Week 4 - Methods and Messages

Question 1

What is a Javadoc comment?

Answer 1

It is a multi line comment that is used in the Java programming language, it allowed web page documentation for the method to be created automatically.
/**      
*
*/

Question 2

What are the three parts that make up the method header?

Answer 2

the first part is the access modifier - it tells the complier what other groups of objects can send the corresponding message.
the second part is the type of value returned - if any.
the final part is the name of the method, followed by ().

Question 3

The following is the method home() in the class Frog.

/**
*Resets the receiver to it "home" position of 1.
*/
public void home()
{
      this.setPosition(1);
} 

write down the comment, the method header and the method body.

Answer 3

a) /**
*Resets the receiver to it “home” position of 1.
*/
b) public void home()
c) {
this.setPosition(1);
}

Question 4

What is the difference between a message and a method?

Answer 4

A method is a piece of code that is executed when an object received a message. A message is the event that results from a message-send; it is a request doe an object to do something (execute a method).

Question 5

What do you need to do to add a new message to the protocol of an object?

Answer 5

Adding a message to the protocol requires editing the related class definition to add a new method and recompiling the class.

Question 6

How can an object send a message to itself?

Answer 6

In a method you can use the keyword this whenever you want to refer to the object currently executing the method. So this is a ‘self’ reference.

Question 7

Where and how must we place Javadoc comments?

Answer 7

Javadoc comments must be placed before the method header and be contained within a /** and */.

Question 8

What is run-time?

Answer 8

while a program is running if a message is sent to it it is said to be sent during run-time.

Question 9

Write down a Java statement to create an instance of the class Frog referenced by a newly declared variable called aFrog.

Answer 9

Frog aFrog = new Frog();

Question 10

a new emthod, yellow() is required, write the code for this.

Answer 10

/**
*Sets the colour of the receiver to Yellow
*/
public void yellow()
{
this.setColour(OUColour.YELLOW);
}

Question 11

What is the declared return type for the method getColour() in the class Frog?

Answer 11

The declared return type is OUColour.

Question 12

Write down the method header for a method that has the name myMethod(), has no arguments, and returns an int value. Objects of any class should be able to send the corresponding message.

Answer 12

public int myMethod()

Question 13

Write down the method header for a method that has the name myMethod() that is accessible to any object, has no arguments, and has no return value.

Answer 13

public void myMethod()

Question 14

Write down the receivers and the arguments of the following message-sends.

a) frog1.sameColourAs(frog3);
b) frog4.setPosition(3);

Answer 14

a) frog1 is the receiver and frog3 is the argument

b) frog4 is the receiver and 3 is the argument.

Question 15

What is the method signature?

Answer 15

It is the name of a method including the parentheses and the type of any arguments used.

For example, the signature of setPosition() is setPosition(int). The signature shows clearly the type of the expected argument.

Question 16

when we say that a method, or method name, has been overloaded, what do we mean?

Answer 16

When methods have the same name but different signatures are present. for example,

public void sameColourAs (Frog aFrog)
public void sameColourAs (Toad aToad)

Question 17

Why would these two methods not be allowed?

public void doSomething (int marks)
private int doSomething (int comission)

Answer 17

They have the same signature, namely doSomething(int). The access modifier and return type is not part of the signature, nor are the names of any formal arguments.

The Java compiler would report an error if you attempted to compile a class that defined two such methods.

Question 18

What are the signatures of the getColour() and setColour() methods in the Frog class?

Answer 18

Since the getter takes no arguments, its signature is getColor(). The setter has the signature setColour(OUColour).

Question 19

What is the signature of the method:

public void setPositionAndColour(int aPosition, OUColour aColour)

Answer 19

setPositionAndColour(int, OUColour)

note that the order of the arguments is important.

Question 20

Give two examples of class members.

Answer 20

Instance variables and methods are examples of what are called class members. Each instance variable and method is a class member.

Question 21

Summarise the the meaning of the access modifiers private and public.

Answer 21

• if a member is declared as private, only code within the class itself can directly access it;
• if a member is declared a public, code within any class (and in M250 code within the OUWorkspace) can directly access it.

Question 22

How is encapsulation implemented in Java?

Answer 22

In Java you can defined classes that are templates for the creation of objects. These objects then encapsulate both state and behaviour.

Question 23

How do you implement data hiding in Java?

Answer 23

Data Hiding is implemented by declaring instance variable as private.

Question 24

When does composition occur?

Answer 24

This occurs when objects of one class have instance variables that reference objects of other classes.