Why Does Java's Method Overloading Ignore Runtime Parameter Types?

Overloaded Method Selection: Understanding the Role of Actual Parameter Types

Java's dynamic method dispatch mechanism has nuances that can be easily overlooked. This article explores a specific misunderstanding regarding the influence of actual parameter types on method selection.

The Issue:

In the provided code, the Callee interface defines three overloaded methods foo. The expectation is that when invoking foo with an actual "Integer i," the method foo(Integer i) will be selected. However, the observed behavior is that foo(Object o) is always executed, regardless of the actual parameter type.

The Explanation:

Contrary to expectations, Java's method selection process solely considers the compile-time types of the arguments. This means that overloading based on the actual parameter types, a concept known as "runtime type checking," is not supported in Java.

The Official Word:

The Java Language Specification explicitly states:

When a method is invoked, the number of actual arguments (and any explicit type arguments) and **the compile-time types of the arguments** are used, at compile time, to determine the signature of the method that will be invoked.

Therefore, Java's method selection mechanism disregards the actual runtime type of objects passed as method arguments.

Implications:

This behavior can have significant implications. For example, when dealing with polymorphic objects or hierarchies of classes, the expected overloading behavior based on actual parameter types may not occur.

Alternatives:

If runtime type checking is desired, Java provides alternative mechanisms such as reflection and generics, which offer greater flexibility in handling different object types.

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