Why does passing `null` to an overloaded method in Java call the method with the `String` parameter?

Method Overloading and Null Parameter Selection

In Java, when a method is overloaded, multiple methods exist with the same name but different parameter lists. When a method invocation is made, the compiler determines which overloaded method to call based on the number and types of the actual arguments passed.

Consider the following code snippet:

public class MoneyCalc {

   public void method(Object o) {
      System.out.println("Object Verion");
   }

   public void method(String s) {
      System.out.println("String Version");
   }

   public static void main(String args[]) {
      MoneyCalc question = new MoneyCalc();
      question.method(null);
   }
}

Question: Why does passing a null value to the overloaded method method call the method(String) version?

Answer:

In Java, null is a special constant value that represents a reference to an object that does not exist. It is not a value of any particular type, but it can be assigned to a variable of any reference type, including Object and String.

When the method invocation is made with null as the argument, the compiler chooses the method(String) version because it is more specific than the method(Object) version. According to the Java Language Specification (JLS), the most specific overload is chosen based on its applicability and specificity.

Applicability refers to whether the arguments can be passed to the method without a compile-time error. In this case, null is applicable to both methods as it can be assigned to variables of both Object and String types.

Specificity measures how specialized the method is for the given arguments. In this case, the method(String) overload is more specific because it is designed to handle arguments of type String only. The method(Object) overload is more general as it can handle arguments of type Object or any of its subclasses.

Therefore, the compiler selects the method(String) version as it is more specific and applicable to the provided argument, which is a null reference.

Note: If a second overloaded method with StringBuffer as the parameter type is added to the MoneyCalc class, a compilation error would occur. This is because neither method(String) nor method(StringBuffer) would be more specific than the other in this case, leading to an ambiguity in method selection.

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