Method dispatch (overloading and overriding)

Java has a number of mechanisms in which, when a program calls a method on an object, either the compiler or the runtime must decide which method should be executed for that method invocation. This is a known as method dispatch.

• Method overloading (static dispatch)
• Static Type and Dynamic Type
  • First look at inheritance
• Back to method dispatch
  • Example 1
  • Example 2
  • Example 3
  • Example 4
• Method overriding (dynamic dispatch)
  • Example 5

Method overloading (static dispatch)

Method overloading allows us to perform the same task (call the same function) in multiple different ways, depending on the inputs that are given (the arguments that are given to the function).

More concretely, it allows the same class to define several methods that have the same name but different parameter lists. In Java, a method’s signature is its name, its parameter list, and any modifiers applied to it, like public or static. Those first two pieces—the method name and parameter list—together uniquely identify a method within a class. Within the same class, there can never be more than one method with a given name and parameter list.

However, we can have several methods with the same name, but different parameter lists.

How our program chooses between all of those methods with the same names is a matter of static or dynamic dispatch.

Consider the two classes in the example below: Circle and Reporter.

The Reporter class above contains three static methods called report.¹ Each one takes a different list of parameter types.

When we write code to call one of the methods, our toolchain needs to decide which method should get called. This process is called method dispatch.

Why do you think they are static methods?¹

So, for example, if we were to make the following function call:

Circle circle = new Circle(10);
Reporter.report(circle);

At compile time, the compiler determines which of the report methods best matches the provided input type. In this case, the argument we’ve given to the report method is a single Circle object. The compiler looks at all the report methods and identifies that, yes, there is a report method that expects a single Circle object as an argument.

After the code above is run, the text "Circle: Radius: 10" would be printed out.

If, instead, we called

Reporter.report(23, 2);

The compiler would call the report method that takes two ints as inputs, and print "Two ints: 23 2"

However, if we did something like this…

Circle circle = new Circle(10);
Circle anotherCircle = new Circle(20);
Reporter.report(circle, anotherCircle);

…we would run into a compiler error. The compiler is unable to resolve the report call above, i.e., it cannot find a matching method in the Reporter class.

Since all of the above happens at compile time, i.e., before we ran the program, this is a form of static dispatch.

Static Type and Dynamic Type

At this point, it’s useful to take a little detour.

We’ve talked about how Java is statically-typed. Take a minute to remind yourself of what it means to be statically-typed. Put simply, it means that the types of variables are pretty much settled at compile time, as opposed to run time.

However, this is not quite the complete picture in Java. In some situations, what the compiler thinks is the type of a variable may not necessarily be the same as what the runtime² thinks the type is (with some limitations).

But wait! Wasn’t using a statically-typed language meant to save us from these ambiguities? How can this be?

First look at inheritance

The Java standard library defines the Object class. The Object class defines the behaviours that all objects are expected to support. For example, the Object class defines these instance methods, among others:

• equals(Object): boolean — This method checks if the calling object is equal to the specified Object
• toString(): String — This method returns a string representation of the object

Every reference type in Java is a child type of Object. That is, every reference type in Java is an Object, and can perform all of the actions defined in the Object class, even if those methods were not defined for that reference type.

This is due to inheritance. All objects in Java inherit certain behaviours from the Object class, whether or not you, the programmer, explicitly told them to.

So, a String is an Object. A Point is an Object. The CsCohort and Pitcher classes that we created in previous lessons are Objects, even though we didn’t explicitly mark them as such.

With that in mind, consider the following assignment statement:

Object whatAmI = new Circle(3);

The type on the left-hand-side of the assignment operator (Object) does not match the type on the right-hand-side ( Circle). But our compiler is still happy with that assignment statement.

This is because Circle is a subtype of Object, i.e., it can do everything that an Object can do. Our compiler works this out due to the inheritance relationship between Object and Circle. So our compiler is totally happy to say “ok, I will treat this variable as an Object, because I know it can do Object things”.

So:

• The compiler sees the variable whatAmI as an Object. This is known as the variable’s static type.
• The runtime sees the variable whatAmI as a Circle. This is known as the variable’s dynamic type.

I’ll say that again, because that’s important: the variable is treated slightly differently by the compiler and the runtime. As far as the compiler knows, it’s an Object, but the actual thing that is stored in memory is a Circle.

If, instead, we did…

Circle circ = new Circle(3);

…then we have no differences. The static type and the dynamic type of the variable are the same.

As a counterexample, consider the following.

// This will break!
Circle whatAmI = new Object();

The line above would not compile, because we are declaring whatAmI to be a Circle, but then we’re giving it an Object as a value. That is, we’re saying we want to make the variable do Circle actions (like, say, computing its area). But in actuality, at runtime, the variable is an Object, which cannot compute an area — it doesn’t even have a radius! Our compiler is able to work out this incongruence, because it understands the direction of the relationship between Circle and Object; all Circles are Objects, but not all Objects are Circles.

Back to method dispatch

All right, with that background, let’s get back to method overloading.

In the Reporter example, we have the following two report functions:

• One that takes in a Circle as a parameter.
• One that takes in an Object as a parameter.

Let’s look at some function calls with different inputs.

Example 1

Circle circle = new Circle(3);
Reporter.report(circle);

• The input’s static type is Circle.
• The input’s dynamic type is Circle.

What will be printed? Based on the static type of the variable, the compiler will decide which function should be called. In this case, it will decide on the Reporter.report(Circle) function, because the static type of the input is Circle. So the printed output will be: "Circle: Radius: 3"

Example 2

Object obj = new Object();
Reporter.report(obj);

• The input’s static type is Object.
• The input’s dynamic type is Object.

What will be printed? The compiler will choose the Reporter.report(Object) function, because the static type of the input is Object. So the printed output will be something like: "Object: Object@324618"³

Example 3

Object circ = new Circle(3);
Reporter.report(circ);

• The input’s static type is Object.
• The input’s dynamic type is Circle.

What will be printed? The compiler will choose the Reporter.report(Object) function, because the static type of the input is Object. However, the dynamic type is still Circle. That is, even if the compiler thinks it’s an Object, the object that’s sitting memory is still a Circle.

So we get the following output:

"Object: Radius: 3"

Let’s think about how we got that, step by step:

• When we say the object in memory is a Circle, we are referring to the data held by a Circle (radius) and behaviours the Circle can perform (computing the area using getArea, or getting a string representation of the object using toString).
• So when the print statement in the report function implicitly calls the toString method, the following two things happen:
  1. The compiler checks if the static type (Object) has an instance method called toString. It does (see earlier in this lesson). If it didn’t the code would not compile, and we wouldn’t be able to run it at all. And this is a good check, because if the Object has a toString, we know that the Circle must also have a toString, because of their inheritance relationship.
  2. The runtime needs to decide which toString method to call. We know that there is one in the Object class, and we have one in the Circle class. Just like before, we have to choose one, i.e., we have to go through the process of method dispatch. Since the dispatch is now occurring at run time, it is known as dynamic dispatch.
  3. In this case, since the object is a Circle at run time, we choose the Circle’s toString method. And we print the Radius: 3 message.

Putting all of that together, we get the output: "Object: Radius: 3"

The "Object" in the string above came from the static dispatch, i.e., the choice of which report method to call. The "Radius: 3" came from the dynamic dispatch, i.e., the choice of which toString method to call.

Example 4

Let’s consider a final example. Suppose, for a moment, that the report(Circle) method did not exist in Reporter. If you are following along with the code on your own machine, go ahead and delete or comment out that method now. And suppose we did the following:

Circle circ = new Circle(3);
Reporter.report(circ);

Now, our compiler cannot find a report method that takes a Circle as a parameter, because in this example it doesn’t exist. So the compiler will now “look upward”, and instead look for a method that takes as its parameter a parent class of Circle (i.e., Object). So the example above would compile, and it would print out "Object: Circle@eb2857" (or something similar).³

Main takeway with method overloading/static dispatch.

When a method has many overloads, the compiler checks the following rules, in this order, to determine which method should be called based on the arguments that are provided.

1. First, it looks for a method whose parameter type is an exact match with the input’s static type.
2. If that is not successful, it looks for a method whose parameter has the closest “is a” relationship with the given input’s static type. In Example 4 above, a Circle is an Object, so the compiler would resolve to calling report(Object) if report(Circle) was not present.
3. If neither of the above resolves to a method, the compiler checks if the static type of the input can be implicitly converted to the expected parameter type for the method. For example, if the method takes in a double as a parameter, and we attempt to call it with an int as a parameter, the compiler will implicitly convert that int into a double and call the double method instead.

Method overriding (dynamic dispatch)

Closely related to method overloading is the confusingly similarly named method overriding.

When a superclass (parent class) defines a behaviour and the subclass (child class) does not, the subclass inherits that behaviour from the superclass. For example, if we did not write a toString method in our Circle class, it would have inherited the Object toString behaviour instead.

Method overriding is when a method in a subclass has the same name and parameter list as a method in the superclass. The effect of this is that the subclass’s method overrides the superclass’s method, thus replacing the superclass’s behaviour that the subclass would otherwise inherit.

Example 5

Let’s illustrate this with an example.

Continuing with our running example, suppose we had not written a toString method in the Circle class.

The Circle class would then inherit the toString behaviour from the Object class. If we did not want the Object’s toString behaviour, we need to override it, by writing our own toString class in Circle.

Here are the two classes again for the sake of convenience. For this example, Circle has no toString method.

// Reporter.java
public class Reporter {
    public static void report(Object obj) {
        System.out.println("Object:   " + obj);
    }

    public static void report(int x, int y) {
        System.out.println("Two ints: " + x + " " + y);
    }
}

// Circle.java
public class Circle {
    private final double radius;

    public Circle(double radius) {
        this.radius = radius;
    }

    // This time with no toString method
}

Suppose we run the following code:

Object obj = new Circle(3);
Reporter.report(obj);

• The input’s static type is Object.
• The input’s dynamic type is Circle.

The following will take place:

First, the compiler will look for a report function that takes an Object parameter. All good so far.

Next, within the report function, we reach the print statement. Inside that print statement, obj.toString() is implicitly called.

Next, we should remember that, at runtime, obj is a Circle, because that was the dynamic type of our original input. But Circle has no toString method now.

So what happens?

This is where the inheritance relationship between Circle and Object comes into play. When the runtime fails to find a toString method in the Circle object, it will “search upward”, checking the ancestors of Circle—in this case, Object.

So, what we will see printed is:

"Circle: Object@eb78402" (or something similar).³

If we were to give Circle a toString method of its own (as in the very first code snippets in this lesson), we would replace the “default” toString behaviour of all Objects with a specific toString method for Circles. This process is known as method overriding.