3.9 this Keyword

1. this: a reference to the current object

Inside an instance method or a constructor, the keyword this acts as a special variable that holds a reference to the current object: the object whose method or constructor is being called 3.9.A.1.

The most common use is in a constructor whose parameters have the same names as the instance variables. The parameter hides the instance variable (topic 3.8), and this. names the instance variable.

public class Runner
{
    private String name;
    private double kilometers;

    public Runner(String name, double kilometers)
    {
        this.name = name;               // instance variable = parameter
        this.kilometers = kilometers;
    }

    public void run(double kilometers)
    {
        this.kilometers += kilometers;  // add the parameter to the total
    }

    public String getName()
    {
        return name;
    }

    public double getKilometers()
    {
        return kilometers;
    }

    public String toString()
    {
        return name + " " + kilometers + " km";
    }
}

Runner r = new Runner("Ivy", 5.0);
r.run(3.5);
System.out.println(r);

Output:

Ivy 8.5 km

(println(r) uses the class’s toString method; writing toString is background, outside the AP® exam.) In the call r.run(3.5), the current object is the one r refers to, so inside run, this and r are references to the same object.

r    [ ref ] ---+
                +---> Runner object: name "Ivy", kilometers 8.5
this [ ref ] ---+    (inside run, during the call r.run(3.5))

2. Which object is this?

The same method can run for many objects. Each time, this refers to the object the method was called on 3.9.A.1. Comparing this with a parameter using == checks whether they are the same object.

public class Badge
{
    private String owner;

    public Badge(String owner)
    {
        this.owner = owner;
    }

    public String whoAmI()
    {
        return "I belong to " + this.owner;
    }

    public boolean isSameBadge(Badge other)
    {
        return this == other;
    }
}

Badge a = new Badge("Omar");
Badge b = new Badge("Jun");
Badge c = a;
System.out.println(a.whoAmI());
System.out.println(b.whoAmI());
System.out.println(a.isSameBadge(c) + " " + a.isSameBadge(b));

Output:

I belong to Omar
I belong to Jun
true false

In a.isSameBadge(c), this refers to the object a refers to, and other is a copy of the reference in c. Both point to the same object, so == is true.

Watch out: in whoAmI, writing owner or this.owner gives the same result, because no parameter or local variable is named owner. this. is required only when a local name hides the instance variable.

3. Passing this as an argument

The keyword this can be used to pass the current object as an argument in a method call 3.9.A.2. That lets another object store or use a reference to the current object.

public class Room
{
    private int number;
    private Guest occupant;

    public Room(int number)
    {
        this.number = number;
        occupant = null;
    }

    public void assign(Guest g)
    {
        occupant = g;
    }

    public String describe()
    {
        if (occupant == null)
        {
            return "Room " + number + ": empty";
        }
        return "Room " + number + ": " + occupant.getName();
    }
}

public class Guest
{
    private String name;

    public Guest(String name)
    {
        this.name = name;
    }

    public String getName()
    {
        return name;
    }

    public void checkInto(Room r)
    {
        r.assign(this);   // pass the current Guest object to the room
    }
}

Room r12 = new Room(12);
Room r14 = new Room(14);
Guest maya = new Guest("Maya");
maya.checkInto(r12);
System.out.println(r12.describe());
System.out.println(r14.describe());

Output:

Room 12: Maya
Room 14: empty

During maya.checkInto(r12), this refers to the Guest object that maya refers to. r.assign(this) passes that reference, so the room’s occupant and maya become aliases of one Guest object.

4. Class methods have no this

Class methods (static methods) are called on the class, not on an object, so they do not have a this reference 3.9.A.3.

public class Counter
{
    private int count;

    public static void resetAll()
    {
        this.count = 0;
    }
}

Compiler says:

error: non-static variable this cannot be referenced from a static context

Where the code is	Is there a this?
Constructor	Yes: the object being constructed 3.9.A.1
Instance method	Yes: the object the method was called on 3.9.A.1
Class (static) method	No 3.9.A.3