Settings(); //Compilation error! :(
Now, if a class can't be instantiated, what could it be for? The most likely reason for this is that the class would return instances of itself from a static method, probably as a singleton. The settings are normally global to the program, so a singleton pattern really fits here. So there would be a static method that goes kind of like this
static private TheOnlySettings = null;
static public getSettings()
{
if(TheOnlySettings == null)
TheOnlySettings = new Settings(); //Legal, since it's inside the Settings class
return TheOnlySettings;
}
See if that's indeed the case.
As other have mentioned, a class having private constructors cannot be instantiated from outside the class. A static method can be used in this case.
class Demo
{
private Demo()
{
}
static void createObjects()
{
Demo o = new Demo();
}
}
class Test
{
public static void main (String ...ar)
{
Demo.createObjects();
}
}
We can have private constructor . Below program depicts the use of private constructor with a static function
class PrivateConstructor {
private:
PrivateConstructor(){
cout << "constructor called" << endl;
}
public:
static void display() {
PrivateConstructor();
}
};
int main() {
PrivateConstructor::display();
}
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.
Given this code snippet, could you explain why it woks?
The thing is that the class constructor is marked private, so should not it prevent us to call it with new operator?
public class Alpha {
protected Alpha() {}
}
class SubAlpha extends Alpha {
private SubAlpha() {System.out.println("ok");}
public static void main(String args[]) {
new SubAlpha();
}
}
It all works because the static method is part of the class and it can see all private fields and methods, right? Outside this "new" initialization would never work?
The only private constructor in your question is SubAlpha, which SubAlpha itself is calling. There's no issue, a class can call its own private methods. The Alpha constructor is protected, so SubAlpha has access to it.
Edit: Re your edit: Yes, exactly. A separate class (whether a subclass or not) would not have access to SubAlpha's private constructor and could not successfully construct a new SubAlpha().
Example 1:
public class Beta
{
public static final void main(String[] args)
{
new SubAlpha();
// ^--- Fails with a "SubAlpha() has private access in SubAlpha"
// compilation error
}
}
Example 2:
public class SubSubAlpha extends SubAlpha
{
private subsubAlpha()
{
// ^== Fails with a "SubAlpha() has private access in SubAlpha"
// compilation error because of the implicit call to super()
}
}
This is, of course, constructor-specific, since scope is always member-specific. If a class has a different constructor with a different signature and a less restrictive scope, then a class using it (including a subclass) can use that other constructor signature. (In the case of a subclass, that would require an explicit call to super(args);.)
The code works as the main method is also in the same class. You may not be able to initialize SubAplha from a different class.
I've come across some odd behavior in assignment of final variables. You can assign a final varible in a constructor to initialize it, which makes sense. However you can't do the same in a subclass, even if the final variable is a member of the subclass -
public class FinalTest {
public final String name;
public FinalTest()
{
name = "FinalTest";
}
public static class FinalTestSubclass extends FinalTest {
public FinalTestSubclass()
{
name = "FinalTestSubclass"; //<---- this won't compile, assignment to final variable.
}
}
}
Can someone think of a good reason why this should/would work this way?
Every constructor of a subclass must invoke a constructor of the superclass as its first operation. Every final member variable must be initialized before a constructor completes. A final variable can be assigned only once. Given those rules, it is impossible for a subclass constructor to directly assign a value to a final superclass' member.
Making exceptions would increase complexity and create "gotchas" in exchange for limited additional utility.
A practical solution is to provide a superclass constructor that takes a value to be assigned to the final member. This can be protected or package-private if desired. If the superclass is outside of your control, there's a good chance that allowing derived classes to break its assumptions about the finality of its members would cause other problems.
If you were allowed to assign a value to name in FinalTestSubClass it would mean that the value assigned in FinalTest was not actually the final value.
If your example was valid, then this would mean that name could have different values (based upon which class was instantiated), making the final modifier pretty much redundant.
A better question is, why should the behavior you desire be allowed?
informally, final fields should have been initialized when the constructor is finished.
in your subclass constructor, super() has been called implicitly, the constructor of the super class is finished, the final fields in the super class should not be modified.
you may want this instead:
class A
final String s;
A(String s){ this.s = s; }
A() { this("default"); }
class B extends A
B(){ super("B's default"); }
This is standard behavior in Java
The key word final can by used in multiple way, for class close the possibility to inherite from it, for method to override it, for variable allow to be assigned only once in simply words.
For your case this variable is allready assigned in super class,
what You can do is
public class FinalTest {
public final String name = "FinalTest";
public FinalTest()
{
}
public static class FinalTestSubclass extends FinalTest {
public final String name = "FinalTestSubclass";
public FinalTestSubclass()
{
}
}
}
Read more about final variables
In reply to your comment to matt's answer; you can achieve determining the constant in the subclass by passing it in the constructor:
public class FinalTest {
public final String name;
public FinalTest()
{
this("FinalTest");
}
protected FinalTest(String nameConstant)
{
name = nameConstant;
}
public static class FinalTestSubclass extends FinalTest {
public FinalTestSubclass()
{
super("FinalTestSubclass");
}
}
}