Default Values and the ClassRoom Class

In lab, you were asked to execute a program like this:

using System;
 
class Program
{
  static void Main(string[] args)
  {
    Rectangle myRect = new Rectangle();
    Console.WriteLine($"Length is {myRect.GetLength()}");
    Console.WriteLine($"Width is {myRect.GetWidth()}");
  }
}

Note that we create a Rectangle object, but do not use the SetLength or SetWidth methods to assign values to its instance variables. It displays the following output:

Length is 0
Width is 0

This works because the instance variables length and width have a default value of 0, even if you never assign them a value
Local variables, like the ones we write in the Main method, do not have default values. You must assign them a value before using them in an expression.
- For example, this code will produce a compile error:
```
int myVar1;
int myVar2 = myVar1 + 5;
```
  You cannot assume myVar1 will be 0; it has no value at all until you use an assignment statement.
When you create (instantiate) a new object, its instance variables will be assigned specific default values based on their type:

Type Default Value
Numeric types 0
string null
objects null
bool false
char '\0'
Remember, null is the value of a reference-type variable that refers to “nothing” - it does not contain the location of any object at all. You cannot do anything with a reference variable containing null.

Type	Default Value
Numeric types	0
`string`	`null`
objects	`null`
`bool`	`false`
`char`	`'\0'`

A class we will use for subsequent examples

ClassRoom: Represents a room in a building on campus
UML Diagram:
- There are two attributes: the name of the building (a string) and the room number (an int)
- Each attribute will have a “getter” and “setter” method
Implementation:
```
class ClassRoom
{
  private string building;
  private int number;
 
  public void SetBuilding(string buildingParam)
  {
    building = buildingParam;
  }
 
  public string GetBuilding()
  {
    return building;
  }
 
  public void SetNumber(int numberParam)
  {
    number = numberParam;
  }
 
  public int GetNumber()
  {
    return number;
  }
}
```
- Each attribute is implemented by an instance variable with the same name
- To write the “setter” for the building attribute, we write a method whose return type is void, with a single string-type parameter. Its body assigns the building instance variable to the value in the parameter buildingParam
- To write the “getter” for the building attribute, we write a method whose return type is string, and whose body returns the instance variable building

Creating an object and using its default values:

using System;
 
class Program
{
  static void Main(string[] args)
  {
    ClassRoom english = new ClassRoom();
    Console.WriteLine(
      $"Building is {english.GetBuilding()}"
    );
    Console.WriteLine(
      $"Room number is {english.GetNumber()}"
    );
  }
}

This will print the following output:

Building is
Room number is 0

Remember that the default value of a string variable is null. When you use string interpolation on null, you get an empty string.

Constructors

Instantiation syntax requires you to write parentheses after the name of the class, like this:
```
ClassRoom english = new ClassRoom();
```
Parentheses indicate a method call, like in Console.ReadLine() or english.GetBuilding()
In fact, the instantiation statement new ClassRoom() does call a method: the constructor
Constructor: A special method used to create an object. It “sets up” a new instance by initializing its instance variables.
If you do not write a constructor in your class, C# will generate a “default” constructor for you – this is what’s getting called when we write new ClassRoom() here
The default constructor initializes each instance variable to its default value – that’s where default values come from

Writing a constructor

Example for ClassRoom:

public ClassRoom(string buildingParam, int numberParam)
{
    building = buildingParam;
    number = numberParam;
}

To write a constructor, write a method whose name is exactly the same as the class name
This method has no return type, not even void. It does not have a return statement either
For ClassRoom, this means the constructor’s header starts with public ClassRoom
- You can think of this method as “combining” the return type and name. The name of the method is ClassRoom, and its output is of type ClassRoom, since the return value of new ClassRoom() is always a ClassRoom object
- You do not actually write a return statement, though, because new will always return the new object after calling the constructor
A custom constructor usually has parameters that correspond to the instance variables: for ClassRoom, it has a string parameter named buildingParam, and an int parameter named numberParam
- Note that when we write a method with two parameters, we separate the parameters with a comma
The body of a constructor must assign values to all instance variables in the object
Usually this means assigning each parameter to its corresponding instance variable: initialize the instance variable to equal the parameter
- Very similar to calling both “setters” at once
Using a constructor
An instantiation statement will call a constructor for the class being instantiated
Arguments in parentheses must match the parameters of the constructor

Example with the ClassRoom constructor:

using System;
 
class Program
{
  static void Main(string[] args)
  {
    ClassRoom csci = new ClassRoom("Allgood East", 356);
    Console.WriteLine($"Building is {csci.GetBuilding()}");
    Console.WriteLine($"Room number is {csci.GetNumber()}");
  }
}

This program will produce this output:

Building is Allgood East
Room number is 356

The instantiation statement new ClassRoom("Allgood East", 356) first creates a new “empty” object of type ClassRoom, then calls the constructor to initialize it. The first argument, “Allgood East”, becomes the constructor’s first parameter (buildingParam), and the second argument, 356, becomes the constructor’s second parameter (numberParam).
After executing the instantiation statement, the object referred to by csci has its instance variables set to these values, even though we never called SetBuilding or SetNumber

Methods with multiple parameters

The constructor we wrote is an example of a method with two parameters
The same syntax can be used for ordinary, non-constructor methods, if we need more than one input value

For example, we could write this method in the Rectangle class:

public void MultiplyBoth(int lengthFactor, int widthFactor)
{
    length *= lengthFactor;
    width *= widthFactor;
}

The first parameter has type int and is named lengthFactor. The second parameter has type int and is named widthFactor
You can call this method by providing two arguments, separated by a comma:
```
Rectangle myRect = new Rectangle();
myRect.SetLength(5);
myRect.SetWidth(10);
myRect.MultiplyBoth(3, 5);
```
The first argument, 3, will be assigned to the first parameter, lengthFactor. The second argument, 5, will be assigned to the second parameter, widthFactor
The order of the arguments matters when calling a multi-parameter method. If you write myRect.MultiplyBoth(5, 3), then lengthFactor will be 5 and widthFactor will be 3.
The type of each argument must match the type of the corresponding parameter. For example, when you call the ClassRoom constructor we just wrote, the first argument must be a string and the second argument must be an int

Writing multiple constructors

Remember that if you do not write a constructor, C# generates a “default” one with no parameters, so you can write new ClassRoom()
Once you add a constructor to your class, C# will not generate a default constructor
- This means once we write the ClassRoom constructor (as shown earlier), this statement will produce a compile error: ClassRoom english = new ClassRoom();
- The constructor we wrote has 2 parameters, so now you always need 2 arguments to instantiate a ClassRoom
If you still want the option to create an object with no arguments (i.e. new ClassRoom()), you must write a constructor with no parameters

A class can have more than one constructor, so it would look like this:

class ClassRoom
{
    //...
    public ClassRoom(string buildingParam, int numberParam)
    {
        building = buildingParam;
        number = numberParam;
    }
    public ClassRoom()
    {
        building = null;
        number = 0;
    }
    //...
}

The “no-argument” constructor must still initialize all the instance variables, even though it has no parameters
- You can pick any “default value” you want, or use the same ones that C# would use (0 for numeric variables, null for object variables, etc.)
When a class has multiple constructors, the instantiation statement must decide which constructor to call
The instantiation statement will call the constructor whose parameters match the arguments you provide
- For example, each of these statements will call a different constructor:
```
ClassRoom csci = new ClassRoom("Allgood East", 356);
ClassRoom english = new ClassRoom();
```
  The first statement calls the two-parameter constructor we wrote, since it has a string argument and an int argument (in that order), and those match the parameters (string buildingParam, int numberParam). The second statement calls the zero-parameter constructor since it has no arguments.
- If the arguments do not match any constructor, it is still an error:
```
ClassRoom csci = new ClassRoom(356, "Allgood East");
```
  This will produce a compile error, because the instantiation statement has two arguments in the order int, string, but the only constructor with two parameters needs the first parameter to be a string.

Writing `ToString` Methods

ToString recap
- String interpolation automatically calls the ToString method on each variable or value
- ToString returns a string “equivalent” to the object; for example, if num is an int variable containing 42, num.ToString() returns “42”.
- C# datatypes already have a ToString method, but you need to write a ToString method for your own classes to use them in string interpolation
Writing a ToString method
- To add a ToString method to your class, you must write this header: public override string ToString()
- The access modifier must be public (so other code, like string interpolation, can call it)
- The return type must be string (ToString must output a string)
- It must have no parameters (the string interpolation code will not know what arguments to supply)
- The keyword override means your class is “overriding,” or providing its own version of, a method that is already defined elsewhere – ToString is defined by the base object type, which is why string interpolation “knows” it can call ToString on any object
  - If you do not use the keyword override, then the pre-existing ToString method (defined by the base object type) will be used instead, which only returns the name of the class
- The goal of ToString is to return a “string representation” of the object, so the body of the method should use all of the object’s attributes and combine them into a string somehow
- Example ToString method for ClassRoom:
```
public override string ToString()
{
    return building + " " + number;
}
```
  - There are two instance variables, building and number, and we use both of them
  - A natural way to write the name of a classroom is the building name followed by the room number, like “University Hall 124”, so we concatenate the variables in that order
  - Note that we add a space between the variables
  - Note that building is already a string, but number is an int, so string concatenation will implicitly call number.ToString() – ToString methods can call other ToString methods
  - Another way to write the body would be return $"{building} {number}";
Using a ToString method
- Any time an object is used in string interpolation or concatenation, its ToString method will be called
- You can also call ToString by name using the “dot operator,” like any other method
- This code will call the ToString method we just wrote for ClassRoom:
```
ClassRoom csci = new ClassRoom("Allgood East", 356);
Console.WriteLine(csci);
Console.WriteLine($"The classroom is {csci}");
Console.WriteLine("The classroom is " + csci.ToString());
```

Method Signatures and Overloading

Name uniqueness in C#

In general, variables, methods, and classes must have unique names, but there are several exceptions
Variables can have the same name if they are in different scopes
- Two methods can each have a local variable with the same name
- A local variable (scope limited to the method) can have the same name as an instance variable (scope includes the whole class), but this will result in shadowing
Classes can have the same name if they are in different namespaces
- This is one reason C# has namespaces: you can name your classes anything you want. Otherwise, if a library (someone else’s code) used a class name, you would be prevented from using that name
- For example, imagine you were using a “shapes library” that provided a class named Rectangle, but you also wanted to write your own class named Rectangle
- The library’s code would use its own namespace, like this:
```
namespace ShapesLibrary
{
    class Rectangle
    {
        //instance variables, methods, etc.
    }
}
```
  Then your own code could have a Rectangle class in your own namespace:
```
namespace MyProject
{
    class Rectangle
    {
        //instance variables, methods, etc.
    }
}
```
- You can use both Rectangle classes in the same code, as long as you specify the namespace, like this:
```
MyProject.Rectangle rect1 = new MyProject.Rectangle();
ShapesLibrary.Rectangle rect2 = new ShapesLibrary.Rectangle();
```
Methods can have the same name if they have different signatures; this is called overloading
- We’ll explain signatures in more detail in a minute
- Briefly, methods can have the same name if they have different parameters
- For example, you can have two methods named Multiply in the Rectangle class, as long as one has one parameter and the other has two parameters:
```
public void Multiply(int factor)
{
    length *= factor;
    width *= factor;
}
public void Multiply(int lengthFactor, int widthFactor)
{
    length *= lengthFactor;
    width *= widthFactor;
}
```
  C# understands that these are different methods, even though they have the same name, because their parameters are different. If you write myRect.Multiply(2) it can only mean the first “Multiply” method, not the second one, because there is only one argument.
- We have used overloading already when we wrote multiple constructors – constructors are methods too. For example, these two constructors have the same name, but different parameters:
```
public ClassRoom(string buildingParam, int numberParam)
{
    building = buildingParam;
    number = numberParam;
}
public ClassRoom()
{
    building = null;
    number = 0;
}
```

Method signatures

A method’s signature has 3 components: its name, the type of each parameter, and the order the parameters appear in
Methods are unique if their signatures are unique, which is why they can have the same name
Signature examples:
- public void Multiply(int lengthFactor, int widthFactor) – the signature is Multiply(int, int) (name is Multiply, parameters are int and int type)
- public void Multiply(int factor) – signature is Multiply(int)
- public void Multiply(double factor) – signature is Multiply(double)
- These could all be in the same class since they all have different signatures
Parameter names are not part of the signature, just their types
- Note that the parameter names are omitted when I write down the signature
- That means these two methods are not unique and could not be in the same class:
```
public void SetWidth(int widthInMeters)
{
    //...
}
public void SetWidth(int widthInFeet)
{
    //...
}
```
  Both have the same signature, SetWidth(int), even though the parameters have different names. You might intend the parameters to be different (i.e. represent feet vs. meters), but any int-type parameter is the same to C#
The method’s return type is not part of the signature
- So far all the examples have the same return type (void), but changing it would not change the signature
- The signature of public int Multiply(int factor) is Multiply(int), which is the same as public void Multiply(int factor)
- The signature “begins” with the name of the method; everything “before” that does not count (i.e. public, int)
The order of parameters is part of the signature, as long as the types are different
- Since parameter name is not part of the signature, only the type can determine the order
- These two methods have different signatures:
```
public int Update(int number, string name)
{
    //...
}
public int Update(string name, int number)
{
    //..
}
```
  The signature of the first method is Update(int, string). The signature of the second method is Update(string, int).
- These two methods have the same signature, and could not be in the same class:
```
public void Multiply(int lengthFactor, int widthFactor)
{
    //...
}
public void Multiply(int widthFactor, int lengthFactor)
{
    //...
}
```
  The signature for both methods is Multiply(int, int), even though we switched the order of the parameters – the name does not count, and they are both int type
Constructors have signatures too
- The constructor ClassRoom(string buildingParam, int numberParam) has the signature ClassRoom(string, int)
- The constructor ClassRoom() has the signature ClassRoom()
- Constructors all have the same name, but they are unique if their signatures (parameters) are different

Calling overloaded methods

Previously, when you used the dot operator and wrote the name of a method, the name was enough to determine which method to execute – myRect.GetLength() would call the GetLength method
When a method is overloaded, you must use the entire signature to determine which method gets executed
A method call has a “signature” too: the name of the method, and the type and order of the arguments
C# will execute the method whose signature matches the signature of the method call
Example: myRect.Multiply(4); has the signature Multiply(int), so C# will look for a method in the Rectangle class that has the signature Multiply(int). This matches the method public void Multiply(int factor)
Example: myRect.Multiply(3, 5); has the signature Multiply(int, int), so C# will look for a method with that signature in the Rectangle class. This matches the method public void Multiply(int lengthFactor, int widthFactor)
The same process happens when you instantiate a class with multiple constructors: C# calls the constructor whose signature matches the signature of the instantiation
If no method or constructor matches the signature of the method call, you get a compile error. You still cannot write myRect.Multiply(1.5) if there is no method whose signature is Multiply(double).

Constructors in UML

Now that we can write constructors, they should be part of the UML diagram of a class
- No need to include the default constructor, or one you write yourself that takes no arguments
- Non-default constructors go in the operations section (box 3) of the UML diagram
- Similar syntax to a method: [+/-] <<constructor>> [name]([parameter name]: [parameter type])
- Note that the name will always match the class name
- No return type, ever
- Annotation “<<constructor>>” is nice, but not necessary: if the method name matches the class name, it is a constructor
Example for ClassRoom:

ℙrinℂomℙ

Explorer

More Advanced Object Concepts

Default Values and the ClassRoom Class

A class we will use for subsequent examples

Constructors

Writing a constructor

Methods with multiple parameters

Writing multiple constructors

Writing `ToString` Methods

Method Signatures and Overloading

Name uniqueness in C#

Method signatures

Calling overloaded methods

Constructors in UML

Graph View

Table of Contents

Backlinks

ℙrinℂomℙ

Explorer

More Advanced Object Concepts

Default Values and the ClassRoom Class

A class we will use for subsequent examples

Constructors

Writing a constructor

Methods with multiple parameters

Writing multiple constructors

Writing ToString Methods

Method Signatures and Overloading

Name uniqueness in C#

Method signatures

Calling overloaded methods

Constructors in UML

Graph View

Table of Contents

Backlinks

Writing `ToString` Methods