Difference between this and super keywords in Java

Question

What is the difference between the keywords this and super   Both are used to access constructors of class right  Can any of you explain

User · Answer

super is used to access methods of the base class while this is used to access methods of the current class   Extending the notion  if you write super    it refers to constructor of the base class  and if you write this    it refers to the constructor of the very class where you are writing this code

User · Answer

From your question  I take it that you are really asking about the use of this and super in constructor chaining  e g   public class A extends B       public A                this                             versus  public class A extends B       public A                super                             The difference is simple    The this form chains to a constructor in the current class  i e  in the A class    The super form chains to a constructor in the immediate superclass  i e  in the B class

User · Answer

this is a reference to the object typed as the current class  and super is a reference to the object typed as its parent class   In the constructor  this   calls a constructor defined in the current class   super   calls a constructor defined in the parent class   The constructor may be defined in any parent class  but it will refer to the one overridden closest to the current class   Calls to other constructors in this way may only be done as the first line in a constructor   Calling methods works the same way   Calling this method   calls a method defined in the current class where super method   will call the same method as defined in the parent class

User · Answer

Lets consider this situation  class Animal     void eat         System out println  animal   eat           class Dog extends Animal     void eat         System out println  dog   eat          void anotherEat         super eat           public class Test     public static void main String   args        Animal a   new Animal        a eat        Dog d   new Dog        d eat        d anotherEat            The output is going to be  animal   eat dog   eat animal   eat   The third line is printing  animal eat  because we are calling super eat    If we called this eat    it would have printed as  dog eat

User · Answer

super    amp  this     super   - to call parent class constructor  this   -  to call same class constructor    NOTE    We can use super   and this   only in constructor not anywhere else  any attempt to do so will lead to compile-time error  We have to keep either super   or this   as the first line of the constructor but NOT both simultaneously    super  amp  this keyword   super - to call parent class members variables and methods   this - to call same class members variables and methods     NOTE  We can use both of them anywhere in a class except static areas static block or method   any    attempt to do so will lead to compile-time error

User · Answer

this is used to access the methods and fields of the current object  For this reason  it has no meaning in static methods  for example   super allows access to non-private methods and fields in the super-class  and to access constructors from within the class  constructors only

User · Answer

When writing code you generally don t want to repeat yourself   If you have an class that can be constructed with various numbers of parameters a common solution to avoid repeating yourself is to simply call another constructor with defaults in the missing arguments   There is only one annoying restriction to this - it must be the first line of the declared constructor   Example   MyClass        this default1  default2      MyClass arg1  arg2       validate arguments  etc       note that your validation logic is only written once now     As for the super   constructor  again unlike super method   access it must be the first line of your constructor   After that it is very much like the this   constructors  DRY  Don t Repeat Yourself   if the class you extend has a constructor that does some of what you want then use it and then continue with constructing your object  example   YourClass extends MyClass      YourClass arg1  arg2  arg3             super arg1  arg2     calls MyClass arg1  arg2        validate and process arg3             Additional information   Even though you don t see it  the default no argument constructor always calls super   first   Example   MyClass         is equivalent to  MyClass        super        I see that many have mentioned using the this and super keywords on methods and variables - all good   Just remember that constructors have unique restrictions on their usage  most notable is that they must be the very first instruction of the declared constructor and you can only use one

User · Answer

this refers to a reference of the current class   super refers to the parent of the current class  which called the super keyword    By doing this  it allows you to access methods attributes of the current class  including its own private methods attributes    super allows you to access public protected method attributes of parent base  class  You cannot see the parent s private method attributes

User · Answer

this keyword use to call constructor in the same class  other overloaded constructor   syntax  this  args list     compatible with args list in other constructor in the same class   super keyword use to call constructor in the super class    syntax  super  args list     compatible with args list in the constructor of the super class   Ex   public class Rect   int x1  y1  x2  y2   public Rect int x1  int y1  int x2  int y2     1st constructor          code to build a rectangle      public Rect           2nd constructor this  0 0 width height     call 1st constructor  because it has   4 int args     this is another way to build a rectangle      public class DrawableRect extends Rect    public DrawableRect  int a1  int b1  int a2  int b2    super  a1 b1 a2 b2     call super class constructor  Rect class

[java] Difference between "this" and"super" keywords in Java

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