1 / 38

Comp1004: Building Better Objects I

Comp1004: Building Better Objects I. Methods. Coming up. Methods and Parameters Why Parameterise? Call by value, call by reference Return Types Methods as a Function Overloading. Methods and Parameters. Why Parameterise?. public class Account{ int balance = 100;

dafydd
Download Presentation

Comp1004: Building Better Objects I

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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 passed into a method are called arguments Values received by a method are called parameters. Within the method they can be used like any other local variable

  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, “SotonUni 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 a int a; a = 10; Elephant b; b = new Elephant(); 10 Elephant int

  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 Visitors is a primitive, so when it is sent to a method it is pass by value 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 }

  13. Parameters, Primitives and Objects Visitors is a primitive, so when it is sent to a method it is pass by value visitors 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 } 0 int

  14. Parameters, Primitives and Objects Visitors is a primitive, so when it is sent to a method it is pass by value visitors v 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 } 0 int int 0

  15. Parameters, Primitives and Objects Visitors is a primitive, so when it is sent to a method it is pass by value visitors v 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 } 0 int int 1

  16. Parameters, Primitives and Objects Visitors is a primitive, so when it is sent to a method it is pass by value visitors 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 } 0 int

  17. Parameters, Primitives and Objects Visitors is a primitive, so when it is sent to a method it is pass by value visitors 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 } 0 int So this line will print “0”

  18. Parameters, Primitives and Objects elephant is an object reference, so when it is sent to a method it is pass by reference 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 }

  19. Parameters, Primitives and Objects elephant is an object reference, so when it is sent to a method it is pass by reference elephant 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

  20. Parameters, Primitives and Objects elephant is an object reference, so when it is sent to a method it is pass by reference elephant e 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 Elephant

  21. Parameters, Primitives and Objects elephant is an object reference, so when it is sent to a method it is pass by reference elephant e 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 Elephant

  22. Parameters, Primitives and Objects elephant is an object reference, so when it is sent to a method it is pass by reference elephant 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 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 Inputs Output

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

  27. Return types What will happen? public class CashRegister{ public static void main(String[] args){ CashRegisterreg = 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.

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

  34. Variations on a Method public class CashRegister{ public static void main(String[] args){ CashRegisterreg = 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); } } 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 • intcalcVAT(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 • intcalcVAT(float price, char band) OK • intcalcVAT(float price) not OK clashes

  37. Overloading Variations on a Method public class CashRegister{ public static void main(String[] args){ CashRegisterreg = 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); } } When a method is called Java invokes the method with the matching signature

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

More Related