I have the following code:
public class BookLib {
void f() {
final int x = 5; // Line 1
class MyCLass {
void print() {
System.out.println(x);
}
}
}
}
I don't understand why should use final variable in this case (Line 1)?
You've created an inner class here. Since the life-time of objects of this class can potentially be much greater than the runtime of the method invocation (i.e. the object can still exist long after the method has returned), it needs to "preserve" the state of local variables that it can access.
This preserving is done by creating an (invisible, synthetic) copy inside the inner class and automatically replacing all references to the local variable with references to that copy. This could lead to strange effects when the local variable were modified after the inner class object was created.
To avoid this, there is a requirement that all local variables that you access this way are final: this ensures that there is only ever one possible value for the local variable and no inconsistencies are observed.
This specific rule can be found in §8.1.3 Inner Classes and Enclosing Instances of the JLS:
Any local variable, formal method parameter or exception handler parameter used but not declared in an inner class must be declared final. Any local variable, used but not declared in an inner class must be definitely assigned (§16) before the body of the inner class.
Here you create an inner class. In order to access the context of execution, the references to the variables should stay intact, otherwise an error behaviour is observed. In order to guarantee this, you should declare your variable final: thus one can't change it.
See detailed explanation here.
From kathy sierra scjp Book
The local variables of the method live on the stack, and exist only for the lifetime of the method. We already know that the scope of a local variable is limited to the method the variable is declared in. When the method ends, the stack frame is blown away and the variable is history. But even after the method completes, the inner class object created within it might still be alive on the heap if, for example, a reference to it was passed into some other code and then stored in an instance variable. Because the local variables aren't guaranteed to be alive as long as the method-local inner class object, the inner class object can't use them. Unless the local variables are marked final!
So you have a local final variable and method-local inner class. A method-local inner class cannot use non-final variables declared within method (including parameters).
To go more deeply into this:
Method-local inner class can be instantiated only within method where the class is defined. The local variables live on stack for the lifetime of the method. As the method ends, the local variables disappear, however the inner class object created still lives.
It's a matter of lifetime/scope.
Related
I know that method variables are stored on stack of the memory but slightly confused over final. I had browsed many links like this could not get proper understanding? below is the example of inner class in which final variables are accessed and local non-final variables are not as they are stored in stack
class Employee {
public void getAddress(){
final int location = 13;
int notFinalVar = 13;
class Address {
System.out.println (location);
System.out.println (notFinalVar); // compiler error
}
}
Update: Just now came to know about hidden fields called synthetic field( inner class heap memory area) in which copy of final variables are stored so does it finally means that final variables are stored in finally Stack memory Area?
Reading through some answers of SO and articles, My understanding is :
The answer is stack. All local variable (final or not) stored into the stack and go out of scope when the method execution is over.
But about final variable JVM take these as a constant as they will not change after initiated . And when a inner class try to access them compiler create a copy of that variable (not that variable it self) into the heap and create a synthetic field inside the inner class so even when the method execution is over
it is accessible because the inner class has it own copy.
so does it finally means that final variables are stored in finally Stack memory Area?
final variable also stored in stack but the copy that variable which a inner class have stored in heap.
synthetic field are filed which actually doesn't exist in the source code but compiler create those fields in some inner classes to make those field accessible. In simple word hidden field.
References :
How does marking a variable as final allow inner classes to access them?
Cannot refer to a non-final variable inside an inner class defined in a different method
Mystery of Accessibility in Local Inner Classes
According to the doc Local variables in java are declared in methods, constructors, or blocks.
In the below Class A isn't x a local variable too since it is in blocks({}) i know they are called as instance variable but i am confused? If yes Access modifiers cannot be used for local variables but i am sure i can add public private protected ? It also says that local variable are stored in stack but as per the below code x will be stored in heap right since they are part of the instance?
class A{
private int x = 5; // Isn't this a local varibale too since it is in blocks
}
.
class A{
public void function(){
int x = 5; // this is a local variable since it is declared in a function
private int x2=5; // Error Access modifiers cannot be used for local variables
}
}
In the below Class A isn't x a local variable too since it is in blocks({})
No. It's not in a block. It's in a class declaration, but that's not a block as such. "Block" isn't synonymous with "text in braces".
To be a bit clearer, local variables are declared in:
Methods
Constructors
Static initializers
Instance initializers
If you look at the production for a class declaration, that's not a Block (unlike the production for static initializers and instance initializers).
In the below Class A isn't x a local variable too since it is in
blocks({})
Your x is not a local variable, it's an instance variable. Block means, instance initialization block or static block or try catch block.
Access modifiers cannot be used for local variables but i am sure i
can add public private protected ?
It doesn't make sense to have access modifiers to method local variables, since, method local variable can only be accessed inside the method.
I'm not entirely sure where you are reading. Local variables are values declared inside a method.
There is no special keyword designating a variable as local; that determination comes entirely from the location in which the variable is declared — which is between the opening and closing braces of a method.
Taken from here.
A local variable is a variable that has a local scope and is generally not expected to last a long time.
If you declare a variable inside a method, that variable will only be relevant while that method is being executed, and after the method finishes the variable will be discarded.
An instance variable is a variable that is bound to an instance of "something" and is generally considered to last as long as that "something" lasts.
If you declare a variable inside a class, then when you create a new instance of the class the variable will also be created and pinned onto the class. The variable will be there as long as the instance object of the class that you created exists, if you decide to discard the instance of the class you made, then the instance variable gets discarded as well.
Hope this was intuitive.
The Java documentation for Local Classes says that:
In addition, a local class has access to local variables. However, a
local class can only access local variables that are declared final.
When a local class accesses a local variable or parameter of the
enclosing block, it captures that variable or parameter. For example,
the PhoneNumber constructor can access the local variable numberLength
because it is declared final; numberLength is a captured variable.
What is captured variable,what is its use and why is that needed? Please help me in understanding the concept of it.
What is captured variable,what is its use and why is that needed?
A captured variable is one that has been copied so it can be used in a nested class. The reason it has to be copied is the object may out live the current context. It has to be final (or effectively final in Java 8) so there is no confusion about whether changes to the variable will be seen (because they won't)
Note: Groovy does have this rule and a change to the local variable can mean a change to the value in the enclosing class which is especially confusing if multiple threads are involved.
An example of capture variable.
public void writeToDataBase(final Object toWrite) {
executor.submit(new Runnable() {
public void run() {
writeToDBNow(toWrite);
}
});
// if toWrite were mutable and you changed it now, what would happen !?
}
// after the method returns toWrite no longer exists for the this thread...
Here is a post describing it: http://www.devcodenote.com/2015/04/variable-capture-in-java.html
Here is a snippet from the post:
”It is imposed as a mandate by Java that if an inner class defined within a method references a local variable of that method, that local variable should be defined as final.”
This is because the function may complete execution and get removed from the process stack, with all the variables destroyed but it may be the case that objects of the inner class are still on the heap referencing a particular local variable of that function. To counter this, Java makes a copy of the local variable and gives that as a reference to the inner class. To maintain consistency between the 2 copies, the local variable is mandated to be “final” and non-modifiable.
A captured variable is one from the outside of your local class - one declared in the surrounding block. In some languages this is called a closure.
In the example from the Oracle Docs (simplified) the variable numberLength, declared outside of class PhoneNumber, is "captured".
final int numberLength = 10; // in JDK7 and earlier must be final...
class PhoneNumber {
// you can refer to numberLength here... it has been "captured"
}
This question already has answers here:
Why are only final variables accessible in anonymous class?
(15 answers)
Closed 9 years ago.
If I had this anonymous method I should declare x variable as final.
private void testMethod (ListField<BeanModel> listField){
final ListLoader<BeanModel> loader = new PagedListLoader<BeanModel>();
listField.addListener(Events.Attach, new Listener<ListViewEvent<BeanModel>>() {
#Override
public void handleEvent(ListViewEvent<BeanModel> be) {
loader.load();
}
});
}
However, if loader was a class field, it wouldn't be necessary to declare it as final:
public class testClass{
private ListLoader<BeanModel> loader = new PagedListLoader<BeanModel>();
private void testMethod (ListField<BeanModel> listField){
listField.addListener(Events.Attach, new Listener<ListViewEvent<BeanModel>>() {
#Override
public void handleEvent(ListViewEvent<BeanModel> be) {
loader.load();
}
});
//Could I modify loader's reference here, before the method executes?
//loader = null;
}
}
Does anyone know the reason why they guarantee local variables not to change when they're accessed but don't do it for class fields?
Accroding to java docs
An anonymous class has access to the members of its enclosing class.
An anonymous class cannot access local variables in its enclosing scope that are not declared as final or effectively final (Effectively final means that the variable is never changed after it is initialized. Method parameters are often effectually final.)
The reason for this restriction becomes apparent if we shed some light on how local classes are implemented. An anonymous local class can use local variables because the compiler automatically gives the class a private instance field to hold a copy of each local variable the class uses. The compiler also adds hidden parameters to each constructor to initialize these automatically created private fields. Thus, a local class does not actually access local variables, but merely its own private copies of them. The only way this can work correctly is if the local variables are declared final, so that they are guaranteed not to change. With this guarantee in place, the local class is assured that its internal copies of the variables accurately reflect the actual local variables.
The local variable is allocated in the stack, and it will fall out of scope after testMethod(). Making the variable final ensures that it is ok to just pass a reference to it to the anonymous class. If it was not final, a later assignment to it in testMethod() could change the value later with confusing results. (The user might expect the later assigned value used, but that would be impossible).
A field of the parent class, however can be accessed through the parent reference of the anonymous class, so any later assignments can be handled without confusion.
Anonymous classes get local variables implicitly through constructors. That is they get copies of local vars they use. So if we changed variable value in the main code the anonymous class would not see this change. Declaring local vars final helps avoid this ambiguity.
Have a look at Lambdas and Conjures in java.
An anonymous inner class has no information around it - you have to specify that it is final so you can guarantee its existence.
This could be something to do with the nature of a ListLoader but I am unexperienced with using this library.
I hope I pointed you into the right direction.
I was watching a tutorial on youtube and the topic was private variables. We usually set variables in java like this:
class hello {
public static void main(String args[]) {
String x;
x = "Hello"
}
}
but in that tutorial, the string type was declared out of the method like this:
class hello {
private String x;
public void apples() {
x = "this is a private variable.";
System.out.println(x);
}
}
As you can see it was not the main method, but i want to ask that do private variables always have to be out of method or what?
I am a beginner so this will be really helpful to know as i don't want to cram up knowledge to prevent confusion and also because it is a matter of fact that people who cram up code never become a good programmer.
do private variables always have to be out of method or what?
That's right. A variable inside a method is a local variable and can not have any access modifiers such as private, public or protected. These modifiers can only be applied to member variables, i.e. variables that are declared in the class scope.
If you think about it, it makes a lot of sense, since local variables can't be accessed by another class anyway. Not even another object of the same class or another method in the same object.
Related question:
What is the difference between a member variable and a local variable?
Cass variables must be declared to be one of the following types:
Public
Protected
Public
In the first example, the variable is local to the function: that is, it's specifically bound to method hello.main().
In that case, it's only accessible within that method function. It's not a class variable, so it doesn't need its access level to be set.
In the second example, the variable is a class variable. When you have a class variable, you can set it to private (can only be accessed by an object of that class), protected (accessed by inherited classes), or public (can be accessed outside of the object). The many methods possible inside the class can then access that class variable.
When you have a variable set inside a class definition, and not inside a method, it is called a "field" or "property" or "attribute." The way you define the accessibility of the field is required since multiple methods within the class can refer to it.
When you have a variable set within any method, it can only be accessed inside of that method, and can't be accessed outside (unless you use a reference pointer or pass it through method arguments).
A private variable in Java is a variable Globally accessible to the class and only to that class (and of course any other methods in the containing class).
A variable declared within a method can be accessed only within the scope of the method. A variable declared inside an if statement is only accessible inside the if statement .... and so on.
Its best to have as little private variable as possible because of performance issues. Lets say you have 100 private variables declared in a class. Your class contains 10 methods and each method utilizes 10 variables. When instantiating an object then your object is created instantiating olso the 100 private variables. If you make you variables local to your methods, then no variable is created on instantiation of the class, and 10 variables are used each time you access a method....
There are also other types of variables in java, to better comprehend you can start from here http://docs.oracle.com/javase/tutorial/java/nutsandbolts/variables.html
I think what's confusing you is the distinction between a local variable (declared in a method), and a member variable (declared in a class, outside of the class's methods).
A local variable exists just while the method that it's declared in is running. It comes into existence at the point that it's declared, then goes out of scope later, when the next } character occurs (other than those that match { characters that aren't opened yet). This effectively means that the variable disappears in a puff of smoke - it can't be used once it's gone out of scope.
But a member variable lives inside an object. That means it gets created when the object is created, and destroyed when the object is destroyed. So it typically lives for a much longer duration than the local variables do. Member variables can sometimes be used by objects other than the object that they belong to; and there are some quite complex rules around when it's possible to do this.
The private modifier on a member variable just means that it can only be accessed by code that's in the class that the object belongs to.
but i want to ask that do private variables always have to be out of method or what?.
Well, It doesn't make sense actually to make a variable private in a method. Because variables declared in a method are stack variables and they have narrower scope than a private one. As they can be accessed only in the method they are declared in and private variables have scope in the whole class they are declared in.
Any variable created with in a scope ( code block bounded inside { } ) is a local variable of that scope; and not accessible out side the block.
Also private variables is term which comes into picture when you talk about classes and define a member which is not accessible outside class.