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
2. The toString Method
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.
4. Javier Gonzalez-Sanchez | CSE110 | Summer 2020 | 4 Example
5. Javier Gonzalez-Sanchez | CSE110 | Summer 2020 | 5 Example
6. Previously Getters and Setters
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; } }
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; } }
9. Javier Gonzalez-Sanchez | CSE110 | Summer 2020 | 9 Getters and Setters Do this: example.setNumber(1); System.out.println( example.getNumber() );
10. Javier Gonzalez-Sanchez | CSE110 | Summer 2020 | 10 Example
11. Javier Gonzalez-Sanchez | CSE110 | Summer 2020 | 11 Example
12. The null reference
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.
14. Javier Gonzalez-Sanchez | CSE110 | Summer 2020 | 14 Example
15. Javier Gonzalez-Sanchez | CSE110 | Summer 2020 | 15 Example
16. The this keyword
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.
18. Javier Gonzalez-Sanchez | CSE110 | Summer 2020 | 18 Example
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.