What is dispatching in JAVA?

Question

I am confused on what exactly dispatching is. Especially when it comes to double dispatching. Is there a simple way that I can grasp this concept?

Answer 1

Dispatch is the way a language links calls to function/method definitions.

In java, a class may have multiple methods with the same name but different parameter types, and the language specifies that method calls are dispatched to the method with the right number of parameters that has the most specific types that the actual parameters could match. That's static dispatch.

For example,

void foo(String s) { ... }
void foo(Object o) { ... }
{ foo("");           }  // statically dispatched to foo(String)
{ foo(new Object()); }  // statically dispatched to foo(Object)
{ foo((Object) "");  }  // statically dispatched to foo(Object)

Java also has virtual method dispatch. A subclass can override a method declared in a superclass. So at run-time, the JVM has to dispatch the method call to the version of the method that is appropriate to the run-time type of this.

For example,

class Base { void foo() { ... } }
class Derived extends Base { @Override void foo() { ... } }


{ new Derived().foo(); }  // Dynamically dispatched to Derived.foo.
{
  Base x = new Base();
  x.foo();                // Dynamically dispatched to Base.foo.
  x = new Derived();      // x's static type is still Base.
  x.foo();                // Dynamically dispatched to Derived.foo.
}

Double-dispatch is the combination of static and run-time(also called dynamic) dispatches.