The main method tries to access var, but results in ambiguous call. Why? Instance variable var in Base1 isn't accessible (visible?) from static context anyway.
class Base1 {
int var;
}
interface Base2 {
public static final int var = 0;
}
class Test extends Base1 implements Base2 {
public static void main(String args[]) {
System.out.println("var:" + var);
}
}
The JLS rule for field access ambiguity is
If the identifier names several accessible (ยง6.6) member fields in
type T, then the field access is ambiguous and a compile-time error
occurs.
And on the subject of accessibility
A member (class, interface, field, or method) of a reference type, or
a constructor of a class type, is accessible only if the type is
accessible and the member or constructor is declared to permit access:
It doesn't make a distinction about whether the instance field access would cause a compile error in a static context.
Note that you could have had
public static void main(String args[]) {
Test test = new Test();
System.out.println("var:" + test.var);
}
You'd still have the ambiguity.
To make it unambiguous put the interface name as qualifying prefix:
class Test extends Base1 implements Base2 {
public static void main(String args[]) {
System.out.println("var:" + Base2.var);
}
}
Inititally at step one compiler will look for variable var in the class which you extend and the interface you implement. Since it finds a variable at both places in step two, it shows ambiguity.
The static and non static contexts do not rule on how variables are permitted to be accessed
The access modifiers are the ones which actually rule this...
change the access modifier for var in the Base1 to private and the ambiguity disappears, though this might not be the way you want it to shape up but access modfiers actually dictate the reference to instance variables rather than the static no static contexts.
class Base1 {
private int var;
//static int var=5;
}
interface Base2 {
public static final int var = 0;
}
class ambiguousNonStaticCall extends Base1 implements Base2 {
public static void main(String args[]) {
System.out.println("var:" + var);
}
}
The above code compiles fine.
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
I have the following class:
public abstract class A()
{
public static final SomeString = null;
static()
{
SomeString = "aaa";
}
}
When is this static method invoked and how?
What is the purpose in creating such a static method (without name / return type)?
That is not a method, it's a static initialization block, and your syntax is wrong
public abstract class A()
{
public static String SomeString = null;
static
{
SomeString = "aaa";
}
}
The easiest way of initializing fields (static or instance) in Java at the time of their declaration is simply by providing a compile time constant value of a compatible data type. For example:
public class InitializationWithConstants{
public static int staticIntField = 100;
private boolean instanceBoolField = true;
}
This type of initialization has its limitation due to its simplicity and it can not support initialization based even on some moderately complex logic - like initializing only selected elements of a complex array using some logic in a for loop.
Here comes the need for static initialization blocks and initializer blocks for initializing static and instance fields, respectively.
It's a normal block of code enclosed within a pair of braces and preceded by a 'static' keyword. These blocks can be anywhere in the class definition where we can have a field or a method. The Java runtime guarantees that all the static initialization blocks are called in the order in which they appear in the source code and this happens while loading of the class in the memory.
public class InitializationWithStaticInitBlock{
public static int staticIntField;
private boolean instanceBoolField = true;
static{
//compute the value of an int variable 'x'
staticIntField = x;
}
}
Since static initialization blocks are actually code-blocks so they will allow us to initialize even those static fields which require some logical processing to be done for them to get their initial values.
Your syntax is incorrect; it should be:
public abstract class A()
{
public static final String SomeString;
static
{
SomeString = "aaa";
}
}
The static block allows static variables to be initialised when the class is loaded when that initialisation is more complication than simply = something;.
Reference
Syntax aside, you're looking at a static initializer block. They're mentioned here.
Essentially, a static initializer block is a piece of code that executes when a class is loaded, and can be used to initialize static fields in the class.
Example:
public abstract class A
{
public static final String SomeString;
static
{
System.out.println("static{}");
SomeString = "aaa";
}
public static void main(String[]args)
{
System.out.println("main");
}
}
Output:
static{}
main
yeas it's not a method. it is static block and i'st evaluated once when the owner class is loaded.
u can use it for initialize static variable dynamically ...
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.
Why can we have static final members but cant have static method in an non static inner class ?
Can we access static final member variables of inner class outside the outer class without instantiating inner class ?
YOU CAN have static method in a static "inner" class.
public class Outer {
static String world() {
return "world!";
}
static class Inner {
static String helloWorld() {
return "Hello " + Outer.world();
}
}
public static void main(String args[]) {
System.out.println(Outer.Inner.helloWorld());
// prints "Hello world!"
}
}
To be precise, however, Inner is called a nested class according to JLS terminology (8.1.3):
Inner classes may inherit static members that are not compile-time constants even though they may not declare them. Nested classes that are not inner classes may declare static members freely, in accordance with the usual rules of the Java programming language.
Also, it's NOT exactly true that an inner class can have static final members; to be more precise, they also have to be compile-time constants. The following example illustrates the difference:
public class InnerStaticFinal {
class InnerWithConstant {
static final int n = 0;
// OKAY! Compile-time constant!
}
class InnerWithNotConstant {
static final Integer n = 0;
// DOESN'T COMPILE! Not a constant!
}
}
The reason why compile-time constants are allowed in this context is obvious: they are inlined at compile time.