We Keep Coding

why should we use final to mark the constructor parameter in enum? [duplicate]

I can't understand where the final keyword is really handy when it is used on method parameters.
If we exclude the usage of anonymous classes, readability and intent declaration then it seems almost worthless to me.
Enforcing that some data remains constant is not as strong as it seems.
If the parameter is a primitive then it will have no effect since the parameter is passed to the method as a value and changing it will have no effect outside the scope.
If we are passing a parameter by reference, then the reference itself is a local variable and if the reference is changed from within the method, that would not have any effect from outside of the method scope.
Consider the simple test example below.
This test passes although the method changed the value of the reference given to it, it has no effect.
public void testNullify() {
Collection<Integer> c = new ArrayList<Integer>();
nullify(c);
assertNotNull(c);
final Collection<Integer> c1 = c;
assertTrue(c1.equals(c));
change(c);
assertTrue(c1.equals(c));
}
private void change(Collection<Integer> c) {
c = new ArrayList<Integer>();
}
public void nullify(Collection<?> t) {
t = null;
}

Stop a Variable’s Reassignment
While these answers are intellectually interesting, I've not read the short simple answer:
Use the keyword final when you want the compiler to prevent a
variable from being re-assigned to a different object.
Whether the variable is a static variable, member variable, local variable, or argument/parameter variable, the effect is entirely the same.
Example
Let’s see the effect in action.
Consider this simple method, where the two variables (arg and x) can both be re-assigned different objects.
// Example use of this method:
// this.doSomething( "tiger" );
void doSomething( String arg ) {
String x = arg; // Both variables now point to the same String object.
x = "elephant"; // This variable now points to a different String object.
arg = "giraffe"; // Ditto. Now neither variable points to the original passed String.
}
Mark the local variable as final. This results in a compiler error.
void doSomething( String arg ) {
final String x = arg; // Mark variable as 'final'.
x = "elephant"; // Compiler error: The final local variable x cannot be assigned.
arg = "giraffe";
}
Instead, let’s mark the parameter variable as final. This too results in a compiler error.
void doSomething( final String arg ) { // Mark argument as 'final'.
String x = arg;
x = "elephant";
arg = "giraffe"; // Compiler error: The passed argument variable arg cannot be re-assigned to another object.
}
Moral of the story:
If you want to ensure a variable always points to the same object,
mark the variable final.
Never Reassign Arguments
As good programming practice (in any language), you should never re-assign a parameter/argument variable to an object other than the object passed by the calling method. In the examples above, one should never write the line arg = . Since humans make mistakes, and programmers are human, let’s ask the compiler to assist us. Mark every parameter/argument variable as 'final' so that the compiler may find and flag any such re-assignments.
In Retrospect
As noted in other answers…
Given Java's original design goal of helping programmers to avoid dumb mistakes such as reading past the end of an array, Java should have been designed to automatically enforce all parameter/argument variables as 'final'. In other words, Arguments should not be variables. But hindsight is 20/20 vision, and the Java designers had their hands full at the time.
So, always add final to all arguments?
Should we add final to each and every method parameter being declared?
In theory, yes.
In practice, no.➥ Add final only when the method’s code is long or complicated, where the argument may be mistaken for a local or member variable and possibly re-assigned.
If you buy into the practice of never re-assigning an argument, you will be inclined to add a final to each. But this is tedious and makes the declaration a bit harder to read.
For short simple code where the argument is obviously an argument, and not a local variable nor a member variable, I do not bother adding the final. If the code is quite obvious, with no chance of me nor any other programmer doing maintenance or refactoring accidentally mistaking the argument variable as something other than an argument, then don’t bother. In my own work, I add final only in longer or more involved code where an argument might mistaken for a local or member variable.
#Another case added for the completeness
public class MyClass {
private int x;
//getters and setters
}
void doSomething( final MyClass arg ) { // Mark argument as 'final'.
arg = new MyClass(); // Compiler error: The passed argument variable arg cannot be re-assigned to another object.
arg.setX(20); // allowed
// We can re-assign properties of argument which is marked as final
}
record
Java 16 brings the new records feature. A record is a very brief way to define a class whose central purpose is to merely carry data, immutably and transparently.
You simply declare the class name along with the names and types of its member fields. The compiler implicitly provides the constructor, getters, equals & hashCode, and toString.
The fields are read-only, with no setters. So a record is one case where there is no need to mark the arguments final. They are already effectively final. Indeed, the compiler forbids using final when declaring the fields of a record.
public record Employee( String name , LocalDate whenHired ) // 🡄 Marking `final` here is *not* allowed.
{
}
If you provide an optional constructor, there you can mark final.
public record Employee(String name , LocalDate whenHired) // 🡄 Marking `final` here is *not* allowed.
{
public Employee ( final String name , final LocalDate whenHired ) // 🡄 Marking `final` here *is* allowed.
{
this.name = name;
whenHired = LocalDate.MIN; // 🡄 Compiler error, because of `final`.
this.whenHired = whenHired;
}
}

Sometimes it's nice to be explicit (for readability) that the variable doesn't change. Here's a simple example where using final can save some possible headaches:
public void setTest(String test) {
test = test;
}
If you forget the 'this' keyword on a setter, then the variable you want to set doesn't get set. However, if you used the final keyword on the parameter, then the bug would be caught at compile time.

Yes, excluding anonymous classes, readability and intent declaration it's almost worthless. Are those three things worthless though?
Personally I tend not to use final for local variables and parameters unless I'm using the variable in an anonymous inner class, but I can certainly see the point of those who want to make it clear that the parameter value itself won't change (even if the object it refers to changes its contents). For those who find that adds to readability, I think it's an entirely reasonable thing to do.
Your point would be more important if anyone were actually claiming that it did keep data constant in a way that it doesn't - but I can't remember seeing any such claims. Are you suggesting there's a significant body of developers suggesting that final has more effect than it really does?
EDIT: I should really have summed all of this up with a Monty Python reference; the question seems somewhat similar to asking "What have the Romans ever done for us?"

Let me explain a bit about the one case where you have to use final, which Jon already mentioned:
If you create an anonymous inner class in your method and use a local variable (such as a method parameter) inside that class, then the compiler forces you to make the parameter final:
public Iterator<Integer> createIntegerIterator(final int from, final int to)
{
return new Iterator<Integer>(){
int index = from;
public Integer next()
{
return index++;
}
public boolean hasNext()
{
return index <= to;
}
// remove method omitted
};
}
Here the from and to parameters need to be final so they can be used inside the anonymous class.
The reason for that requirement is this: Local variables live on the stack, therefore they exist only while the method is executed. However, the anonymous class instance is returned from the method, so it may live for much longer. You can't preserve the stack, because it is needed for subsequent method calls.
So what Java does instead is to put copies of those local variables as hidden instance variables into the anonymous class (you can see them if you examine the byte code). But if they were not final, one might expect the anonymous class and the method seeing changes the other one makes to the variable. In order to maintain the illusion that there is only one variable rather than two copies, it has to be final.

I use final all the time on parameters.
Does it add that much? Not really.
Would I turn it off? No.
The reason: I found 3 bugs where people had written sloppy code and failed to set a member variable in accessors. All bugs proved difficult to find.
I'd like to see this made the default in a future version of Java. The pass by value/reference thing trips up an awful lot of junior programmers.
One more thing.. my methods tend to have a low number of parameters so the extra text on a method declaration isn't an issue.

Using final in a method parameter has nothing to do with what happens to the argument on the caller side. It is only meant to mark it as not changing inside that method. As I try to adopt a more functional programming style, I kind of see the value in that.

Personally I don't use final on method parameters, because it adds too much clutter to parameter lists.
I prefer to enforce that method parameters are not changed through something like Checkstyle.
For local variables I use final whenever possible, I even let Eclipse do that automatically in my setup for personal projects.
I would certainly like something stronger like C/C++ const.

Since Java passes copies of arguments I feel the relevance of final is rather limited. I guess the habit comes from the C++ era where you could prohibit reference content from being changed by doing a const char const *. I feel this kind of stuff makes you believe the developer is inherently stupid as f*** and needs to be protected against truly every character he types. In all humbleness may I say, I write very few bugs even though I omit final (unless I don't want someone to override my methods and classes). Maybe I'm just an old-school dev.

Short answer: final helps a tiny bit but... use defensive programming on the client side instead.
Indeed, the problem with final is that it only enforces the reference is unchanged, gleefully allowing the referenced object members to be mutated, unbeknownst to the caller. Hence the best practice in this regard is defensive programming on the caller side, creating deeply immutable instances or deep copies of objects that are in danger of being mugged by unscrupulous APIs.

I never use final in a parameter list, it just adds clutter like previous respondents have said. Also in Eclipse you can set parameter assignment to generate an error so using final in a parameter list seems pretty redundant to me.
Interestingly when I enabled the Eclipse setting for parameter assignment generating an error on it caught this code (this is just how I remember the flow, not the actual code. ) :-
private String getString(String A, int i, String B, String C)
{
if (i > 0)
A += B;
if (i > 100)
A += C;
return A;
}
Playing devil's advocate, what exactly is wrong with doing this?

One additional reason to add final to parameter declarations is that it helps to identify variables that need to be renamed as part of a "Extract Method" refactoring. I have found that adding final to each parameter prior to starting a large method refactoring quickly tells me if there are any issues I need to address before continuing.
However, I generally remove them as superfluous at the end of the refactoring.

Follow up by Michel's post. I made myself another example to explain it. I hope it could help.
public static void main(String[] args){
MyParam myParam = thisIsWhy(new MyObj());
myParam.setArgNewName();
System.out.println(myParam.showObjName());
}
public static MyParam thisIsWhy(final MyObj obj){
MyParam myParam = new MyParam() {
#Override
public void setArgNewName() {
obj.name = "afterSet";
}
#Override
public String showObjName(){
return obj.name;
}
};
return myParam;
}
public static class MyObj{
String name = "beforeSet";
public MyObj() {
}
}
public abstract static class MyParam{
public abstract void setArgNewName();
public abstract String showObjName();
}
From the code above, in the method thisIsWhy(), we actually didn't assign the [argument MyObj obj] to a real reference in MyParam. In instead, we just use the [argument MyObj obj] in the method inside MyParam.
But after we finish the method thisIsWhy(), should the argument(object) MyObj still exist?
Seems like it should, because we can see in main we still call the method showObjName() and it needs to reach obj. MyParam will still use/reaches the method argument even the method already returned!
How Java really achieve this is to generate a copy also is a hidden reference of the argument MyObj obj inside the MyParam object ( but it's not a formal field in MyParam so that we can't see it )
As we call "showObjName", it will use that reference to get the corresponding value.
But if we didn't put the argument final, which leads a situation we can reassign a new memory(object) to the argument MyObj obj.
Technically there's no clash at all! If we are allowed to do that, below will be the situation:
We now have a hidden [MyObj obj] point to a [Memory A in heap] now live in MyParam object.
We also have another [MyObj obj] which is the argument point to a [Memory B in heap] now live in thisIsWhy method.
No clash, but "CONFUSING!!" Because they are all using the same "reference name" which is "obj".
To avoid this, set it as "final" to avoid programmer do the "mistake-prone" code.

Tracing method invocation arguments in bytecode using ASM

How can I inspect the bytecode of a class (using something such as ASM) to learn which initial values were passed through to a method?
For example: Given some methods that pass values to each other:
void m1(Object o) {
Object v = o;
m2(v);
m2("box");
}
void m2(Object o) {
Object v = o;
m3(x);
}
void m3(Object o) {
}
And some method calls, all defined in the same class:
{
Object foo = "foo";
m1(foo);
m2("bar");
m3("baz");
}
How can I inspect the class' bytecode to learn that m3 will be called 4 times with the values "foo", "box", "bar" and "baz"?

Using ASM, you can in theory trace for each method if another method of the same class is invoked from within it. The visitor API's method that is responsible for defining method invocations is visitMethodIns. Assuming that your class was called bar.Foo, you would need to trace:
visitMethodIns(<any>, "bar.Foo", <any>, <any>)
You would then need to build a transitive relation of methods calling each other where the last two parameters allow you to build such a relation hierarchy. Additionally, you would need to trace the arguments of these method invocations, what is more tricky but not impossible either.
The reason it is more complex is the number of possible ways an argument can be loaded onto the operand stack. For your example, you only need to pay attention to the visitIns and the visitLCDIns callbacks.
When calling a method on a constant pool value (LCD), the resolution of the argument is rather trivial. You would however need to trace the entire instruction chain before calling a method to learn of the local variable assignment in order to know that you are calling the method on the method parameter. Thus, you could find out that
ALOAD_0 / ASTORE_1 / ALOAD_1 => ALOAD_0
is an effective result of a sequence of reads/writes form the methods local variable array.
With all this, from parsing the byte code, you would learn about the following call-transitions:
m1(Ljava/lang/Object)V -> m2(Ljava/lang/Object)V [ALOAD 0]
-> m2(Ljava/lang/Object)V [LCD "box"]
m2(Ljava/lang/Object)V -> m3(Ljava/lang/Object)V [ALOAD 0]
You could then use these results to parse your block where you find out about these method calls and their implications. You would however have created a quite fragile solution where indirections such as:
{
Foo foo = this;
foo.m1("bar");
}
would not be discovered. As pointed out in the comments, you basically need to emulate the Java virtual machine in order to "run" your code.
And even if you implement a complex solution to trace all this, you could still not be sure of your result. What happens when I invoke an interface method from within an implementation. Or a method of a subclass? Thanks to the dynamic dispatch of methods, you can never be sure of the target that is called.

Object creation (state initialisation) and thread safety

I am look into the book "Java Concurrency in Practice" and found really hard to believe below quoted statement (But unfortunately it make sense).
http://www.informit.com/store/java-concurrency-in-practice-9780321349606
Just wanted to get clear about this 100%
public class Holder {
private int n;
public Holder(int n) { this.n = n; }
public void assertSanity() {
if (n != n)
throw new AssertionError("This statement is false.");
}
}
While it may seem that field values set in a constructor are the first
values written to those fields and therefore that there are no "older"
values to see as stale values, the Object constructor first
writes the default values to all fields before subclass
constructors run. It is therefore Possible to see the default value
for a field as a stale value
Regarding bolded statement in above,
I am aware that the behaviour BUT now it is clear that this calling hierarchy of constructors is NOT guarantee to be ATOMIC (calling super constructors in single synchronised block that is guarded by a lock), but what would be the solution? imagine a class hierarchy that has more than one level (even it is not recommended, lets assume as it is possible). The above code snippest is a kind of a prototype that we see everyday in most of the projects.

You misread the book. It explicitely says:
The problem here is not the Holder class itself, but that the Holder is not properly published.
So the above construct if fine. What's not fine is to improperly publish such an object to other threads. The book explains that in details.

When creating a new object things happen sequentially. I don't know the precise order, but it's something like: allocate the space and initialize it to zeroes, then set the fields that get constant values, then set the fields that get calculated values, then run the constructor code. And, of course, it's got to initialize the subclasses in there somewhere.
So if you try to work with an object that is still being constructed, you can see odd, invalid values in the fields. This doesn't usually happen, but ways to do it:
Reference a field that doesn't yet have a value during an assignment to another field.
Reference a value in the constructor that doesn't get assigned till later in the constructor.
Reference a field in an object in a field in an object that was just read from an ObjectInputStream. (OIS often takes a long time to put values in objects it's read.)
Before Java 5, something like:
public volatile MyClass myObject;
...
myObject = new MyClass( 10 );
could make trouble because another thread could grab the reference to myObject before the MyClass constructor was finished and it would see bad values (zero instead of 10, in this case) inside the object. With Java 5, the JVM is not allowed to make myObject non-null until the constructor is finished.
And today you can still set myObject to this within the constructor and accomplish the same thing.
If you're clever, you can also get hold of Class fields before they've been initialized.
In your code example, (n != n) would be true if something changed the value between the two reads of n. I guess the point is n starts as zero, get's set to something else by the constructor, and assertSanity is called during the construction. In this case, n is not volatile so I don't think the assert will ever be triggered. Make it volatile and it will happen once every million times or so if you time everything precisely right. In real life this kind of problem happens just often enough to wreak havoc but rarely enough that you can't reproduce it.

I guess theoretically it is possible. It is similar to double checked locking problem.
public class Test {
static Holder holder;
static void test() {
if (holder == null) {
holder = new Holder(1);
}
holder.assertSanity();
}
...
If test() is called by 2 threads, thread-2 might see the holder in a state when initialization is still in progress so n != n may happen to be true. Here is bytecode for n != n:
ALOAD 0
GETFIELD x/Holder.n : I
ALOAD 0
GETFIELD x/Holder.n : I
IF_ICMPEQ L1
as you can see JVM loads field n to operand stack twice. So it may happen that the first var gets value before init and the seccond after init

the comment:
the Object constructor first writes the default values to all fields
before subclass constructors run
seems wrong. My prior experience is that the default values for a class are set before its constructor is run. that is a super class will see its init-ed variables set before its constructor runs and does things. This was root of bug a friend looked at where a base class was calling a method during construction that the super class implemented and set a reference that was defined with initialization to null in the super class. the item would be there until entry to the constructor at which time the init set it to null value.
references to the object are not available to another thread (assuming none generated in the constructor) until it completes construction and object reference is returned.

What's the point of the "final" keyword for method arguments in Java? [duplicate]

I can't understand where the final keyword is really handy when it is used on method parameters.
If we exclude the usage of anonymous classes, readability and intent declaration then it seems almost worthless to me.
Enforcing that some data remains constant is not as strong as it seems.
If the parameter is a primitive then it will have no effect since the parameter is passed to the method as a value and changing it will have no effect outside the scope.
If we are passing a parameter by reference, then the reference itself is a local variable and if the reference is changed from within the method, that would not have any effect from outside of the method scope.
Consider the simple test example below.
This test passes although the method changed the value of the reference given to it, it has no effect.
public void testNullify() {
Collection<Integer> c = new ArrayList<Integer>();
nullify(c);
assertNotNull(c);
final Collection<Integer> c1 = c;
assertTrue(c1.equals(c));
change(c);
assertTrue(c1.equals(c));
}
private void change(Collection<Integer> c) {
c = new ArrayList<Integer>();
}
public void nullify(Collection<?> t) {
t = null;
}

Stop a Variable’s Reassignment
While these answers are intellectually interesting, I've not read the short simple answer:
Use the keyword final when you want the compiler to prevent a
variable from being re-assigned to a different object.
Whether the variable is a static variable, member variable, local variable, or argument/parameter variable, the effect is entirely the same.
Example
Let’s see the effect in action.
Consider this simple method, where the two variables (arg and x) can both be re-assigned different objects.
// Example use of this method:
// this.doSomething( "tiger" );
void doSomething( String arg ) {
String x = arg; // Both variables now point to the same String object.
x = "elephant"; // This variable now points to a different String object.
arg = "giraffe"; // Ditto. Now neither variable points to the original passed String.
}
Mark the local variable as final. This results in a compiler error.
void doSomething( String arg ) {
final String x = arg; // Mark variable as 'final'.
x = "elephant"; // Compiler error: The final local variable x cannot be assigned.
arg = "giraffe";
}
Instead, let’s mark the parameter variable as final. This too results in a compiler error.
void doSomething( final String arg ) { // Mark argument as 'final'.
String x = arg;
x = "elephant";
arg = "giraffe"; // Compiler error: The passed argument variable arg cannot be re-assigned to another object.
}
Moral of the story:
If you want to ensure a variable always points to the same object,
mark the variable final.
Never Reassign Arguments
As good programming practice (in any language), you should never re-assign a parameter/argument variable to an object other than the object passed by the calling method. In the examples above, one should never write the line arg = . Since humans make mistakes, and programmers are human, let’s ask the compiler to assist us. Mark every parameter/argument variable as 'final' so that the compiler may find and flag any such re-assignments.
In Retrospect
As noted in other answers…
Given Java's original design goal of helping programmers to avoid dumb mistakes such as reading past the end of an array, Java should have been designed to automatically enforce all parameter/argument variables as 'final'. In other words, Arguments should not be variables. But hindsight is 20/20 vision, and the Java designers had their hands full at the time.
So, always add final to all arguments?
Should we add final to each and every method parameter being declared?
In theory, yes.
In practice, no.➥ Add final only when the method’s code is long or complicated, where the argument may be mistaken for a local or member variable and possibly re-assigned.
If you buy into the practice of never re-assigning an argument, you will be inclined to add a final to each. But this is tedious and makes the declaration a bit harder to read.
For short simple code where the argument is obviously an argument, and not a local variable nor a member variable, I do not bother adding the final. If the code is quite obvious, with no chance of me nor any other programmer doing maintenance or refactoring accidentally mistaking the argument variable as something other than an argument, then don’t bother. In my own work, I add final only in longer or more involved code where an argument might mistaken for a local or member variable.
#Another case added for the completeness
public class MyClass {
private int x;
//getters and setters
}
void doSomething( final MyClass arg ) { // Mark argument as 'final'.
arg = new MyClass(); // Compiler error: The passed argument variable arg cannot be re-assigned to another object.
arg.setX(20); // allowed
// We can re-assign properties of argument which is marked as final
}
record
Java 16 brings the new records feature. A record is a very brief way to define a class whose central purpose is to merely carry data, immutably and transparently.
You simply declare the class name along with the names and types of its member fields. The compiler implicitly provides the constructor, getters, equals & hashCode, and toString.
The fields are read-only, with no setters. So a record is one case where there is no need to mark the arguments final. They are already effectively final. Indeed, the compiler forbids using final when declaring the fields of a record.
public record Employee( String name , LocalDate whenHired ) // 🡄 Marking `final` here is *not* allowed.
{
}
If you provide an optional constructor, there you can mark final.
public record Employee(String name , LocalDate whenHired) // 🡄 Marking `final` here is *not* allowed.
{
public Employee ( final String name , final LocalDate whenHired ) // 🡄 Marking `final` here *is* allowed.
{
this.name = name;
whenHired = LocalDate.MIN; // 🡄 Compiler error, because of `final`.
this.whenHired = whenHired;
}
}

Sometimes it's nice to be explicit (for readability) that the variable doesn't change. Here's a simple example where using final can save some possible headaches:
public void setTest(String test) {
test = test;
}
If you forget the 'this' keyword on a setter, then the variable you want to set doesn't get set. However, if you used the final keyword on the parameter, then the bug would be caught at compile time.

Yes, excluding anonymous classes, readability and intent declaration it's almost worthless. Are those three things worthless though?
Personally I tend not to use final for local variables and parameters unless I'm using the variable in an anonymous inner class, but I can certainly see the point of those who want to make it clear that the parameter value itself won't change (even if the object it refers to changes its contents). For those who find that adds to readability, I think it's an entirely reasonable thing to do.
Your point would be more important if anyone were actually claiming that it did keep data constant in a way that it doesn't - but I can't remember seeing any such claims. Are you suggesting there's a significant body of developers suggesting that final has more effect than it really does?
EDIT: I should really have summed all of this up with a Monty Python reference; the question seems somewhat similar to asking "What have the Romans ever done for us?"

Let me explain a bit about the one case where you have to use final, which Jon already mentioned:
If you create an anonymous inner class in your method and use a local variable (such as a method parameter) inside that class, then the compiler forces you to make the parameter final:
public Iterator<Integer> createIntegerIterator(final int from, final int to)
{
return new Iterator<Integer>(){
int index = from;
public Integer next()
{
return index++;
}
public boolean hasNext()
{
return index <= to;
}
// remove method omitted
};
}
Here the from and to parameters need to be final so they can be used inside the anonymous class.
The reason for that requirement is this: Local variables live on the stack, therefore they exist only while the method is executed. However, the anonymous class instance is returned from the method, so it may live for much longer. You can't preserve the stack, because it is needed for subsequent method calls.
So what Java does instead is to put copies of those local variables as hidden instance variables into the anonymous class (you can see them if you examine the byte code). But if they were not final, one might expect the anonymous class and the method seeing changes the other one makes to the variable. In order to maintain the illusion that there is only one variable rather than two copies, it has to be final.

I use final all the time on parameters.
Does it add that much? Not really.
Would I turn it off? No.
The reason: I found 3 bugs where people had written sloppy code and failed to set a member variable in accessors. All bugs proved difficult to find.
I'd like to see this made the default in a future version of Java. The pass by value/reference thing trips up an awful lot of junior programmers.
One more thing.. my methods tend to have a low number of parameters so the extra text on a method declaration isn't an issue.

Using final in a method parameter has nothing to do with what happens to the argument on the caller side. It is only meant to mark it as not changing inside that method. As I try to adopt a more functional programming style, I kind of see the value in that.

Personally I don't use final on method parameters, because it adds too much clutter to parameter lists.
I prefer to enforce that method parameters are not changed through something like Checkstyle.
For local variables I use final whenever possible, I even let Eclipse do that automatically in my setup for personal projects.
I would certainly like something stronger like C/C++ const.

Since Java passes copies of arguments I feel the relevance of final is rather limited. I guess the habit comes from the C++ era where you could prohibit reference content from being changed by doing a const char const *. I feel this kind of stuff makes you believe the developer is inherently stupid as f*** and needs to be protected against truly every character he types. In all humbleness may I say, I write very few bugs even though I omit final (unless I don't want someone to override my methods and classes). Maybe I'm just an old-school dev.

Short answer: final helps a tiny bit but... use defensive programming on the client side instead.
Indeed, the problem with final is that it only enforces the reference is unchanged, gleefully allowing the referenced object members to be mutated, unbeknownst to the caller. Hence the best practice in this regard is defensive programming on the caller side, creating deeply immutable instances or deep copies of objects that are in danger of being mugged by unscrupulous APIs.

I never use final in a parameter list, it just adds clutter like previous respondents have said. Also in Eclipse you can set parameter assignment to generate an error so using final in a parameter list seems pretty redundant to me.
Interestingly when I enabled the Eclipse setting for parameter assignment generating an error on it caught this code (this is just how I remember the flow, not the actual code. ) :-
private String getString(String A, int i, String B, String C)
{
if (i > 0)
A += B;
if (i > 100)
A += C;
return A;
}
Playing devil's advocate, what exactly is wrong with doing this?

One additional reason to add final to parameter declarations is that it helps to identify variables that need to be renamed as part of a "Extract Method" refactoring. I have found that adding final to each parameter prior to starting a large method refactoring quickly tells me if there are any issues I need to address before continuing.
However, I generally remove them as superfluous at the end of the refactoring.

Follow up by Michel's post. I made myself another example to explain it. I hope it could help.
public static void main(String[] args){
MyParam myParam = thisIsWhy(new MyObj());
myParam.setArgNewName();
System.out.println(myParam.showObjName());
}
public static MyParam thisIsWhy(final MyObj obj){
MyParam myParam = new MyParam() {
#Override
public void setArgNewName() {
obj.name = "afterSet";
}
#Override
public String showObjName(){
return obj.name;
}
};
return myParam;
}
public static class MyObj{
String name = "beforeSet";
public MyObj() {
}
}
public abstract static class MyParam{
public abstract void setArgNewName();
public abstract String showObjName();
}
From the code above, in the method thisIsWhy(), we actually didn't assign the [argument MyObj obj] to a real reference in MyParam. In instead, we just use the [argument MyObj obj] in the method inside MyParam.
But after we finish the method thisIsWhy(), should the argument(object) MyObj still exist?
Seems like it should, because we can see in main we still call the method showObjName() and it needs to reach obj. MyParam will still use/reaches the method argument even the method already returned!
How Java really achieve this is to generate a copy also is a hidden reference of the argument MyObj obj inside the MyParam object ( but it's not a formal field in MyParam so that we can't see it )
As we call "showObjName", it will use that reference to get the corresponding value.
But if we didn't put the argument final, which leads a situation we can reassign a new memory(object) to the argument MyObj obj.
Technically there's no clash at all! If we are allowed to do that, below will be the situation:
We now have a hidden [MyObj obj] point to a [Memory A in heap] now live in MyParam object.
We also have another [MyObj obj] which is the argument point to a [Memory B in heap] now live in thisIsWhy method.
No clash, but "CONFUSING!!" Because they are all using the same "reference name" which is "obj".
To avoid this, set it as "final" to avoid programmer do the "mistake-prone" code.

What is the possible benefit (if any) of allowing recursive constructors?

In Java, constructors cannot be recursive. Compile time error: "recursive constructor invocation". Let's assume that we did not have this restriction.
Things to keep in mind:
The return type of a constructor is void. Since it is a void method you can't harness the complete power of recursion.
A constructor can invoke itself (or any other constructor) using this(). But a "call to this must be first statement in constructor"
We could use non local data between consecutive calls to still have some possible gain from recursive constructors.
Would there be any benefit from allowing recursive constructors?

Constructors (when they are calling each other) are like methods that return void. Consequently the only way they can produce results is by side-effects. This is then limited to mutating the object they are constructing or by mutating the values passed in as parameters. The latter is a pretty nasty idea in a constructor; a constructor usually takes information from its parameters without mutating them.
So mutating the object being constructed is the only option in order to have any way to track the progress of the recursion, in order for it to terminate eventually. And it's very hard to see how that would be easier to write, clearer to read, etc. than a simple loop inside an ordinary constructor.
Calling another constructor (with this) from within a constructor is of course totally different from using a new expression within a constructor:
class Node
{
Node _left, _right;
public Node(Node left, Node right)
{
_left = left != null ? new Node(left._left, left._right) : null;
_right = right != null ? new Node(right._left, right._right) : null;
}
}
Here the Node constructor calls itself, but via a new expression. This is the crucial difference. A new expression produces a value, so this is purely "functional", non-mutating stuff, and provides a convenient way to make a deep copy of the tree of nodes.

Constructors can be recursive. (That's in C#, but you can do the same thing in Java)

Let's look at this problem. First of all, what happens when you invoke new MyClass("foo");? Well there are two things happening. First of all, the virtual machine will allocate the memory needed to store an object of type MyClass. Then, the constructor gets called. The job of a constructor is to initialise this just allocated memory. Therefore, a constructor does not have a return type at all (not even void). The value returned by the new operator is a reference to the allocated memory, so the constructor can not return as well.
Then, what would be the benefit of recursive constructor invocation. The only benefit of such invocation would be to handle certain constructor parameters like others, and doing so by re-invoking the constructor. While this is possible it is generally easy just to adjust the values in the constructor itself (using non-final parameters), and after that initialise the object attributes (in short you don't need recursion for this).
Second, you can do recursion fairly easily by offloading all the work to a worker method that can recurse as much as you want.
A more interesting question is the restriction on super or this invocation being the first statement of the constructor. This restriction was probably put in to discourage sloppy or unsafe programming practices. Statement is put in bold here though as it is possible (although not beautiful) to work around this restriction. If you remember that expressions may have side effects (e.g. variable assignments), and expressions used for parameters are invoked before the call itself it is possible to create complicated expressions that do all your calculations before invoking the delegate constructor.
The general reason why you want to have a delegate/super constructor invocation later in the constructor body is parameter manipulation. You can do that with (static) helper functions that do these calculations and provide the correct values. This is generally cleaner but not in all cases. The actual execution speed should not be affected as hotspot can inline these things very well.
That means that in the end the consideration boils down to providing the flexibility of free placement of delegate/super calls versus the added safety provided by making incorrect practices quite much harder. The choice made by Java's designers (and the general Java philosophy) is to go for making it harder to do the wrong things at the cost of raw language power at the hand of experts (with increased complexity). The choice made is to me a valid one albeit I personally would like the power (one can always implement a java++ language on the JVM that does not have these restrictions).

You might not be able to write a recursive constructor, but you can call a recursive function from your constructor. I have never had to do this before, and I can't think of a situation where you might need to, but you can do it if you want.

What do you mean allow? You can have recursive constructors in Java. They allow you to reuse code and design your constructors in a more hierarchical fashion.
In the following recursive constructor example, I can call new User() or new User("Marcus") and with either constructor that I use, newUser is set to true.
public class User() {
public String userName;
public boolean newUser;
User() {
newUser = true;
}
User(String userName) {
// Recursively call no-argument constructor
this();
this.userName = userName;
}
}
Here's the same thing without recursive constructors. Notice the duplicate line of code:
public class User() {
public String userName;
public boolean newUser;
User() {
newUser = true;
}
User(String userName) {
newUser = true;
this.userName = userName;
}
}
In the following non-recursive constructor example, if I don't pass a name into the constructor, then the name is set to "New User". I would only want to call the no-argument constructor if I'm not setting the name. If I did a recursive constructor call here, I would end up setting the userName twice:
public class User() {
public String userName;
User() {
this.userName = "New User";
}
User(String userName) {
this.userName = userName;
}
}
You will only use recursive constructors if you:
Have more than one constructor
Have code in your constructors
Want to recursively use code that's in another constructor

The return type of a constructor is
void.
No it isn't.
A constructor can invoke itself (or any other constructor) using this()
No. It can only invoke other constructors, and only if that won't lead to a recursive invocation of the current constructor. That's why you get the error message you referred to.
We could use non local data between
consecutive calls to still have some
possible gain from recursive
constructors.
How? Why would you want to re-initialize an object? When can't you do it sequentially in one pass? Never had this problem in 39 years of computer programming, and 20 years of OO.
Would there be any benefit from
allowing recursive constructors?
You haven't come up with any ...