How O bjects Behave

1. How Objects Behave

2. Overview • Methods use object state • Arguments and return types in methods • Java passes by value • Getters and Setters • Encapsulation • Using references in an Array

3. Methods use object state • In a non-OOP language, functions need to have lots of parameters • Alternatively, you can have lots of functions • Using methods, many objects contain instance variables that can be used to change methods.

4. Example if (dog_a_weight < 14){ small_bark(); } else { large_bark(); } • We had to have more functions for each ‘state’ of dog. We would also need to do this for each dog!

5. Java Example • The Java example on the webpage shows how a dog’s barks changes with size

6. Arguments and return types in methods • Just like in C functions, Java methods can take in arguments and return values • The syntax is identical to C • E.g. int somefunction(int somevalue){ //Some code that uses the variable somevalue return 45; // Returns 45 }

7. Arguments and return types in methods • You can send multiple arguments as well • The order of the arguments matters void someotherfunction(int x, int y)

8. Java passes by value • When calling a method, a copy of each argument is made for the function • In C, you can pass pointers where the function manipulates the original value • If you want a function to manipulate a value in Java, the value needs to be returned, or be an instance variable

9. Getters and Setters • Getters = Accessors • Setters = Mutators • Getters and Setters are simple methods that let programs manipulate an objects instance variables • Why does this matter? Cant we access these values already with the dot operator?

10. Encapsulation • This is the concept of keeping our data safe so that only valid value can be put in instance variables • We set instance variables to private • We set setters and getters to public

11. Example • Lets modify our previous example to use the concept of encapsulation • We will even add some simple error checking

12. Instance Variables vs. Local Variables • Just like in C, where variables are defined sets their ‘scope’ • Instance variables are accessible by all methods and are declared in the class outside of any method • Local variables only apply to one method and are defined at the top of the method they are used in

13. Comparing Objects • Simple comparison of primitives can be done with ‘==‘ • How can we do this with objects? • To see if two object reference variables are the same, still use ‘==‘ • To see of the instance values in two objects are the same, write an ‘equals()’ method and use it