CSE110 Lecture 20

Principles of Programming with Java
null and this
(202006)

Transcript

1. CSE110
   Principles of Programming
   with Java
   Lecture 20:
   toString method, null and this
   Javier Gonzalez-Sanchez
   javiergs@asu.edu
   javiergs.engineering.asu.edu | javiergs.com
   Office Hours: By appointment
   

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2. The toString Method
   

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3. Javier Gonzalez-Sanchez | CSE110 | Summer 2020 | 3
   The toString Method
   • The toString method is a method that takes no
   parameter and returns a string
   • A returned string usually contains information on
   instance variables of its class.
   • Each class has a default toString method that
   contains its class object name and hash number.
   • When an object is used with System.out.println
   method, its toString method will be called.
   

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4. Javier Gonzalez-Sanchez | CSE110 | Summer 2020 | 4
   Example
   

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5. Javier Gonzalez-Sanchez | CSE110 | Summer 2020 | 5
   Example
   

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6. Previously
   Getters and Setters
   

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7. Javier Gonzalez-Sanchez | CSE110 | Summer 2020 | 7
   Getters and Setters
   public class Example {
   private int number;
   //Getter or Accessor method
   public int getNumber() {
   return number;
   }
   //Setter or Mutator method
   public void setNumber(int numberFromOutside){
   number = numberFromOutside;
   }
   }
   

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8. Javier Gonzalez-Sanchez | CSE110 | Summer 2020 | 8
   Getters and Setters
   public class Example {
   private int number;
   //Getter or Accessor method
   public int getNumber() {
   return number;
   }
   //Setter or Mutator method
   public void setNumber(int numberFromOutside){
   number = numberFromOutside;
   }
   }
   

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9. Javier Gonzalez-Sanchez | CSE110 | Summer 2020 | 9
   Getters and Setters
   Do this:
   example.setNumber(1);
   System.out.println( example.getNumber() );
   

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10. Javier Gonzalez-Sanchez | CSE110 | Summer 2020 | 10
    Example
    

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11. Javier Gonzalez-Sanchez | CSE110 | Summer 2020 | 11
    Example
    

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12. The null reference
    

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13. Javier Gonzalez-Sanchez | CSE110 | Summer 2020 | 13
    null
    • The null reference – the reference variable that
    does not currently point to an object.
    • Any variable of a class that was not
    instantiated/created using its constructor is a null.
    • If a program tries to access methods or variables,
    then you will get NullPointerException error during
    the execution of the program. In that case, check
    your program to see if it called a constructor to
    instantiate it.
    

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14. Javier Gonzalez-Sanchez | CSE110 | Summer 2020 | 14
    Example
    

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15. Javier Gonzalez-Sanchez | CSE110 | Summer 2020 | 15
    Example
    

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16. The this keyword
    

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17. Javier Gonzalez-Sanchez | CSE110 | Summer 2020 | 17
    this
    • The this reference refers to its class itself and can be
    used to refer to the instance variables of an object
    of the class.
    • You can create an instance variable and a local
    variable using a same name (compiles), but it is
    confusing!
    • In such case, local variables will be used, and
    instance variables will be ignored. We can use the
    this reference to refer to instance variables to
    distinguish them.
    

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18. Javier Gonzalez-Sanchez | CSE110 | Summer 2020 | 18
    Example
    

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19. CSE110 - Principles of Programming
    Javier Gonzalez-Sanchez
    javiergs@asu.edu
    Summer 2020
    Disclaimer. These slides can only be used as study material for the class CSE110 at ASU. They cannot be distributed or used for another purpose.
    

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