When should I use this in a class

Question

I know that this refers to a current object  But I do not know when I really need to use it  For example  will be there any difference if I use x instead of this x in some of the methods  May be x will refer to a variable which is local for the considered method  I mean variable which is seen only in this method   What about this method    Can I use it  Should I use it  If I just use method    will it not be  by default  applied to the current object

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This    keyword in java is used to refer current class objects   There are 6 uses of    this    keyword in java   Accessing class level variable  mostly used if local and class level variable is same Accessing class methods   this is default behavior and can be ignored  For calling other constructor of same class   Using    this    keyword as return value   for returning current instance from method Passing    this    keyword as argument to method Passing   for passing current class instance as argument  this keyword as argument to constructor   for passing current class instance as argument    ref  https   stacktraceguru com java this-keyword-in-java

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There are a lot of good answers  but there is another very minor reason to put this everywhere  If you have tried opening your source codes from a normal text editor  e g  notepad etc   using this will make it a whole lot clearer to read   Imagine this   public class Hello       private String foo          Some 10k lines of codes      private String getStringFromSomewhere                                   More codes      public class World           private String bar              Another 10k lines of codes          public void doSomething                    More codes             foo    FOO                  More codes             String s   getStringFromSomewhere                   More codes             bar   s                      This is very clear to read with any modern IDE  but this will be a total nightmare to read with a regular text editor   You will struggle to find out where foo resides  until you use the editor s  find  function  Then you will scream at getStringFromSomewhere   for the same reason  Lastly  after you have forgotten what s is  that bar   s is going to give you the final blow   Compare it to this   public void doSomething            More codes     Hello this foo    FOO          More codes     String s   Hello this getStringFromSomewhere           More codes     this bar   s       You know foo is a variable declared in outer class Hello  You know getStringFromSomewhere   is a method declared in outer class as well  You know that bar belongs to World class  and s is a local variable declared in that method    Of course  whenever you design something  you create rules  So while designing your API or project  if your rules include  if someone opens all these source codes with a notepad  he or she should shoot him herself in the head   then you are totally fine not to do this

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this is a reference to the current object  It is used in the constructor to distinguish between the local and the current class variable which have the same name  e g    public class circle       int x      circle int x           this x  x            class variable  local variable             this can also be use to call one constructor from another constructor  e g     public class circle       int x       circle              this 1              circle int x            this x   x

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With respect to William Brendel s posts and dbconfessions question  regarding case 2  Here is an example   public class Window      private Window parent     public Window  Window parent        this parent   parent         public void addSubWindow         Window child   new Window this       list add child          public void printInfo         if  parent    null          System out println  root          else         System out println  child                   I ve seen this used  when building parent-child relation s with objects  However  please note that it is simplified for the sake of brevity

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this  is also useful when calling one constructor from another   public class MyClass       public MyClass String foo            this foo  null             public MyClass String foo  String bar

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The only need to use the this  qualifier is when another variable within the current scope shares the same name and you want to refer to the instance member  like William describes   Apart from that  there s no difference in behavior between x and this x

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Will be there any difference if I use  x  instead of  this x  in some of the methods   Usually not  But it makes a difference sometimes     class A        private int i       public A int i            this i   i     this i can be used to disambiguate the i being referred to              If I just use  method     will it not be  by default  applied to the current object   Yes  But if needed  this method   clarifies that the call is made by this object

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this does not affect resulting code - it is compilation time operator and the code generated with or without it will be the same  When you have to use it  depends on context  For example you have to use it  as you said  when you have local variable that shadows class variable and you want refer to class variable and not local one   edit  by  resulting code will be the same  I mean of course  when some variable in local scope doesn t hide the one belonging to class  Thus  class POJO      protected int i      public void modify           i   9           public void thisModify           this i   9           resulting code of both methods will be the same  The difference will be if some method declares local variable with the same name    public void m           int i        i   9      i refers to variable in method s scope       this i   9     i refers to class variable

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The this keyword is primarily used in three situations  The first and most common is in setter methods to disambiguate variable references  The second is when there is a need to pass the current class instance as an argument to a method of another object  The third is as a way to call alternate constructors from within a constructor   Case 1  Using this to disambiguate variable references  In Java setter methods  we commonly pass in an argument with the same name as the private member variable we are attempting to set  We then assign the argument x to this x  This makes it clear that you are assigning the value of the parameter  name  to the instance variable  name    public class Foo       private String name       public void setName String name            this name   name            Case 2  Using this as an argument passed to another object   public class Foo       public String useBarMethod             Bar theBar   new Bar            return theBar barMethod this              public String getName             return  Foo            public class Bar       public void barMethod Foo obj            obj getName              Case 3  Using this to call alternate constructors  In the comments  trinithis correctly pointed out another common use of this  When you have multiple constructors for a single class  you can use this arg0  arg1       to call another constructor of your choosing  provided you do so in the first line of your constructor   class Foo       public Foo             this  Some default value for bar               optional other lines            public Foo String bar               Do something with bar           I have also seen this used to emphasize the fact that an instance variable is being referenced  sans the need for disambiguation   but that is a rare case in my opinion

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Google turned up a page on the Sun site that discusses this a bit   You re right about the variable  this can indeed be used to differentiate a method variable from a class field       private int x      public void setX int x            this x x          However  I really hate that convention   Giving two different variables literally identical names is a recipe for bugs   I much prefer something along the lines of       private int x      public void setX int newX            x newX          Same results  but with no chance of a bug where you accidentally refer to x when you really meant to be referring to x instead   As to using it with a method  you re right about the effects  you ll get the same results with or without it   Can you use it   Sure   Should you use it   Up to you  but given that I personally think it s pointless verbosity that doesn t add any clarity  unless the code is crammed full of static import statements   I m not inclined to use it myself

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this is useful in the builder pattern   public class User        private String firstName      private String surname       public User Builder builder           firstName   builder firstName          surname   builder surname             public String getFirstName            return firstName             public String getSurname            return surname             public static class Builder           private String firstName          private String surname           public Builder setFirstName String firstName                this firstName   firstName              return this                     public Builder setSurname String surname                this surname   surname              return this                     public User build                return new User this                         public static void main String   args            User Builder builder   new User Builder            User user   builder setFirstName  John   setSurname  Doe   build

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You only need to use this - and most people only use it - when there s an overlapping local variable with the same name   Setter methods  for example    Of course  another good reason to use this is that it causes intellisense to pop up in IDEs

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Unless you have overlapping variable names  its really just for clarity when you re reading the code

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The second important use of this  beside hiding with a local variable as many answers already say  is when accessing an outer instance from a nested non-static class   public class Outer     protected int a     public class Inner       protected int a       public int foo          return Outer this a             public Outer getOuter          return Outer this

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Following are the ways to use    this    keyword in java     Using this keyword to refer current class instance variables Using this   to invoke current class constructor Using this keyword to return the current class instance Using this keyword as method parameter   https   docs oracle com javase tutorial java javaOO thiskey html

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To make sure that the current object s members are used   Cases where thread safety is a concern  some applications may change the wrong objects member values  for that reason this should be applied to the member so that the correct object member value is used   If your object is not concerned with thread safety then there is no reason to specify which object member s value is used

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when there are two variables one instance variable and other local variable of the same name then we use this  to refer current executing object to avoid the conflict between the names

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William Brendel answer provided three different use cases in nice way    Use case 1   Offical java documentation page on this provides same use-cases       Within an instance method or a constructor  this is a reference to the current object     the object whose method or constructor is being called  You can refer to any member of the current object from within an instance method or a constructor by using this    It covers two examples     Using this with a Field and Using this with a Constructor  Use case 2   Other use case which has not been quoted in this post  this can be used to synchronize the current object in a multi-threaded application to guard critical section of data  amp  methods   synchronized this          Do some thing       Use case 3   Implementation of Builder pattern depends on use of this to return the modified object   Refer to this post  Keeping builder in separate class  fluent interface

[java] When should I use "this" in a class?

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