Encapsulation: private fields, getters, setters

Protect your data

• Encapsulation means hiding an object's data and only changing it through methods.
• This keeps the data safe. Other code cannot put a bad value into it.
• The main tools are the words private and public.

private fields

• Mark instance variables private so code outside the class cannot touch them directly.
• Methods inside the same class can still use them.
• This is the normal style for AP CSA: fields are private.

public class BankAccount {
    private double balance;   // hidden from outside code

    public BankAccount(double start) {
        this.balance = start;
    }
}

Accessors (getters)

• A field is private, so we add a public method to read it.
• A method that returns a field is called an accessor or getter.
• It usually starts with get, like getBalance().

public class BankAccount {
    private double balance;

    public BankAccount(double start) {
        this.balance = start;
    }

    public double getBalance() {
        return this.balance;
    }
}

Mutators (setters)

• A method that changes a field is called a mutator or setter.
• It can check the new value first and refuse a bad one.
• Below, deposit ignores a negative amount, so the balance stays safe.

public class BankAccount {
    private double balance;

    public BankAccount(double start) {
        this.balance = start;
    }

    public void deposit(double amount) {
        if (amount > 0) {
            this.balance = this.balance + amount;
        }
    }

    public double getBalance() {
        return this.balance;
    }
}

static vs instance

• An instance field belongs to one object. Each object has its own.
• A static field belongs to the class itself — it is shared by all objects.
• Below, count rises by 1 every time any Robot is built.

public class Robot {
    private static int count = 0;   // shared by all robots

    public Robot() {
        count = count + 1;
    }

    public static int getCount() {
        return count;
    }
}

toString()

• toString() returns text that describes the object.
• Java calls it automatically when you print the object or join it with +.
• It takes no parameters and returns a String.

public class Point {
    private int x;
    private int y;

    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }

    public String toString() {
        return "(" + this.x + ", " + this.y + ")";
    }
}

Now you try

• Fill in each class so its private data is changed only through safe methods.
• A hidden Harness builds the object, calls the methods, and checks the results.
• Press Run to compile, then Check answer.