Why would one mark local variables and method parameters as final in Java

Question

In Java  you can qualify local variables and method parameters with the final keyword   public static void foo final int x      final String qwerty    bar        Doing so results in not being able to reassign x and qwerty in the body of the method   This practice nudges your code in the direction of immutability which is generally considered a plus  But  it also tends to clutter up code with  final  showing up everywhere  What is your opinion of the final keyword for local variables and method parameters in Java

User · Answer

We do it here for the local variables if we think they will not be reassigned or should not be reassigned.

The parameters are not final since we have a Checkstyle-Check which checks for reassigning parameters. Of course nobody would ever want to reassign a parameter variable.

User · Answer

Yes do it   It s about readability   It s easier to reason about the possible states of the program when you know that variables are assigned once and only once   A decent alternative is to turn on the IDE warning when a parameter is assigned  or when a variable  other than a loop variable  is assigned more than once

User · Answer

In the case of local variables  I tend to avoid this  It causes visual clutter  and is generally unnecessary - a function should be short enough or focus on a single impact to let you quickly see that you are modify something that shouldn t be   In the case of magic numbers  I would put them as a constant private field anyway rather than in the code   I only use final in situations where it is necessary  e g   passing values to anonymous classes

User · Answer

Yes do it   It s about readability   It s easier to reason about the possible states of the program when you know that variables are assigned once and only once   A decent alternative is to turn on the IDE warning when a parameter is assigned  or when a variable  other than a loop variable  is assigned more than once

User · Answer

In the case of local variables  I tend to avoid this  It causes visual clutter  and is generally unnecessary - a function should be short enough or focus on a single impact to let you quickly see that you are modify something that shouldn t be   In the case of magic numbers  I would put them as a constant private field anyway rather than in the code   I only use final in situations where it is necessary  e g   passing values to anonymous classes

User · Answer

My personal opinion is that it is a waste of time  I believe that the visual clutter and added verbosity is not worth it   I have never been in a situation where I have reassigned  remember  this does not make objects immutable  all it means is that you can t reassign another reference to a variable  a variable in error   But  of course  it s all personal preference  -

User · Answer

You should try to do this  whenever it is appropriate  Besides serving to warn you when you  accidentally  try to modify a value  it provides information to the compiler that can lead to better optimization of the class file  This is one of the points in the book   Hardcore Java  by Robert Simmons  Jr  In fact  the book spends all of its second chapter on the use of final to promote optimizations and prevent logic errors  Static analysis tools such as PMD and the built-in SA of Eclipse flag these sorts of cases for this reason

User · Answer

Why would you want to  You wrote the method  so anyone modifying it could always remove the final keyword from qwerty and reassign it  As for the method signature  same reasoning  although I m not sure what it would do to subclasses of your class    they may inherit the final parameter and even if they override the method  be unable to de-finalize x  Try it and find out if it would work   The only real benefit  then  is if you make the parameter immutable and it carries over to the children  Otherwise  you re just cluttering your code for no particularly good reason  If it won t force anyone to follow your rules  you re better off just leaving a good comment as you why you shouldn t change that parameter or variable instead of giving if the final modifier   Edit  In response to a comment  I will add that if you are seeing performance issues  making your local variables and parameters final can allow the compiler to optimize your code better  However  from the perspective of immutability of your code  I stand by my original statement

User · Answer

Making a parameter final guarantees that the value used at any location in the method refers to the value passed   Otherwise you have to parse mentally all the code above a given location to know what value the parameter has at that point   Hence  not using final makes your code less readable  and maintainable  all by itself     Final local variables depend on intent  and is less important in my point of view   Depends on what goes on

User · Answer

Yes do it   It s about readability   It s easier to reason about the possible states of the program when you know that variables are assigned once and only once   A decent alternative is to turn on the IDE warning when a parameter is assigned  or when a variable  other than a loop variable  is assigned more than once

User · Answer

In the case of local variables  I tend to avoid this  It causes visual clutter  and is generally unnecessary - a function should be short enough or focus on a single impact to let you quickly see that you are modify something that shouldn t be   In the case of magic numbers  I would put them as a constant private field anyway rather than in the code   I only use final in situations where it is necessary  e g   passing values to anonymous classes

User · Answer

We do it here for the local variables if we think they will not be reassigned or should not be reassigned.

The parameters are not final since we have a Checkstyle-Check which checks for reassigning parameters. Of course nobody would ever want to reassign a parameter variable.

User · Answer

Making a parameter final guarantees that the value used at any location in the method refers to the value passed   Otherwise you have to parse mentally all the code above a given location to know what value the parameter has at that point   Hence  not using final makes your code less readable  and maintainable  all by itself     Final local variables depend on intent  and is less important in my point of view   Depends on what goes on

User · Answer

We do it here for the local variables if we think they will not be reassigned or should not be reassigned.

The parameters are not final since we have a Checkstyle-Check which checks for reassigning parameters. Of course nobody would ever want to reassign a parameter variable.

User · Answer

final has three good reasons    instance variables set by constructor only become immutable methods not to be overridden become final  use this with real reasons  not by default local variables or parameters to be used in anonimous classes inside a method need to be final   Like methods  local variables and parameters need not to be declared final  As others said before  this clutters the code becoming less readable with very little efford for compiler performace optimisation  this is no real reason for most code fragments

User · Answer

In the case of local variables  I tend to avoid this  It causes visual clutter  and is generally unnecessary - a function should be short enough or focus on a single impact to let you quickly see that you are modify something that shouldn t be   In the case of magic numbers  I would put them as a constant private field anyway rather than in the code   I only use final in situations where it is necessary  e g   passing values to anonymous classes

User · Answer

I let Eclipse do it for me when they are being used in an anonymous class  which is increasing due to my use of Google Collection API

User · Answer

Because of the  occasionally  confusing nature of Java s  pass by reference  behavior I definitely agree with finalizing parameter var s    Finalizing local var s seems somewhat overkill IMO

User · Answer

Why would you want to  You wrote the method  so anyone modifying it could always remove the final keyword from qwerty and reassign it  As for the method signature  same reasoning  although I m not sure what it would do to subclasses of your class    they may inherit the final parameter and even if they override the method  be unable to de-finalize x  Try it and find out if it would work   The only real benefit  then  is if you make the parameter immutable and it carries over to the children  Otherwise  you re just cluttering your code for no particularly good reason  If it won t force anyone to follow your rules  you re better off just leaving a good comment as you why you shouldn t change that parameter or variable instead of giving if the final modifier   Edit  In response to a comment  I will add that if you are seeing performance issues  making your local variables and parameters final can allow the compiler to optimize your code better  However  from the perspective of immutability of your code  I stand by my original statement

User · Answer

You should try to do this  whenever it is appropriate  Besides serving to warn you when you  accidentally  try to modify a value  it provides information to the compiler that can lead to better optimization of the class file  This is one of the points in the book   Hardcore Java  by Robert Simmons  Jr  In fact  the book spends all of its second chapter on the use of final to promote optimizations and prevent logic errors  Static analysis tools such as PMD and the built-in SA of Eclipse flag these sorts of cases for this reason

User · Answer

Making local variables and method parameters final is essential if you want to pass those parameters into anonymous classes - like you instantiate an anonymous Thread and want to access those params in the body of the run   method   Apart from that I am not sure of the performance benefits w r t better performance through compiler optimization  It is up to the specific compiler implementation whether it wants to optimize it at all     It will be good to know of any performance stats from using final

User · Answer

final has three good reasons    instance variables set by constructor only become immutable methods not to be overridden become final  use this with real reasons  not by default local variables or parameters to be used in anonimous classes inside a method need to be final   Like methods  local variables and parameters need not to be declared final  As others said before  this clutters the code becoming less readable with very little efford for compiler performace optimisation  this is no real reason for most code fragments

User · Answer

I let Eclipse do it for me when they are being used in an anonymous class  which is increasing due to my use of Google Collection API

User · Answer

Making local variables and method parameters final is essential if you want to pass those parameters into anonymous classes - like you instantiate an anonymous Thread and want to access those params in the body of the run   method   Apart from that I am not sure of the performance benefits w r t better performance through compiler optimization  It is up to the specific compiler implementation whether it wants to optimize it at all     It will be good to know of any performance stats from using final

User · Answer

Because of the  occasionally  confusing nature of Java s  pass by reference  behavior I definitely agree with finalizing parameter var s    Finalizing local var s seems somewhat overkill IMO

User · Answer

I let Eclipse do it for me when they are being used in an anonymous class  which is increasing due to my use of Google Collection API

User · Answer

You should try to do this  whenever it is appropriate  Besides serving to warn you when you  accidentally  try to modify a value  it provides information to the compiler that can lead to better optimization of the class file  This is one of the points in the book   Hardcore Java  by Robert Simmons  Jr  In fact  the book spends all of its second chapter on the use of final to promote optimizations and prevent logic errors  Static analysis tools such as PMD and the built-in SA of Eclipse flag these sorts of cases for this reason

User · Answer

My personal opinion is that it is a waste of time  I believe that the visual clutter and added verbosity is not worth it   I have never been in a situation where I have reassigned  remember  this does not make objects immutable  all it means is that you can t reassign another reference to a variable  a variable in error   But  of course  it s all personal preference  -

User · Answer

Although it creates a little clutter  it is worth putting final  Ides e g eclipse can automatically put the final if you configure it to do so

User · Answer

We do it here for the local variables if we think they will not be reassigned or should not be reassigned.

The parameters are not final since we have a Checkstyle-Check which checks for reassigning parameters. Of course nobody would ever want to reassign a parameter variable.

User · Answer

Why would you want to  You wrote the method  so anyone modifying it could always remove the final keyword from qwerty and reassign it  As for the method signature  same reasoning  although I m not sure what it would do to subclasses of your class    they may inherit the final parameter and even if they override the method  be unable to de-finalize x  Try it and find out if it would work   The only real benefit  then  is if you make the parameter immutable and it carries over to the children  Otherwise  you re just cluttering your code for no particularly good reason  If it won t force anyone to follow your rules  you re better off just leaving a good comment as you why you shouldn t change that parameter or variable instead of giving if the final modifier   Edit  In response to a comment  I will add that if you are seeing performance issues  making your local variables and parameters final can allow the compiler to optimize your code better  However  from the perspective of immutability of your code  I stand by my original statement

User · Answer

final has three good reasons    instance variables set by constructor only become immutable methods not to be overridden become final  use this with real reasons  not by default local variables or parameters to be used in anonimous classes inside a method need to be final   Like methods  local variables and parameters need not to be declared final  As others said before  this clutters the code becoming less readable with very little efford for compiler performace optimisation  this is no real reason for most code fragments

User · Answer

Because of the  occasionally  confusing nature of Java s  pass by reference  behavior I definitely agree with finalizing parameter var s    Finalizing local var s seems somewhat overkill IMO

User · Answer

Yes do it   It s about readability   It s easier to reason about the possible states of the program when you know that variables are assigned once and only once   A decent alternative is to turn on the IDE warning when a parameter is assigned  or when a variable  other than a loop variable  is assigned more than once

User · Answer

You should try to do this  whenever it is appropriate  Besides serving to warn you when you  accidentally  try to modify a value  it provides information to the compiler that can lead to better optimization of the class file  This is one of the points in the book   Hardcore Java  by Robert Simmons  Jr  In fact  the book spends all of its second chapter on the use of final to promote optimizations and prevent logic errors  Static analysis tools such as PMD and the built-in SA of Eclipse flag these sorts of cases for this reason

User · Answer

My personal opinion is that it is a waste of time  I believe that the visual clutter and added verbosity is not worth it   I have never been in a situation where I have reassigned  remember  this does not make objects immutable  all it means is that you can t reassign another reference to a variable  a variable in error   But  of course  it s all personal preference  -

User · Answer

Because of the  occasionally  confusing nature of Java s  pass by reference  behavior I definitely agree with finalizing parameter var s    Finalizing local var s seems somewhat overkill IMO

User · Answer

Why would you want to  You wrote the method  so anyone modifying it could always remove the final keyword from qwerty and reassign it  As for the method signature  same reasoning  although I m not sure what it would do to subclasses of your class    they may inherit the final parameter and even if they override the method  be unable to de-finalize x  Try it and find out if it would work   The only real benefit  then  is if you make the parameter immutable and it carries over to the children  Otherwise  you re just cluttering your code for no particularly good reason  If it won t force anyone to follow your rules  you re better off just leaving a good comment as you why you shouldn t change that parameter or variable instead of giving if the final modifier   Edit  In response to a comment  I will add that if you are seeing performance issues  making your local variables and parameters final can allow the compiler to optimize your code better  However  from the perspective of immutability of your code  I stand by my original statement

User · Answer

My personal opinion is that it is a waste of time  I believe that the visual clutter and added verbosity is not worth it   I have never been in a situation where I have reassigned  remember  this does not make objects immutable  all it means is that you can t reassign another reference to a variable  a variable in error   But  of course  it s all personal preference  -

User · Answer

final has three good reasons    instance variables set by constructor only become immutable methods not to be overridden become final  use this with real reasons  not by default local variables or parameters to be used in anonimous classes inside a method need to be final   Like methods  local variables and parameters need not to be declared final  As others said before  this clutters the code becoming less readable with very little efford for compiler performace optimisation  this is no real reason for most code fragments

User · Answer

Although it creates a little clutter  it is worth putting final  Ides e g eclipse can automatically put the final if you configure it to do so

User · Answer

I let Eclipse do it for me when they are being used in an anonymous class  which is increasing due to my use of Google Collection API

[java] Why would one mark local variables and method parameters as "final" in Java?

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