Properties and Encapsulation

Encapsulated State

Methods can change private state while the class controls what is allowed.

encapsulation Encapsulation keeps state changes inside the object that owns the state.

Encapsulated State

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

class Wallet
{
    private int cents;

    public int Cents
    {
        get { return cents; }
    }

    public void Deposit(int amount)
    {
        if (amount > 0)
        {
            cents += amount;
        }
    }

    public bool Spend(int amount)
    {
        if (amount <= cents)
        {
            cents -= amount;
            return true;
        }

        return false;
    }
}

class Program
{
    static void Main()
    {
        int spend = 30;
        Wallet wallet = new Wallet();
        wallet.Deposit(50);
        bool paid = wallet.Spend(spend);

        Console.WriteLine($"spend={spend}");
        Console.WriteLine($"paid={paid}");
        Console.WriteLine($"remaining={wallet.Cents}");
    }
}

using System;

class Wallet
{
    private int cents;

    public int Cents
    {
        get { return cents; }
    }

    public void Deposit(int amount)
    {
        if (amount > 0)
        {
            cents += amount;
        }
    }

    public bool Spend(int amount)
    {
        if (amount <= cents)
        {
            cents -= amount;
            return true;
        }

        return false;
    }
}

class Program
{
    static void Main()
    {
        int spend = 20;
        Wallet wallet = new Wallet();
        wallet.Deposit(50);
        bool paid = wallet.Spend(spend);

        Console.WriteLine($"spend={spend}");
        Console.WriteLine($"paid={paid}");
        Console.WriteLine($"remaining={wallet.Cents}");
    }
}

using System;

class Wallet
{
    private int cents;

    public int Cents
    {
        get { return cents; }
    }

    public void Deposit(int amount)
    {
        if (amount > 0)
        {
            cents += amount;
        }
    }

    public bool Spend(int amount)
    {
        if (amount <= cents)
        {
            cents -= amount;
            return true;
        }

        return false;
    }
}

class Program
{
    static void Main()
    {
        int spend = 80;
        Wallet wallet = new Wallet();
        wallet.Deposit(50);
        bool paid = wallet.Spend(spend);

        Console.WriteLine($"spend={spend}");
        Console.WriteLine($"paid={paid}");
        Console.WriteLine($"remaining={wallet.Cents}");
    }
}

  1. spend ← 30, wallet ← Wallet

    33{34    static void Main()35    {36        int spend→ 30 = 30; //@spend=20, 8037        Wallet wallet→ Wallet = new Wallet();38        walletWallet.Deposit(50);39        bool paid = wallet.Spend(spend);

  2. public void Deposit(int amount)

    12public void Deposit(int amount50)13{14    if (amount > 0)

  3. cents ← 50

    13{14    if (amount50 > 0)15    {16        cents→ 50 += amount50;17    }

  4. wallet.Deposit(50);

    37Wallet wallet = new Wallet();38walletWallet.Deposit(50);39bool paid = walletWallet.Spend(spend30);

  5. public bool Spend(int amount)

    20public bool Spend(int amount30)21{22    if (amount <= cents)

  6. cents ← 20

    21{22    if (amount30 <= cents50)23    {24        cents→ 20 -= amount30;25        return true;26    }

  7. paid ← True

    38wallet.Deposit(50);39bool paid→ True = walletWallet.Spend(spend30);4041Console.WriteLine($"spend={spend30}");42Console.WriteLine($"paid={paidTrue}");43Console.WriteLine($"remaining={wallet.Cents}");
    outputspend=30
paid=True

  8. get

    pass 1 of 5
    8{9    get { return cents20; }10}

  9. Console.WriteLine($"remaining={wallet.Cents}");

    42    Console.WriteLine($"paid={paid}");43    Console.WriteLine($"remaining={wallet.Cents20}");44}

  10. Console.WriteLine($"remaining={wallet.Cents}");

    42    Console.WriteLine($"paid={paid}");43    Console.WriteLine($"remaining={wallet.Cents20}");44}
    outputremaining=20

  1. spend ← 20, wallet ← Wallet

    33{34    static void Main()35    {36        int spend→ 20 = 20;37        Wallet wallet→ Wallet = new Wallet();38        walletWallet.Deposit(50);39        bool paid = wallet.Spend(spend);

  2. public void Deposit(int amount)

    12public void Deposit(int amount50)13{14    if (amount > 0)

  3. cents ← 50

    13{14    if (amount50 > 0)15    {16        cents→ 50 += amount50;17    }

  4. wallet.Deposit(50);

    37Wallet wallet = new Wallet();38walletWallet.Deposit(50);39bool paid = walletWallet.Spend(spend20);

  5. public bool Spend(int amount)

    20public bool Spend(int amount20)21{22    if (amount <= cents)

  6. cents ← 30

    21{22    if (amount20 <= cents50)23    {24        cents→ 30 -= amount20;25        return true;26    }

  7. paid ← True

    38wallet.Deposit(50);39bool paid→ True = walletWallet.Spend(spend20);4041Console.WriteLine($"spend={spend20}");42Console.WriteLine($"paid={paidTrue}");43Console.WriteLine($"remaining={wallet.Cents}");
    outputspend=20
paid=True

  8. get

    pass 1 of 5
    8{9    get { return cents30; }10}

  9. Console.WriteLine($"remaining={wallet.Cents}");

    42    Console.WriteLine($"paid={paid}");43    Console.WriteLine($"remaining={wallet.Cents30}");44}

  10. Console.WriteLine($"remaining={wallet.Cents}");

    42    Console.WriteLine($"paid={paid}");43    Console.WriteLine($"remaining={wallet.Cents30}");44}
    outputremaining=30

  1. spend ← 80, wallet ← Wallet

    33{34    static void Main()35    {36        int spend→ 80 = 80;37        Wallet wallet→ Wallet = new Wallet();38        walletWallet.Deposit(50);39        bool paid = wallet.Spend(spend);

  2. public void Deposit(int amount)

    12public void Deposit(int amount50)13{14    if (amount > 0)

  3. cents ← 50

    13{14    if (amount50 > 0)15    {16        cents→ 50 += amount50;17    }

  4. wallet.Deposit(50);

    37Wallet wallet = new Wallet();38walletWallet.Deposit(50);39bool paid = walletWallet.Spend(spend80);

  5. public bool Spend(int amount)

    20public bool Spend(int amount80)21{22    if (amount <= cents)23    {24        cents -= amount;25        return true;26    }2728    return false;29}

  6. paid ← False

    38wallet.Deposit(50);39bool paid→ False = walletWallet.Spend(spend80);4041Console.WriteLine($"spend={spend80}");42Console.WriteLine($"paid={paidFalse}");43Console.WriteLine($"remaining={wallet.Cents}");
    outputspend=80
paid=False

  7. get

    pass 1 of 5
    8{9    get { return cents50; }10}

  8. Console.WriteLine($"remaining={wallet.Cents}");

    42    Console.WriteLine($"paid={paid}");43    Console.WriteLine($"remaining={wallet.Cents50}");44}

  9. Console.WriteLine($"remaining={wallet.Cents}");

    42    Console.WriteLine($"paid={paid}");43    Console.WriteLine($"remaining={wallet.Cents50}");44}
    outputremaining=50