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Cannot refer to a non-final variable inside an inner class defined in a different method
(20 answers)
Closed 7 years ago.
I know,this question has been asked already and I an advance apology for asking it again but I am not able to figure out the exact reason why this is so?Can someone please help me out with it?
For e.g.-
public class File
{
public static void main(String[] main)
{
File obj=new File();
obj.method();
}
void method()
{
for(int i=0;i<10;i++)
{
class Inner
{
void display()
{
System.out.println("i is : "+i); //Gives error,i needs to be declared final
}
}
Inner cls=new Inner();
cls.display();
}
}
}
My questions are-
Why the variable i needs to be declared final if I want to use it in
display() method of Inner class?
If I want to display i inside the display method in each
iteration,then how may I do so?Because if I declare i to be final
with initial value 0,then I will not be able to iterate over it.
Thanks .....
1)That variable needs to be final or effective final because of security reasons. What if the object using in that modifying the passed object??
2) You have to just move your for loop inside the class. No other way around.
An inner class just want to use the the instance members, just use them, that is the only thing you can do inside inner class. You have no command over that. To achieve that functionality, Java made the rule of final or effective final.
Ok, let see what happens when you made the variable final ?
All the copies of those final variables will be used by the inner class not the actual varibles.
If you not heard of the term effectively final here you go : Difference between final and effectively final
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
When should a method be static?
Usually when writing a static method for a class, the method can be accessed using ClassName.methodName. What is the purpose of using 'static' in this simple example and why should/should not use it here? also does private static defeat the purpose of using static?
public class SimpleTest {
public static void main(String[] args) {
System.out.println("Printing...");
// Invoke the test1 method - no ClassName.methodName needed but works fine?
test1(5);
}
public static void test1(int n1) {
System.out.println("Number: " + n1.toString());
}
//versus
public void test2(int n1) {
System.out.println("Number: " + n1.toString());
}
//versus
private static void test3(int n1) {
System.out.println("Number: " + n1.toString());
}
}
I had a look at a few tutorials. E.g. http://docs.oracle.com/javase/tutorial/java/javaOO/classvars.html
My understanding of it is that instead of creating an instance of a class to use that method, you can just use the class name - saves memory in that certain situations there is no point in constructing an object every time to use a particular method.
The purpose of the static keyword is to be able to use a member without creating an instance of the class.
This is what happens here; all the methods (including the private ones) are invoked without creating an instance of SimpleTest.
In this Example,Static is used to directly to access the methods.A private static method defeats the purpose of "Data hiding".
Your main can directly call test1 method as it is also Static,it dosn't require any object to communicate.Main cannot refer non-static members,or any other non-static member cannot refer static member.
"non-static members cannot be referred from a static context"
You can refer This thread for more info about Static members.
static means that the function doesn't require an instance of the class to be called. Instead of:
SimpleTest st = new SimpleTest();
st.test2(5);
you can call:
SimpleTest.test1(5);
You can read more about static methods in this article.
A question about private static has already been asked here. The important part to take away is this:
A private static method by itself does not violate OOP per se, but when you have a lot of these methods on a class that don't need (and cannot*) access instance fields, you are not programming in an OO way, because "object" implies state + operations on that state defined together. Why are you putting these methods on that class, if they don't need any state? -eljenso
static means that the method is not associated with an instance of the class.
It is orthogonal to public/protected/private, which determine the accessibility of the method.
Calling test1 from main in your example works without using the class name because test1 is a static method in the same class as main. If you wanted to call test2 from main, you would need to instantiate an object of that class first because it is not a static method.
A static method does not need to be qualified with a class name when that method is in the same class.
That a method is private (static or not) simply means it can't be accessed from another class.
An instance method (test2 in your example) can only be called on an instance of a class, i.e:
new SimpleTest().test2(5);
Since main is a static method, if you want to call a method of the class without having to instantiate it, you need to make those methods also static.
In regards to making a private method static, it has more readability character than other. There isn't really that much of a difference behind the hoods.
You put in static methods all the computations which are not related to a specific instance of your class.
About the visibility, public static is used when you want to export the functionality, while private static is intended for instance-independent but internal use.
For instance, suppose that you want to assign an unique identifier to each instance of your class. The counter which gives you the next id isn't related to any specific instance, and you also don't want external code to modify it. So you can do something like:
class Foo {
private static int nextId = 0;
private static int getNext () {
return nextId ++;
}
public final int id;
public Foo () {
id = getNext(); // Equivalent: Foo.getNext()
}
}
If in this case you want also to know, from outside the class, how many instances have been created, you can add the following method:
public static int getInstancesCount () {
return nextId;
}
About the ClassName.methodName syntax: it is useful because it specifies the name of the class which provides the static method. If you need to call the method from inside the class you can neglect the first part, as the name methodName would be the closest in terms of namespace.
It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center.
Closed 10 years ago.
public static final String Test = "ABC";
public static String Test = "ABC";
Please show the difference between "final static String" and "static String" in java.
except the final one can't be changed, What are the other differences? Thanks.
when in more complex circumstances like multithreading and inherited, nothing changed?
Although below description taken from Android doc, but might help for this Java tagged question.
Consider the following declaration at the top of a class:
static int intVal = 42;
static String strVal = "Hello, world!";
The compiler generates a class initializer method, called , that is executed when the class is first used. The method stores the value 42 into intVal, and extracts a reference from the classfile string constant table for strVal. When these values are referenced later on, they are accessed with field lookups.
We can improve matters with the "final" keyword:
static final int intVal = 42;
static final String strVal = "Hello, world!";
The class no longer requires a method, because the constants go into static field initializers in the dex file. Code that refers to intVal will use the integer value 42 directly, and accesses to strVal will use a relatively inexpensive "string constant" instruction instead of a field lookup.
Note:
This optimization applies only to primitive types and String
constants, not arbitrary reference types. Still, it's good practice to
declare constants static final whenever possible.
Hope this will clear.
More details at UseFinal
When it comes to variable
final Static String - constant and it is class variable
static String - class variable and not a constant
here's more
static final String s1;
static String s2;
line 1 does not compile, line 2 compiles
and there's more for primitive types.
final int x = 1;
void x() {
int y = x + 1;
}
bytecode
ICONST_2
ISTORE 1
it means int y = 2 because x is a constant (though it's not static)
if we change
int x = 1;
bytecode
ALOAD 0
GETFIELD Test1.x : I
ICONST_1
IADD
ISTORE 1
JVM reads x and computes y
According to wiki
If the variable is a reference, this means that the variable cannot be re-bound to reference another object. But the object that it references is still mutable, if it was originally mutable.
However since String is immutable to begin with, the difference is irrelevant in this case, and it is just a variable that cannot be modified in anyway.
You said it yourself: the final one cannot be assigned. As far as I know, the final modifier provides o other behavior other than that one. The only thing I can think of is that as Test is public, static, and final, the compiler will automatically replace all occurrences of Test in your code and replace it with the String Literal "ABC", although as the String will be interned it will not have any effect on the code. Basically, final prevents the String from being assigned to a different reference, that is all.
A final class can not be subclassed. It's like the sealed class in C#.
A final method can not be overridden.
A final field can only be initialized/assigned once, like a constant, but may not be necessarily known at compile time.
A static member is shared over all instances of a class.
So, static final is usually for something you never change again, but not necessarily hard coded at compile time.
The compiled java class results in faster performance when Declaring a java variable as static final.
Check This
Final variables are instance variables, whose values cannot be changed after it is initialized.
But Static variables belongs to a class. This 'll be shared among all instances. So when you change the value of a static variable in an instance it gets reflected in all the instances.
You can't change final's reference as below
public static final String Test1 = "ABC";
public static String Test2 = "ABC";
public static void testFinal()
{
Test2 = Test1; //ok
Test1 = Test2; //fail
}
Here you go:
1) final static String: When you say final it's a constant and you can not alter it's value throughout the program. It's not a variable, you can not change it's value. In Java we use final keyword to declare constants. We are supposed to follow all uppercase with underscores to separate the words while declaring constants. For e.g.: MAX_INPUT_TIME, CONNECTION_POOL_SIZE, MAX_USER etc.
2) static String: You can use static for member variables of a class and not for local variables. Static variables are static and are accessible even w/o creating a instance of the class. So you can use them either by object.<static_variable_name> or ClassName.<static_variable_name>. They are accessible and hold the same value for all instances of the class.
For e.g.:
Class A {
static int counter; // it will be default to 0
}
Now while using this class:
System.out.println(A.counter); // output: 0
A.counter++;
System.out.println(A.counter); // output: 1
// Declare an instance of class A
A o1 = new A();
o1.counter++;
System.out.println(o1.counter); // output: 1
// Declare another instance of class A
A o2 = new A();
System.out.println(o1.counter); // output: 1
# You still get it one and not 0, if it was non-static you would get it as 0;
Here, for example you can use static variables for the purpose of keeping track on number of objects created by putting a counter increment in constructor itself.
A string can be declared both static and final. The advantages of declaring string with the both access specifiers are narrated hereunder.
The advantages are inclusive of both the affects of final and static.
A string declared final cannot be reassigned. The string works as a constant.
A string declared static can be called without the help of an object or with class name..
A static variable does not maintain encapsulation. Declaring a static variable as final, no object can change the value but can access it.
public class Demo {
static final String str = "Hello";
public static void main(String args[]) {
// str = "world"; // gives error
System.out.println(str); // called without the help of an object
System.out.println(Demo.str);// called with class name
}
}
The first statement in the main() method gives error as string str is declared as final.
The following statement raises compilation error as string str is declared as final.
Demo d1 = new Demo();
d1.str = "World";
Realtime example
static final String truth = "Sun rises in the east";
I think there is no other difference between them in case of strings.
In case of multithreading, what is suggested is we should try to make objects immutable so that there's no risk of un-desirable state changes in the objects' states (due to improper synchronization or to avoid synchronization overhead).
final keyword (at various place like class, fields, methods) helps in achiving immutability if used properly but Strings are by default immutable.
In case of inheritence, final methods can not be overridden, but again this is not the case here.
final Static String - constant and it is class variableit can be accessed but can't changed from any class
static String - class variable and not a constant, it can be accessed and changed from any class
I think the first statement is often used to define a String constant. And the constant variable name is recommended to be "TEST" instead of "Test".
public static final String TEST = "ABC";
The second statement is seldom used for a good reason. It can cause concurrency issue in multi-thread context. We can have private instance String field in our class:
private String test = "ABC";
Hope this helps!
I'm always confused between static and final keywords in java.
How are they different ?
The static keyword can be used in 4 scenarios
static variables
static methods
static blocks of code
static nested class
Let's look at static variables and static methods first.
Static variable
It is a variable which belongs to the class and not to object (instance).
Static variables are initialized only once, at the start of the execution. These variables will be initialized first, before the initialization of any instance variables.
A single copy to be shared by all instances of the class.
A static variable can be accessed directly by the class name and doesn’t need any object.
Syntax: Class.variable
Static method
It is a method which belongs to the class and not to the object (instance).
A static method can access only static data. It can not access non-static data (instance variables) unless it has/creates an instance of the class.
A static method can call only other static methods and can not call a non-static method from it unless it has/creates an instance of the class.
A static method can be accessed directly by the class name and doesn’t need any object.
Syntax: Class.methodName()
A static method cannot refer to this or super keywords in anyway.
Static class
Java also has "static nested classes". A static nested class is just one which doesn't implicitly have a reference to an instance of the outer class.
Static nested classes can have instance methods and static methods.
There's no such thing as a top-level static class in Java.
Side note:
main method is static since it must be be accessible for an application to run before any instantiation takes place.
final keyword is used in several different contexts to define an entity which cannot later be changed.
A final class cannot be subclassed. This is done for reasons of security and efficiency. Accordingly, many of the Java standard library classes are final, for example java.lang.System and java.lang.String. All methods in a final class are implicitly final.
A final method can't be overridden by subclasses. This is used to prevent unexpected behavior from a subclass altering a method that may be crucial to the function or consistency of the class.
A final variable can only be initialized once, either via an initializer or an assignment statement. It does not need to be initialized at the point of declaration: this is called a blank final variable. A blank final instance variable of a class must be definitely assigned at the end of every constructor of the class in which it is declared; similarly, a blank final static variable must be definitely assigned in a static initializer of the class in which it is declared; otherwise, a compile-time error occurs in both cases.
Note: If the variable is a reference, this means that the variable cannot be re-bound to reference another object. But the object that it references is still mutable, if it was originally mutable.
When an anonymous inner class is defined within the body of a method, all variables declared final in the scope of that method are accessible from within the inner class. Once it has been assigned, the value of the final variable cannot change.
static means it belongs to the class not an instance, this means that there is only one copy of that variable/method shared between all instances of a particular Class.
public class MyClass {
public static int myVariable = 0;
}
//Now in some other code creating two instances of MyClass
//and altering the variable will affect all instances
MyClass instance1 = new MyClass();
MyClass instance2 = new MyClass();
MyClass.myVariable = 5; //This change is reflected in both instances
final is entirely unrelated, it is a way of defining a once only initialization. You can either initialize when defining the variable or within the constructor, nowhere else.
note A note on final methods and final classes, this is a way of explicitly stating that the method or class can not be overridden / extended respectively.
Extra Reading
So on the topic of static, we were talking about the other uses it may have, it is sometimes used in static blocks. When using static variables it is sometimes necessary to set these variables up before using the class, but unfortunately you do not get a constructor. This is where the static keyword comes in.
public class MyClass {
public static List<String> cars = new ArrayList<String>();
static {
cars.add("Ferrari");
cars.add("Scoda");
}
}
public class TestClass {
public static void main(String args[]) {
System.out.println(MyClass.cars.get(0)); //This will print Ferrari
}
}
You must not get this confused with instance initializer blocks which are called before the constructor per instance.
The two really aren't similar. static fields are fields that do not belong to any particular instance of a class.
class C {
public static int n = 42;
}
Here, the static field n isn't associated with any particular instance of C but with the entire class in general (which is why C.n can be used to access it). Can you still use an instance of C to access n? Yes - but it isn't considered particularly good practice.
final on the other hand indicates that a particular variable cannot change after it is initialized.
class C {
public final int n = 42;
}
Here, n cannot be re-assigned because it is final. One other difference is that any variable can be declared final, while not every variable can be declared static.
Also, classes can be declared final which indicates that they cannot be extended:
final class C {}
class B extends C {} // error!
Similarly, methods can be declared final to indicate that they cannot be overriden by an extending class:
class C {
public final void foo() {}
}
class B extends C {
public void foo() {} // error!
}
static means there is only one copy of the variable in memory shared by all instances of the class.
The final keyword just means the value can't be changed. Without final, any object can change the value of the variable.
final -
1)When we apply "final" keyword to a variable,the value of that variable remains constant.
(or)
Once we declare a variable as final.the value of that variable cannot be changed.
2)It is useful when a variable value does not change during the life time of a program
static -
1)when we apply "static" keyword to a variable ,it means it belongs to class.
2)When we apply "static" keyword to a method,it means the method can be accessed without creating any instance of the class
Think of an object like a Speaker. If Speaker is a class, It will have different variables such as volume, treble, bass, color etc. You define all these fields while defining the Speaker class. For example, you declared the color field with a static modifier, that means you're telling the compiler that there is exactly one copy of this variable in existence, regardless of how many times the class has been instantiated.
Declaring
static final String color = "Black";
will make sure that whenever this class is instantiated, the value of color field will be "Black" unless it is not changed.
public class Speaker {
static String color = "Black";
}
public class Sample {
public static void main(String args[]) {
System.out.println(Speaker.color); //will provide output as "Black"
Speaker.color = "white";
System.out.println(Speaker.color); //will provide output as "White"
}}
Note : Now once you change the color of the speaker as final this code wont execute, because final keyword makes sure that the value of the field never changes.
public class Speaker {
static final String color = "Black";
}
public class Sample {
public static void main(String args[]) {
System.out.println(Speaker.color); //should provide output as "Black"
Speaker.color = "white"; //Error because the value of color is fixed.
System.out.println(Speaker.color); //Code won't execute.
}}
You may copy/paste this code directly into your emulator and try.
Easy Difference,
Final : means that the Value of the variable is Final and it will not change anywhere. If you say that final x = 5 it means x can not be changed its value is final for everyone.
Static : means that it has only one object. lets suppose you have x = 5, in memory there is x = 5 and its present inside a class. if you create an object or instance of the class which means there a specific box that represents that class and its variables and methods. and if you create an other object or instance of that class it means there are two boxes of that same class which has different x inside them in the memory. and if you call both x in different positions and change their value then their value will be different. box 1 has x which has x =5 and box 2 has x = 6. but if you make the x static it means it can not be created again.
you can create object of class but that object will not have different x in them.
if x is static then box 1 and box 2 both will have the same x which has the value of 5. Yes i can change the value of static any where as its not final. so if i say box 1 has x and i change its value to x =5 and after that i make another box which is box2 and i change the value of box2 x to x=6. then as X is static both boxes has the same x. and both boxes will give the value of box as 6 because box2 overwrites the value of 5 to 6.
Both final and static are totally different. Final which is final can not be changed. static which will remain as one but can be changed.
"This is an example. remember static variable are always called by their class name. because they are only one for all of the objects of that class. so
Class A has x =5, i can call and change it by A.x=6; "
Static and final have some big differences:
Static variables or classes will always be available from (pretty much) anywhere. Final is just a keyword that means a variable cannot be changed. So if had:
public class Test{
public final int first = 10;
public static int second = 20;
public Test(){
second = second + 1
first = first + 1;
}
}
The program would run until it tried to change the "first" integer, which would cause an error. Outside of this class, you would only have access to the "first" variable if you had instantiated the class. This is in contrast to "second", which is available all the time.
Static is something that any object in a class can call, that inherently belongs to an object type.
A variable can be final for an entire class, and that simply means it cannot be changed anymore. It can only be set once, and trying to set it again will result in an error being thrown. It is useful for a number of reasons, perhaps you want to declare a constant, that can't be changed.
Some example code:
class someClass
{
public static int count=0;
public final String mName;
someClass(String name)
{
mname=name;
count=count+1;
}
public static void main(String args[])
{
someClass obj1=new someClass("obj1");
System.out.println("count="+count+" name="+obj1.mName);
someClass obj2=new someClass("obj2");
System.out.println("count="+count+" name="+obj2.mName);
}
}
Wikipedia contains the complete list of java keywords.
I won't try to give a complete answer here. My recommendation would be to focus on understanding what each one of them does and then it should be cleare to see that their effects are completely different and why sometimes they are used together.
static is for members of a class (attributes and methods) and it has to be understood in contrast to instance (non static) members. I'd recommend reading "Understanding Instance and Class Members" in The Java Tutorials. I can also be used in static blocks but I would not worry about it for a start.
final has different meanings according if its applied to variables, methods, classes or some other cases. Here I like Wikipedia explanations better.
Static variable values can get changed although one copy of the variable traverse through the application, whereas Final Variable values can be initialized once and cannot be changed throughout the application.
How and where should we use a Static modifier for:
1. Field and
2. Method?
For example in java.lang.Math class, the fields methods like abs(), atan(), cos() etc are static, i.e. they can be accessed as: Math.abs()
But why is it a good practice?
Say, I don't keep it static and create an object of the class and access it, which anyways I can, I will just get a warning that, you are trying to access a static method in a non static way (as pointed out by #duffymo, not in case of Math class).
UPDATE 1:
So, utility method, should be static, i.e. whose work is only dependent on the method parameters. So, for example, can the method updateString(String inputQuery, String highlightDoc) should have been a static method in this question?
You can think of a 'static' method or field as if it were declared outside the class definition. In other words
There is only one 'copy' of a static field/method.
Static fields/methods cannot access non-static fields/methods.
There are several instances where you would want to make something static.
The canonical example for a field is to make a static integer field which keeps a count across all instances (objects) of a class. Additionally, singleton objects, for example, also typically employ the static modifier.
Similarly, static methods can be used to perform 'utility' jobs for which all the required dependencies are passed in as parameters to the method - you cannot reference the 'this' keyword inside of a static method.
In C#, you can also have static classes which, as you might guess, contain only static members:
public static class MyContainer
{
private static int _myStatic;
public static void PrintMe(string someString)
{
Console.Out.WriteLine(someString);
_myStatic++;
}
public static int PrintedInstances()
{
return _myStatic;
}
}
Static uses less memory since it exists only once per Classloader.
To have methods static may save some time, beacuse you do not have to create an object first so you can call a function. You can/should use static methods when they stand pretty much on their own (ie. Math.abs(X) - there really is no object the function needs.) Basically its a convenience thing (at least as far as I see it - others might and will disagree :P)
Static fields should really be used with caution. There are quite a few patterns that need static fields... but the basics first:
a static field exists only once. So if you have a simple class (kinda pseudocode):
class Simple {
static int a;
int b;
}
and now you make objects with:
Simple myA = new Simple();
Simple myB = new Simple();
myA.a = 1;
myA.b = 2;
myB.a = 3;
myB.b = 4;
System.out.println(myA.a + myA.b + myB.a + myB.b);
you will get 3234 - because by setting myB.a you actually overwrite myA.a as well because a is static. It exists in one place in memory.
You normally want to avoid this because really weird things might happen. But if you google for example for Factory Pattern you will see that there are actually quite useful uses for this behaviour.
Hope that clears it up a little.
Try taking a look at this post, it also gives some examples of when to and when not to use static and final modifiers.
Most of the posts above are similar, but this post might offer some other insight. When to use Static Modifiers
Usually when the method only depends on the function parameters and not on the internal state of the object it's a static method (with singletone being the only exception). I can't imagine where static fields are really used (they're the same as global variables which should be avoided).
Like in your example the math functions only depend on the parameters.
For a field you should keep it static if you want all instances of a given class to have access to its value. For example if I have
public static int age = 25;
Then any instance of the class can get or set the value of age with all pointing to the same value. If you do make something static you run the risk of having two instances overwriting each others values and possibly causing problems.
The reason to create static methods is mostly for utility function where all the required data for the method is passed in and you do not want to take the over head of creating an instance of the class each time you want to call the method.
You can't instantiate an instance of java.lang.Math; there isn't a public constructor.
Try it:
public class MathTest
{
public static void main(String [] args)
{
Math math = new Math();
System.out.println("math.sqrt(2) = " + math.sqrt(2));
}
}
Here's what you'll get:
C:\Documents and Settings\Michael\My Documents\MathTest.java:5: Math() has private access in java.lang.Math
Math math = new Math();
^
1 error
Tool completed with exit code 1
class StaticModifier
{
{
System.out.println("Within init block");//third
}
public StaticModifier()
{
System.out.println("Within Constructor");//fourth
}
public static void main(String arr[])
{
System.out.println("Within Main:");//second
//StaticModifier obj=new StaticModifier();
}
static
{
System.out.print("Within static block");//first
}
}