1. Comp1004: Building Better Objects I Methods

2. Coming up Methods and Parameters – Why Parameterise? – Call by value, call by reference Return Types – Methods as a Function Overloading

3. Methods and Parameters

4. Why Parameterise? public class Account{ int balance = 100; public static void main(String[] args){ Account myAccountObject = new Account(); myAccountObject.withdrawFiver(); myAccountObject.withdrawTenner(); } public void withdrawFiver(){ balance = balance - 5; } public void withdrawTenner(){ int tenner = 10; balance = balance – 10; } These two methods do almost the same thing. It is wasteful (inelegant?) to write them twice

5. Why Parameterise? public class Account{ int balance = 100; public static void main(String[] args){ Account myAccountObject = new Account(); myAccountObject.withdraw(5); myAccountObject.withdraw(10); } public void withdraw(int amount){ balance = balance - amount; } They can be replaced by a single method that behaves differently depending on what values are passed to it

6. Why Parameterise? public class Account{ int balance = 100; public static void main(String[] args){ Account myAccountObject = new Account(); myAccountObject.withdraw(5); myAccountObject.withdraw(10); } public void withdraw(int amount){ balance = balance - amount; } Values received by a method are called parameters. Within the method they can be used like any other local variable Values passed into a method are called arguments

7. Why Parameterise? public class Account{ int balance = 100; public static void main(String[] args){ Account myAccountObject = new Account(); myAccountObject.withdraw(5); myAccountObject.withdraw(10); myAccountObject.withdraw(“ten pounds”); myAccountObject.withdraw(‘5’); } public void withdraw(int amount){ balance = balance - amount; } Each parameter is typed. You will get a compiler error if you try and pass a method a value of the wrong type

8. Why Parameterise? public class Account{ int balance = 100; public static void main(String[] args){ Account myAccountObject = new Account(); myAccountObject.withdraw(5); myAccountObject.withdraw(10); myAccountObject.withdraw(“ten pounds”); myAccountObject.withdraw(‘5’); } public void withdraw(int amount){ balance = balance - amount; } Each parameter is typed. You will get a compiler error if you try and pass a method a value of the wrong type “ten pounds” is of type String ‘5’ is of type char So these lines will not compile

9. Why Parameterise? public class Account{ int balance = 100; public static void main(String[] args){ Account myAccountObject = new Account(); myAccountObject.withdraw(5, “Soton Uni Shop”); myAccountObject.withdraw(10, “ATM”); } public void withdraw(int amount, String desc){ balance = balance - amount; System.out.print(“Withdrew £”); System.out.print(amount); System.out.print(“ via ”); System.out.println(desc); } Methods can take multiple parameters Each is separated by a comma, and has its own name and type

10. Parameters, Primitives and Objects b Elephant a int int a; a = 10; Elephant b; b = new Elephant();

11. Parameters, Primitives and Objects public class Zoo{ public static void main(String[] args){ int visitors = 0; Elephant elephant = new Elephant(); incVisitors(visitors); feedElephant(elephant); System.out.println(visitors); System.out.println(elephant.isHungry()); } public void incVisitors(int v) { if(zooIsOpen()) v++; } public void feedElephant(Elephant e) { if(zooHasFood()) e.feed(); } //some code omitted } Assuming that isHungry returns true or false depending on whether the elephant has been fed, and that the zoo is open and has food - what will be printed here?

12. Parameters, Primitives and Objects public class Zoo{ public static void main(String[] args){ int visitors = 0; Elephant elephant = new Elephant(); incVisitors(visitors); feedElephant(elephant); System.out.println(visitors); System.out.println(elephant.isHungry()); } public void incVisitors(int v) { if(zooIsOpen()) v++; } public void feedElephant(Elephant e) { if(zooHasFood()) e.feed(); } //some code omitted } Visitors is a primitive, so when it is sent to a method it is pass by value

13. Parameters, Primitives and Objects public class Zoo{ public static void main(String[] args){ int visitors = 0; Elephant elephant = new Elephant(); incVisitors(visitors); feedElephant(elephant); System.out.println(visitors); System.out.println(elephant.isHungry()); } public void incVisitors(int v) { if(zooIsOpen()) v++; } public void feedElephant(Elephant e) { if(zooHasFood()) e.feed(); } //some code omitted } Visitors is a primitive, so when it is sent to a method it is pass by value visitors int

14. Parameters, Primitives and Objects public class Zoo{ public static void main(String[] args){ int visitors = 0; Elephant elephant = new Elephant(); incVisitors(visitors); feedElephant(elephant); System.out.println(visitors); System.out.println(elephant.isHungry()); } public void incVisitors(int v) { if(zooIsOpen()) v++; } public void feedElephant(Elephant e) { if(zooHasFood()) e.feed(); } //some code omitted } Visitors is a primitive, so when it is sent to a method it is pass by value visitors int v

15. Parameters, Primitives and Objects public class Zoo{ public static void main(String[] args){ int visitors = 0; Elephant elephant = new Elephant(); incVisitors(visitors); feedElephant(elephant); System.out.println(visitors); System.out.println(elephant.isHungry()); } public void incVisitors(int v) { if(zooIsOpen()) v++; } public void feedElephant(Elephant e) { if(zooHasFood()) e.feed(); } //some code omitted } Visitors is a primitive, so when it is sent to a method it is pass by value visitors int v

16. Parameters, Primitives and Objects public class Zoo{ public static void main(String[] args){ int visitors = 0; Elephant elephant = new Elephant(); incVisitors(visitors); feedElephant(elephant); System.out.println(visitors); System.out.println(elephant.isHungry()); } public void incVisitors(int v) { if(zooIsOpen()) v++; } public void feedElephant(Elephant e) { if(zooHasFood()) e.feed(); } //some code omitted } Visitors is a primitive, so when it is sent to a method it is pass by value visitors int

17. Parameters, Primitives and Objects public class Zoo{ public static void main(String[] args){ int visitors = 0; Elephant elephant = new Elephant(); incVisitors(visitors); feedElephant(elephant); System.out.println(visitors); System.out.println(elephant.isHungry()); } public void incVisitors(int v) { if(zooIsOpen()) v++; } public void feedElephant(Elephant e) { if(zooHasFood()) e.feed(); } //some code omitted } Visitors is a primitive, so when it is sent to a method it is pass by value visitors int So this line will print “0”

18. Parameters, Primitives and Objects public class Zoo{ public static void main(String[] args){ int visitors = 0; Elephant elephant = new Elephant(); incVisitors(visitors); feedElephant(elephant); System.out.println(visitors); System.out.println(elephant.isHungry()); } public void incVisitors(int v) { if(zooIsOpen()) v++; } public void feedElephant(Elephant e) { if(zooHasFood()) e.feed(); } //some code omitted } elephant is an object reference, so when it is sent to a method it is pass by reference

19. Parameters, Primitives and Objects public class Zoo{ public static void main(String[] args){ int visitors = 0; Elephant elephant = new Elephant(); incVisitors(visitors); feedElephant(elephant); System.out.println(visitors); System.out.println(elephant.isHungry()); } public void incVisitors(int v) { if(zooIsOpen()) v++; } public void feedElephant(Elephant e) { if(zooHasFood()) e.feed(); } //some code omitted } elephant is an object reference, so when it is sent to a method it is pass by reference elephant Elephant

20. Parameters, Primitives and Objects public class Zoo{ public static void main(String[] args){ int visitors = 0; Elephant elephant = new Elephant(); incVisitors(visitors); feedElephant(elephant); System.out.println(visitors); System.out.println(elephant.isHungry()); } public void incVisitors(int v) { if(zooIsOpen()) v++; } public void feedElephant(Elephant e) { if(zooHasFood()) e.feed(); } //some code omitted } elephant is an object reference, so when it is sent to a method it is pass by reference elephant Elephant e

21. Parameters, Primitives and Objects public class Zoo{ public static void main(String[] args){ int visitors = 0; Elephant elephant = new Elephant(); incVisitors(visitors); feedElephant(elephant); System.out.println(visitors); System.out.println(elephant.isHungry()); } public void incVisitors(int v) { if(zooIsOpen()) v++; } public void feedElephant(Elephant e) { if(zooHasFood()) e.feed(); } //some code omitted } elephant is an object reference, so when it is sent to a method it is pass by reference elephant Elephant e

22. Parameters, Primitives and Objects public class Zoo{ public static void main(String[] args){ int visitors = 0; Elephant elephant = new Elephant(); incVisitors(visitors); feedElephant(elephant); System.out.println(visitors); System.out.println(elephant.isHungry()); } public void incVisitors(int v) { if(zooIsOpen()) v++; } public void feedElephant(Elephant e) { if(zooHasFood()) e.feed(); } //some code omitted } elephant is an object reference, so when it is sent to a method it is pass by reference elephant Elephant So this line will print “false”

23. Sometimes (often), we (everyone) gets lazy and says we pass a method an object. This really means we pass that object’s reference. Just so you know

24. Return Types

25. Methods as Functions One way to think about methods is like mathematical functions Function InputsOutput

26. Return types public class CashRegister{ public static void main(String[] args){ CashRegister reg = new CashRegister(); float p1 = 10.0; reg.calcVAT(p1); System.out.println(p1); } public void calcVAT(float price) { price = price * 1.2; } What will happen?

27. Return types public class CashRegister{ public static void main(String[] args){ CashRegister reg = new CashRegister(); float p1 = 10.0; reg.calcVAT(p1); System.out.println(p1); } public void calcVAT(float price) { price = price * 1.2; } Because p1 is a float (a primitive) it is pass by value. So this line will not increase the value of p1. The program will print 10.0 on the screen. What will happen?

28. Return types public class CashRegister{ public static void main(String[] args){ CashRegister reg = new CashRegister(); float p1 = 10.0; p1 = reg.calcVAT(p1); System.out.println(p1); } public float calcVAT(float price) { return price * 1.2; } Instead we can specify a return type And use the return keyword to pass back a value to wherever the method was called

29. Return types public class CashRegister{ public static void main(String[] args){ CashRegister reg = new CashRegister(); float p1 = 10.0; p1 = reg.calcVAT(p1); System.out.println(p1); } public float calcVAT(float price) { return price * 1.2; } Instead we can specify a return type And use the return keyword to pass back a value to wherever the method was called Whatever called the method can then assign the return type to a variable (or do anything else with it!)

30. Can I return more than one thing? public int getAgeAndName(){ return age, name; } This is not legal Java Like a mathematical function you can only return one thing So there can only be one return type But....

31. Collections Later in the course we deal with collections (implemented as classes and objects) You can put many objects or primitives into collections So you could pass or return a collection from a method in order to process many values at once

32. Overloading

33. public class CashRegister{ public static void main(String[] args){ CashRegister reg = new CashRegister(); float p1 = 10.0; p1 = reg.calcVAT(p1); System.out.println(p1); } public float calcVAT(float price) { return price * 1.2; } Variations on a Method What if we wanted to pass the VAT rate as one of the parameters?

34. public class CashRegister{ public static void main(String[] args){ CashRegister reg = new CashRegister(); float p1 = 10.0; p1 = reg.calcVAT(p1); p1 = reg.calcVAT(p1, 0.175); System.out.println(p1); } public float calcVAT(float price) { return price * 1.2; } public float calcVAT(float price, float rate) { return price * (1.0 + rate); } Variations on a Method What if we wanted to pass the VAT rate as one of the parameters? We could add it as a second parameter. N.B. that the two methods have the same name

35. This is called Overloading A method is recognised by its signature – (its name, parameters and the order of parameters) When overloading each method must have a unique signature – Remember that the return type is NOT part of the signature – float calcVAT(float price) – float calcVAT(float price, float rate) OK – int calcVAT(float price, char band) OK

36. This is called Overloading A method is recognised by its signature – (its name, parameters and the order of parameters) When overloading each method must have a unique signature – Remember that the return type is NOT part of the signature – float calcVAT(float price) – float calcVAT(float price, float rate) OK – int calcVAT(float price, char band) OK – int calcVAT(float price) not OK clashes

37. public class CashRegister{ public static void main(String[] args){ CashRegister reg = new CashRegister(); float p1 = 10.0; p1 = reg.calcVAT(p1); p1 = reg.calcVAT(p1, 0.175); System.out.println(p1); } public float calcVAT(float price) { return price * 1.2; } public float calcVAT(float price, float rate) { return price * (1.0 + rate); } Variations on a Method When a method is called Java invokes the method with the matching signature Overloading

38. Summary Methods and Parameters – Why Parameterise? – Call by value, call by reference Return Types – Methods as a Function Overloading