Comp1004: Building Better Objects I

Comp1004: Building Better Objects IMethods

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;

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


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


int balance = 100;

public static void main(String[] args){Account myAccountObject = new Account();myAccountObject.withdraw(5);myAccountObject.withdraw(10);

}


}}

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


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’);

}


}}

Each parameter is typed. You will get a compiler error if you try and pass a method a value of the wrong type


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’);

}


}}

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


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

Parameters, Primitives and Objects

b

Elephant

a

int

10int a;a = 10;

Elephant b;b = new Elephant();

Parameters, Primitives and Objectspublic 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?









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










visitors

int

0










visitors

int

0

v

int

0










visitors

int

0

v

int

1










visitors

int

0










visitors

int

0

So this line will print “0”









elephant is an object reference, so when it is sent to a method it is pass by reference










elephant

Elephant










elephant

Elephant

e

Elephant










elephant

Elephant

e

Elephant










elephant

Elephant

So this line will print “false”

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

Return Types

Methods as Functions

• One way to think about methods is like mathematical functions

FunctionInputs Output

Return typespublic 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?




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?




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





}


}

}

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!)

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....

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

Overloading

public class CashRegister{




}


}

}

Variations on a Method

What if we wanted to pass the VAT rate as one of the parameters?




p1 = reg.calcVAT(p1);p1 = reg.calcVAT(p1, 0.175);System.out.println(p1);

}


}

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

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

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




p1 = reg.calcVAT(p1);p1 = reg.calcVAT(p1, 0.175);System.out.println(p1);

}


}

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

Overloading

Summary

• Methods and Parameters– Why Parameterise?– Call by value, call by reference

• Return Types– Methods as a Function

• Overloading

Documents

Comp1004: Building Better Objects I