Classes and Objects

Constructors

A constructor initializes an object when it is created.

constructor A constructor runs when `new` creates an object.

Constructors

width
Constructors.cs
Replay: real traced execution (multi-file project)
using System;

class Rectangle
{
    public int Width;
    public int Height;

    public Rectangle(int width, int height)
    {
        Width = width;
        Height = height;
    }

    public int Area()
    {
        return Width * Height;
    }
}

class Program
{
    static void Main()
    {
        int width = 5;
        Rectangle box = new Rectangle(width, 4);
        int area = box.Area();

        Console.WriteLine($"width={box.Width}");
        Console.WriteLine($"area={area}");
    }
}

using System;

class Rectangle
{
    public int Width;
    public int Height;

    public Rectangle(int width, int height)
    {
        Width = width;
        Height = height;
    }

    public int Area()
    {
        return Width * Height;
    }
}

class Program
{
    static void Main()
    {
        int width = 3;
        Rectangle box = new Rectangle(width, 4);
        int area = box.Area();

        Console.WriteLine($"width={box.Width}");
        Console.WriteLine($"area={area}");
    }
}

using System;

class Rectangle
{
    public int Width;
    public int Height;

    public Rectangle(int width, int height)
    {
        Width = width;
        Height = height;
    }

    public int Area()
    {
        return Width * Height;
    }
}

class Program
{
    static void Main()
    {
        int width = 7;
        Rectangle box = new Rectangle(width, 4);
        int area = box.Area();

        Console.WriteLine($"width={box.Width}");
        Console.WriteLine($"area={area}");
    }
}

  1. width ← 5

    21{22    static void Main()23    {24        int width→ 5 = 5; //@width=3, 725        Rectangle box = new Rectangle(width, 4);26        int area = box.Area();

  2. public Rectangle(int width, int height)

    8public Rectangle(int width5, int height4)9{10    Width = width5;11    Height = height4;12}

  3. box ← Rectangle

    24int width = 5; //@width=3, 725Rectangle box→ Rectangle = new Rectangle(width, 4);26int area = boxRectangle.Area();

  4. area ← 20

    25    Rectangle box = new Rectangle(width, 4);26    int area→ 20 = boxRectangle.Area();2728    Console.WriteLine($"width={box.Width5}");29    Console.WriteLine($"area={area20}");30}
    outputwidth=5
area=20

  1. width ← 3

    21{22    static void Main()23    {24        int width→ 3 = 3;25        Rectangle box = new Rectangle(width, 4);26        int area = box.Area();

  2. public Rectangle(int width, int height)

    8public Rectangle(int width3, int height4)9{10    Width = width3;11    Height = height4;12}

  3. box ← Rectangle

    24int width = 3;25Rectangle box→ Rectangle = new Rectangle(width, 4);26int area = boxRectangle.Area();

  4. area ← 12

    25    Rectangle box = new Rectangle(width, 4);26    int area→ 12 = boxRectangle.Area();2728    Console.WriteLine($"width={box.Width3}");29    Console.WriteLine($"area={area12}");30}
    outputwidth=3
area=12

  1. width ← 7

    21{22    static void Main()23    {24        int width→ 7 = 7;25        Rectangle box = new Rectangle(width, 4);26        int area = box.Area();

  2. public Rectangle(int width, int height)

    8public Rectangle(int width7, int height4)9{10    Width = width7;11    Height = height4;12}

  3. box ← Rectangle

    24int width = 7;25Rectangle box→ Rectangle = new Rectangle(width, 4);26int area = boxRectangle.Area();

  4. area ← 28

    25    Rectangle box = new Rectangle(width, 4);26    int area→ 28 = boxRectangle.Area();2728    Console.WriteLine($"width={box.Width7}");29    Console.WriteLine($"area={area28}");30}
    outputwidth=7
area=28

Follow the Constructor

  1. width starts as 5.
  2. new Rectangle(width, 4) calls the constructor.
  3. The constructor stores Width = 5.
  4. It also stores Height = 4.
  5. box.Area() returns 5 * 4, so area is 20. | property | value | | --- | --- | | box.Width | 5 | | box.Height | 4 | | area | 20 |

Exercise: Constructors.cs

Reproduce width=5 and area=20, then use the pinned width variants 3 and 7 to predict area=12 and area=28.