What is the meaning of this in Java

Question

Normally  I use this in constructors only    I understand that it is used to identify the parameter variable  by using this something   if it have a same name with a global variable    However  I don t know that what the real meaning of this is in Java and what will happen if I use this without dot

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Quoting an article at programming guide     this has two uses in a Java program   1  As a reference to the current object  The syntax in this case usually looks something like  this someVariable   someVariable    This type of use is described here  The  this  reference  with examples   2  To call a different constructor  The syntax in this case typically looks something like  MyClass         this DEFAULT VALUE      delegate to other constructor    MyClass int value                   This type of use is described here  this      constructor call  with examples

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In Java  this  is a predefined variable  If we use  this  in method that s mean we are getting the reference  address  of the currently running object  For an example   this age ---  age of the currently running object

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This refers to the object you   re    in    right now  In other words this refers to the receiving object  You use this to clarify which variable you   re referring to Java whitepaper page  37  class Point extends Object       public double x      public double y       Point                 x   0 0          y   0 0             Point double x  double y                this x   x          this y   y            In the above example code this x this y refers to current class that is Point class x and y variables where   double x double y  are double values passed from different class to assign values to current class

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To be complete  this can also be used to refer to the outer object  class Outer       class Inner           void foo                 Outer o   Outer this

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A quick google search brought this result  http   xahlee org java-a-day this html  Pretty much the  this  keyword is a reference to the current object  itself

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In Swing its fairly common to write a class that implements ActionListener and add the current instance  ie  this   as an ActionListener for components   public class MyDialog extends JDialog implements ActionListener       public MyDialog                 JButton myButton   new JButton  Hello            myButton addActionListener this              public void actionPerformed ActionEvent evt                System out println  Hurdy Gurdy

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this refers to the current object   Each non-static method runs in the context of an object  So if you have a class like this   public class MyThisTest     private int a     public MyThisTest         this 42      calls the other constructor        public MyThisTest int a        this a   a     assigns the value of the parameter a to the field of the same name        public void frobnicate         int a   1       System out println a      refers to the local variable a     System out println this a      refers to the field a     System out println this      refers to this entire object        public String toString         return  MyThisTest a     a     refers to the field a         Then calling frobnicate   on new MyThisTest   will print    1 42 MyThisTest a 42   So effectively you use it for multiple things    clarify that you are talking about a field  when there s also something else with the same name as a field refer to the current object as a whole invoke other constructors of the current class in your constructor

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As everyone said  this represents the current object   current instance  I understand it this way  if its just  quot this quot  - it returns class object  in below ex  Dog if it has this something  something is a method in that class or a variable class Dog   private String breed  private String name   Dog String breed  String name        this breed   breed      this name   name     public Dog getDog            return Dog type     return this

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this is a reference to the current object  http   download oracle com javase tutorial java javaOO thiskey html

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It s  a reference to the object in the current context  effectively  For example  to print out  this object  you might write   System out println this     Note that your usage of  global variable  is somewhat off    if you re using this variableName then by definition it s not a global variable - it s a variable specific to this particular instance

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Instance variables are common to every object that you creating  say  there is two instance variables  class ExpThisKeyWord  int x  int y   public void setMyInstanceValues int a  int b        x  a      y b       System out println  x is    x       System out println  y is    y            class Demo  public static void main String   args    ExpThisKeyWord obj1   new ExpThisKeyWord    ExpThisKeyWord obj2   new ExpThisKeyWord    ExpThisKeyWord obj3   new ExpThisKeyWord     obj1 setMyInstanceValues 1  2   obj2 setMyInstanceValues 11  22   obj3 setMyInstanceValues 111  222            if you noticed above code  we have initiated three objects and three objects are calling SetMyInstanceValues method   How do you think JVM correctly assign the values for every object   there is the trick  JVM will not see this code how it is showed above  instead of that  it will see like below code   public void setMyInstanceValues int a  int b        this x  a    Answer  this keyword denotes the current object that is handled by JVM      this y b       System out println  x is    x       System out println  y is    y

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this can be used inside some method or constructor   It returns the reference to the current object

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I would like to share what I understood from this keyword  This keyword has 6 usages in java as follows -  1  It can be used to refer to the current class variable   Let us understand with a code    Let s understand the problem if we don t use this keyword by the example given below   class Employee    int id no    String name    float salary    Student int id no String name float salary     id no   id no    name name    salary   salary        void display   System out println id no      name      salary          class TestThis1    public static void main String args       Employee s1 new Employee 111  ankit  5000f     Employee s2 new Employee 112  sumit  6000f     s1 display      s2 display             Output -  0 null 0 0 0 null 0 0   In the above example  parameters  formal arguments  and instance variables are same  So  we are using this keyword to distinguish local variable and instance variable   class Employee    int id no    String name    float salary    Student int id no String name float salary     this id no   id no    this name name    this salary   salary        void display   System out println id no      name      salary          class TestThis1    public static void main String args       Employee s1 new Employee 111  ankit  5000f     Employee s2 new Employee 112  sumit  6000f     s1 display      s2 display            output   111 ankit 5000 112 sumit 6000   2  To invoke the current class method   class A    void m   System out println  hello Mandy       void n      System out println  hello Natasha        m     same as this m     this m              class TestThis4    public static void main String args       A a new A      a n             Output   hello Natasha hello Mandy   3  to invoke the current class constructor  It is used to constructor chaining   class A    A   System out println  hello ABCD       A int x     this      System out println x             class TestThis5    public static void main String args       A a new A 10          Output   hello ABCD 10   4  to pass as an argument in the method   class S2      void m S2 obj       System out println  The method is invoked              void p        m this             public static void main String args         S2 s1   new S2        s1 p                  Output   The method is invoked   5  to pass as an argument in the constructor call  class B      A4 obj      B A4 obj         this obj obj            void display          System out println obj data    using data member of A4 class              class A4      int data 10      A4         B b new B this        b display              public static void main String args          A4 a new A4                 Output -  10   6  to return current class instance  class A    A getA      return this        void msg   System out println  Hello           class Test1    public static void main String args       new A   getA   msg                Output -  Hello      Also  this keyword cannot be used without   dot  as it s syntax is invalid

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I know Im late but shh Im being the sneaky boi you never saw me   The  this keyword in most Object Oriented programming languages if not all means that its a reference towards the current object instance of that class  It s essentially the same thing as calling on that object from outside of the method by name  That probably made no sense so Ill elaborate   Outside of the class  in order to call something within that instance of the object  for example a say you have an object called object and you want to get a field you would need to use  object field   Say for instance you are trying to access object field from inside of your class in say  your constructor for example  you could use  this field   The this keyword essentially replaces the object name keyword when being called inside of the class  There usually isn t much of a reason to do this outside of if you have two variables of the same name one of which being a field of the class and the other just being a variable inside of a method  it helps decipher between the two  For example if you have this    Hah  get it  this  Hehe      just me  okay     I ll leave now   public String Name    Constructor for  object  class public object String Name       Name   Name      That would cause some problems  the compiler wouldn t be able to know the difference between the Name variable defined in the parameters for the constructor and the Name variable inside of your class  field declarations so it would instead assign the Name parameter to     the value of the Name parameter which does nothing beneficial and literally has no purpose  This is a common issue that most newer programs do and I was a victim of as well  Anyways  the correct way to define this parameter would be to use   public String Name    Constructor for  object  class public object String Name       this Name   Name      This way  the compiler knows the Name variable you are trying to assign is a part of the class and not a part of the method and assigns it correctly  meaning it assigns the Name field to whatever you put into the constructor   To sum it up  it essentially references a field of the object instance of the class you are working on  hence it being the keyword  this   meaning its this object  or this instance  Its a good practice to use this when calling a field of your class rather than just using the name to avoid possible bugs that are difficult to find as the compiler runs right over them

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It refers to the current instance of a particular object  so you could write something like  public Object getMe         return this        A common use-case of this is to prevent shadowing  Take the following example   public class Person       private final String name       public Person String name               how would we initialize the field using parameter             we can t do  name   name            In the above example  we want to assign the field member using the parameter s value  Since they share the same name  we need a way to distinguish between the field and the parameter  this allows us to access members of this instance  including the field   public class Person       private final String name       public Person String name            this name   name

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I was also looking for the same answer  and somehow couldn t understand the concept clearly  But finally I understood it from this link  this  is a keyword in Java  Which can be used inside method or constructor of  class  It this  works as a reference to a current object whose method or constructor is being invoked  this keyword can be used to refer any member of current object from within an instance method or a constructor   Check the examples in the link for a clear understanding

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The this Keyword is used to refer the current variable of a block  for example consider the below code Just a exampple  so dont expect the standard JAVA Code    Public class test   test int a    this a a     Void print    System out println a         Public static void main String args         test s new test 2       s print           Thats it  the Output will be  2   If We not used the this keyword  then the output will be   0

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It refers to the instance on which the method is called  class A      public boolean is Object o        return o    this          A someA   new A    A anotherA   new A    someA is someA      returns true someA is anotherA      returns false

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Objects have methods and attributes variables  which are derived from classes  in order to specify which methods and variables belong to a particular object the this reserved word is used  in the case of instance variables  it is important to understand the difference between implicit and explicit parameters  Take a look at the fillTank call for the audi object   Car audi  new Car     audi fillTank 5      5 is the explicit parameter and the car object is the implicit parameter    The value in the parenthesis is the implicit parameter and the object itself is the explicit parameter  methods that don t have explicit parameters  use implicit parameters  the fillTank method has both an explicit and an implicit parameter    Lets take a closer look at the fillTank method in the Car class  public class Car        private double tank      public Car              tank   0           public void fillTank double gallons             tank   tank   gallons            In this class we have an instance variable  tank   There could be many objects that use the tank instance variable  in order to specify that the instance variable  tank  is used for a particular object  in our case the  audi  object we constructed earlier  we use the this reserved keyword  for instance variables the use of  this  in a method indicates that the instance variable  in our case  tank   is instance variable of the implicit parameter   The java compiler automatically adds the this reserved word so you don t have to add it  it s a matter of preference  You can not use this without a dot    because those are the rules of java   the syntax     In summary    Objects are defined by classes and have methods and variables The use of this on an instance variable in a method indicates that  the instance variable belongs to the implicit parameter  or that it is an instance variable of the implicit parameter   The implicit parameter is the object the method is called from in this case  audi   The java compiler automatically adds the this reserved word  adding it is a matter of preference this cannot be used without a dot    this is syntactically invalid this can also be used to distinguish between local variables and global variables that have the same name the this reserve word also applies to methods  to indicate a method belongs to a particular object

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The following is a copy  amp  paste from here  but explains very well all different uses of the this keyword   Definition  Java   s this keyword is used to refer the current instance of the method on which it is used   Following are the ways to use this    To specifically denote that the instance variable is used instead of static or local variable  That is   private String javaFAQ  void methodName String javaFAQ        this javaFAQ   javaFAQ      Here this refers to the instance variable  Here the precedence is high for the local variable  Therefore the absence of the this denotes the local variable  If the local variable that is parameter   s name is not same as instance variable then irrespective of this is used or not it denotes the instance variable  This is used to refer the constructors   public JavaQuestions String javapapers         this javapapers  true        This invokes the constructor of the same java class which has two parameters  This is used to pass the current java instance as parameter  obj itIsMe this    Similar to the above this can also be used to return the current instance  CurrentClassName startMethod          return this      Note  This may lead to undesired results while used in inner classes in the above two points  Since this will refer to the inner class and not the outer instance  This can be used to get the handle of the current class  Class className   this getClass       this methodology is preferable in java   Though this can be done by   Class className   ABC class     here ABC refers to the class name and you need to know that     As always  this is associated with its instance and this will not work in static methods

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If the instance variables are same as the variables that are declared in the constructor then we use  this  to assign data   class Example       int assign    instance variable       Example int assign      variable inside constructor           this assign assign             Hope this helps

[java] What is the meaning of "this" in Java?

1. As a reference to the current object

2. To call a different constructor

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