method X is unclear for the type Y in Java

2 years ago

3 minutes

If you are currently reading this, it is likely that you encountered the “The method X is ambiguous for the type Y” error while compiling a Java program in a terminal or any Java Integrated Development Environment (IDE).

The method call in Java is unclear.

In this explanation, I will discuss the reasons behind the occurrence of the ambiguous method call error in Java, supported by a few examples. This error usually arises when the compiler is unable to determine which overloaded method should be utilized, primarily in cases involving method overloading. Consider the following Java program as an example.

package com.scdev.errors;

public class Test {

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

	public void foo(String s) {
		System.out.println("String");
	}
	public static void main(String[] args) {
		new Test().foo(null);
	}

}

The program compiles without any errors and upon execution, it outputs the word “String”. Therefore, it can be concluded that the program called the method foo(String s). The Java compiler determines which method to invoke by selecting the one with the most specific input parameters. Since Object is the superclass of String, the decision was straightforward. This information is supported by the Java Language Specification.

If there are multiple member methods that can be accessed and applied to a method invocation, the Java programming language follows the guideline of selecting the most specific method.

I am passing “null” because it can be used with any type of arguments, making it easier for the java compiler to determine which method to use if we pass any other objects.

The type Y presents ambiguity when using method X.

Now, we will include the following method to the code mentioned above.

public void foo(Integer i){
	System.out.println("Integer");
}

If you have a method called “foo” with a parameter of type “Object” in your class called “Test”, you will encounter a compilation error. This is because the classes “String” and “Integer” both have “Object” as their parent, resulting in no clear inheritance. Consequently, the Java compiler cannot determine which class is more specific, leading to the error of an ambiguous method call.

package com.scdev.strings;

public class Test {

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

	public void foo(Exception e) {
		System.out.println("Exception");
	}

	public void foo(NullPointerException ne) {
		System.out.println("NullPointerException");
	}

	public static void main(String[] args) {
		new Test().foo(null);
	}

}

As mentioned earlier, the foo(NullPointerException ne) method is considered the most specialized because it is inherited from the Exception class. This leads to successful compilation of the code and the output of “NullPointerException” when executed. I trust that this article has resolved any uncertainties regarding the compiler error of ambiguous method calls in Java. Feel free to leave a comment if you have any additional thoughts or further confusion on this matter.