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Java - Is it possible to write public void() in Constructor?

I couldn't find information if it is possible to write public void in constructor section. Is it possible?

At the byte code level, a constructor is always void so it would be redundant to specify it. i.e. the constructor is always called <init>V i.e. the V is the return type where V == void. Similarly the static initialiser is <clinit>V You will see this notation if you take a stack trace (e.g. print an exception) while in these methods.
The constructor actually takes the object to be initialised as an argument as the object is created before calling the constructor. Note: you can create the object without calling a constructor with Unsafe.allocateInstance(Class)
I couldn't find information if it is possible to write public void in constructor section. Is it possible?
It is not possible to write it as Java distinguishes a constructor from a method is that it must have the same name as the class, and it must not specify a return type. If you specify a return type, it assumes it's a method.
The notation x = new Clazz() also limits the number of return values to 1 which is the object. There is no easy way to modify this notation to return more than one object. i.e. supporting a return type for constructors would not be easy.
If you want to define a return type, most likely you are thinking of a factor method like this.
public static MyInterface createObject() {
return new MyClass();
}
Note how the return type is different to the class actually created, but there is still only one reference returned.

The constructor syntax is defined in the Java Language Specification. Anything else is incorrect.

The question is unclear. Peter Lawrey answered one interpretation, this is an answer to another.
You cannot declare methods within a constructor. You can, however, declare a class and declare variables.
Because you are able to declare a class within a constructor, you could declare a method inside of a class and then use the class. If the method isn't static you can construct an object of the class.

No, Java only allows a method to be declared within a class, not within another method or constructor.Indirectly you can do something like this :
public A() {
class B {
public void m() {
}
}
}

Different ways to write getter and setter?

I have seen accessor methods using this and ones that don't. Do they produce the same results?
For example:
public setName(string name){
this.name=name;
}
public setName(string n){
name=n;
}
public void getName{
return name;
}
public void getName{
// is "this" here useless?
return this.name;
}
What are the differences? Which way is better?

You need to use this when you want to reference the class level variable which has the same name as the local variable in the method. It is only required when the variables have the same name. You can always use this if you want, but it isn't necessary. And the difference between
this.s = s;
and
s = myString;
is just a style preference, and not a big important one that wars are fought over.

Yes, they are entirely equivalent in their function and resulting bytecode (afaik).
The ’this’ keyword is only used when referencing an instance variable (a non-static variable that is defined with the class or relevant super-classes), in order to differentiate it from a local variable of the same name. Omitting the ’this’ keyword in such a situation would assign the local variable to itself.
Regarding your edit about what is better, some prefer not naming variables the same, but some like using ’this’ to explicitly refer to an instance variable. It comes down to a matter of taste and readability.

It is the same.
If your method has a variable with the same name of a class field (eg "name") you need to write "this.name" to refer to class field, because writing only "name" you are refering to method variable.
So in the second example you wrote, you have two different names ("name" and "n") and you don't need to write "this.name".
By the way, in the second example, you are free to say "this.name" if you want, but it is not mandatory... (in my opinion it is not mandatory, but it is a great way to make your code more easy to read).
At the end choose what method you prefer.
In my mind the first one is a more "elegant" way to write code.

Is there a name for "this" in Java?

Eclipse will give an error, "The left-hand side of an assignment must be a variable", when I try something like:
public class Thing{
String a1;
int a2;
public void meth(){
Thing A = new Thing();
this = A;
}
}
I had to assign each variable (this.a1 = A.a1; this.a2 = A.a2;) as a work around.
Are there other ways to do this without going through each variable field?
And if this is not a variable what is it called?

this is a pseudo-variable that points to the current instance of the object, it can not be reassigned. It's also considered a keyword in the language, according to section §3.9 of the Java Language Specification.

No, there is no easy shortcut.
And if "this" is not a variable what is it called?
this is not a variable, it's a keyword.
Even though this is special, in many respects it acts like a reference. Therefore, for consistency, this = A would have to be a reference assignment, which doesn't quite make sense.
You seem to be expecting this = A to perform a field-by-field copy from A to this, and indeed Java's designers could choose do that in this case. However, this would be inconsistent with other reference assignments, and the overall benefits of having this as an exception are not at all clear.

this refers to this instance of the class.
You cannot assign to this

this is a java reserved keyword which refers to the current object. its not a variable its a java reserved keyword.
so this = A; is invalid. using this keyword we can refer to any instance variable or method of the current object. you have to refer to the instance variable like:
this.a1 = A.a1;
From Doc:
The most common reason for using the this keyword is because a field
is shadowed by a method or constructor parameter.

You can't assign to this in Java. It's not a variable; it's a keyword.
One thing you might consider, if you don't need a particular instance, is just returning your new instance.
public class Thing{
String a1;
int a2;
public Thing meth(){
Thing A = new Thing();
return A;
}
}
and you'd use it like
whatever = whatever.meth();

According to java lang spec §15.8.3 this is a keyword that is either an expression or statement
When used as a primary expression this denotes a value that is a reference to the object for which the instance method was invoked.
Expression: Something which evaluates to a value. Example: x++
The keyword this is also used in a special explicit constructor invocation statement
Statement: Syntactic elements that control the execution of a program, which are executed for their effect and do not have values Example: if (true)
In either case it is not a variable
Variable: A storage location with an associated type
In your case this is an expression and not a variable. But for all intents an purposes just call it a keyword
Keyword: A character sequence, formed from ASCII letters, are reserved for use ... that cannot be used as a variable name

this refers to the owner of the method.
In this case, the owner is the object itself.
Sometime, this may not refer to the class that you are writing code. Such as in the annoymous class. A common example is the anonymous listener.
button.addActionListener(
new ActionListener() {
public void actionPerformed(ActionEvent e) {
this; // refers to the ActionListener
}
}
);
In addition, you can return this can do method chaining. Supposed you have a class called Homework and it has a method addTask.
public Homework addTask(String task){
return this;
}
you can call the addTask method like
homework.addTask("a").addTask("b").addTask("c");

I think the OP is asking for the ability to assign the contents of one object to another, rather than to assign a new value to the "this" pointer. C++ has this ability -- you can override the assignment operator -- but Java has no such ability.
It would be a nice feature to have in some occasional cases, but it's simply not currently possible, and it doesn't really fit the Java "mold" to provide the function in the future.
The capability would be more useful (and there would be more motivation to provide it) if Java allowed objects to be embedded in other objects (vs simply embedding referenced), but that's not in the cards either.

There is no1 way to copy the values of all fields from one instance onto another in the basic Java language. And you should typically not need it. You can most often just replace the reference to the new instance or work directly on the target instance.
In your case when you want to reset all fields of a object to the initial values (and there is seldomly a need for it) you typically use a reset method which eighter works on its own instance or is a static one working on any given object.
So
class A {
String a1; int a2;
void reset() { a1 = ""; a2 = 0; }
}
would be used as
A a = new A();
// modify a
a.reset();
and
class A {
String a1; int a2;
static void reset(A anotherA) { anotherA.a1 = ""; anotherA.a2 = 0; }
}
and use it like:
A.reset(a);
In both cases it makes sense to use the reset method also for setting the initial values in the constructor: A() { A.reset(this); } or A() { this.reset(); }
1 actually there are some libraries to do it, and you can code it with the help of reflection, the only reason I see it is used is to implement a clone() method or for some kind of wrapping/stubbing.

It sounds to me like what you're trying to do is have a method that reinitializes your object, i.e., set's it back to it's initial values. That's why you want to create a new object, and assign it to the current object, right?
If that's the case, let's try a different way of doing it, since, as has been said, you can't reassign this.
What if, instead of doing that, you tried something like this:
public class Thing {
String a1;
int a2;
public Thing() {
this.meth();
}
public void meth() {
this.a1 = "a1";
this.a2 = 2;
}
}
This way, Thing.meth() actually initializes your object, and the constructor calls it when the object is created. Then you can call it again whenever you'd like.

==Disclaimer, I don't know java==
You would want to assign manually.
I'm not sure why you are trying to create a new instance of Thing inside Thing, but as you don't set the values of a1 and a2 you would need to assign them the way you did.
this is a reserved keyword pointing the class object it is inside.
For example, if you wanted to have another function named fish() your code may look something like this.
public class Thing{
String a1;
int a2;
public Thing meth(){
Thing A = new Thing();
return A;
}
public Thing fish(){
this.a1 = "foo";
this.meth();
return A;
}
}

When you do this = stuff; you are trying to replace the current object instance reference (in this case, the one that you are initializing in the constructor) with another thing, and (in the particular case of java) thats illegal and the language forbids you of doing it.
Think about it, if you could replace the reference to your current instance just like that, then you could incur in some serious memory and security problems (the reference to the constructed object will be lost and overrided by some unknown object).
What is totally valid is referencing members of your current object using the . operator, because they are owned by this, so no problems should arise (at least not evident ones).
The JVM has some inner security measures (e.g., method max stack size verification, class file format validation, etc) that prevents from easy binary manipulation and are enforced by the language syntax. This could be seen as one of those.

Advantage of using "this."

I've read many large projects in OOP, and I notice that a lot of them use this.[variable], [ClassName].[staticVariable]. For example:
public class ABC {
private float mX;
public static float y;
public float getX() {
return this.mX;
}
public float doSomethingWithY() {
return ABC.y;
}
}
And even with Eclipse auto-generated Getters & Setters feature, it also comes with this.[variable], although it's unnecessary, because no local variable is declared there.
Is there any advantage when using these notations, or it's just a code style?
EDIT so some people don't misunderstand. I know what this and [ClassName].[staticVariable] stand for. But in this case, it's unnecessary. The question is: Even if it's unnecessary, why do guru coders still add it? When they need to update/fix a huge project, will there be any advantage and disadvantage?

Basically with this, you KNOW for sure that you are working with a class attribute, not with a variable created inside the method or maybe received as a parameter.
And also, it helps in case you have a local var with the same name.
And the final reason: readability.

It's necessary in some circumstances, for example this. is required when you need to use a member variable rather than a local method parameter of the same name.
It's also necessary for static variables where you need to be specific which class you want to get the static variable from (many classes could define static variables with the same name).
Apart from the necessary cases, it's really a matter of coding style. My recommendation is to use it whenever it helps to resolve potential ambiguity.

In complicated methods, it's sometimes nice to make a distinction between instance variables in this class, and local variables in a particular function. This distinction is immediately obvious when you use "this."

For small pieces of code it doesn't matter but sometimes this can happen:
public float getX() {
....
int mX = someFunc()
...
return mX;
}
In this case, the local value is returned instead of the member variable.
You normally want to be explicit and say this.mX. However, you shouldn't have huge functions anyway.

this.? '?' is a member variable, this is a reference to the current object.
see this

Its syntax,if you want to access instance variable of a class use the (reference of the object).(instance variable name) .Like
A a= new A();// for non static class,this for the current object
a.(instance variable name)
// for non static class do the same but use (class name).variable name

Why should I use the keyword "final" on a method parameter in Java?

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.