Why does a null value choose the `String` overload in Java method overloading?

Overloaded Method Selection with Null Values

In Java, overloaded methods provide multiple versions with different parameters or return types, allowing for flexibility and code reuse. However, choosing the correct method when passing a null literal value can be a source of confusion.

Consider the following example:

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);
   }
}

Here, the method method(null) chooses the method(String) overload, even though null is an object type. To understand this behavior, we must explore Java's approach to overloads.

Method Selection with Null Values

In Java, the compiler selects the most specific overload that can handle the provided arguments without a type error. For example, method(Object) can accept any object type, including null. However, method(String) can only accept a String or null. Since null is a valid String value, method(String) is more specific and is therefore chosen.

Ambiguity and Compile Errors

In the revised example below:

public class MoneyCalc {

   public void method(StringBuffer sb) {
      System.out.println("StringBuffer Verion");
   }

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

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

The compiler throws an error: "The method method(StringBuffer) is ambiguous for the type MoneyCalc." This is because both method(String) and method(StringBuffer) are equally specific for a null argument. Consequently, the compiler cannot determine which overload to choose, resulting in a compile error.

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