“The method X is ambiguous for the type Y” Java ambiguous method call null error

Filed Under: Java

If you are reading this, chances are you got The method X is ambiguous for the type Y error when compiling a java program in terminal or in any Java IDE.

Java ambiguous method call

Here I am going to explain why java ambiguous method call error comes with some examples. This ambiguous method call error always comes with method overloading where compiler fails to find out which of the overloaded method should be used.

Suppose we have a java program like below.


package com.journaldev.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);
	}

}

Above program compiles perfectly and when we run it, it prints “String”.

So the method foo(String s) was called by the program. The reason behind this is java compiler tries to find out the method with most specific input parameters to invoke a method. We know that Object is the parent class of String, so the choice was easy. Here is the excerpt from Java Language Specification.

If more than one member method is both accessible and applicable to a method invocation … The Java programming language uses the rule that the most specific method is chosen.

The reason I am passing “null” is because it works for any type of arguments, if we pass any other objects the choice of method for the java compiler is easy.

The method X is ambiguous for the type Y

Now let’s add below method to the above code.


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


You will get compile time error as The method foo(Object) is ambiguous for the type Test because both String and Integer class have Object as parent class and there is no inheritance. So java compiler doesn’t consider any of them to be more specific, hence the method ambiguous call error.


package com.journaldev.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 above explained, here foo(NullPointerException ne) is the most specific method because it’s inherited from Exception class and hence this code compiles fine and when executed prints “NullPointerException”.

I hope this article clarifies any doubt you have with java ambiguous method call compiler error, please comment if you want to add something or if you have some confusion with this.

Comments

  1. Umesh Soni says:

    Hi All,

    I am using java 8 and in my system haven’t any warning regarding
    B b=new B();
    new B().foo(null);
    And because of
    public void foo(String ne) {
    System.out.println(“String”);
    }
    overloaded method java display ambiguous error.
    eighter use
    public void foo(Exception e) {
    System.out.println(“Exception”);
    }

    public void foo(NullPointerException ne) {
    System.out.println(“NullPointerException”);
    }
    Or use
    public void foo(String ne) {
    System.out.println(“String”);
    }

  2. Kavita says:

    Good explanation on ambiguity in method overloading.
    https://www.youtube.com/watch?v=xlSRf7psJHE

  3. Ravi teja says:

    Class Met
    {
    public void m(int a,float b)
    {
    System.out.println(“first m”);
    }
    Public void m(float a,char b)
    {
    System.out.println(“sec m”);
    }
    }
    Class Main1
    {
    Public static void main (string arg[])
    {
    Met n= new Met();
    n.m( ‘c’,’c’);
    }

    Output is ambiguous ,why it is ambiguous and can I know the reason and procedure for checking it
    Thanks

    1. Anupam says:

      Hi Ravi Teja,

      Here is compilation error because of data type conversion. “char” can be fit into int. and float both so compiler is confused. see the code.

      public class First {

      public void m(int a,float b){

      }
      public void m(float a,char c){

      }
      public static void main(String[] args) {
      First f= new First();
      f.m((float)2, ‘c’);
      f.m(2, ‘c’);//error because int can be fit into float also.

      Hope u understand, if not then ask again.

  4. Bhargav says:

    What will be the out put for below program ?

    class B{

    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 void foo(String ne) {
    System.out.println(“String”);
    }

    }

    class A extends B {

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

    }

    Could you please explain ?

    Thanks,
    Bhargav

    1. Arthi says:

      Hello Sir,
      The result will be a compile time error, because the compiler cannot choose between foo(String ne) and foo(NullPointerException ne).
      This is what I think the answer is. Please correct me if I am wrong. By the way, a really intuitive question.

    2. Sowmya says:

      The first method you define in class B will be shown in the error.
      In the above program, public void foo(Object o) is the first method and it shows the error:
      The method foo(Object) is ambiguous for the type A.
      If you put the other method say public void foo(NullPointerException ne) error would be:
      The method foo(NullPointerException) is ambiguous for the type A.

      1. Sowmya says:

        In the above example, ‘new’ operator is used twice. Is it valid?
        B b=new B();
        new B().foo(null);

        And b is not used anywhere. you may get a warning ‘ The value of the local variable b is not used’.

  5. shef says:

    Thank you so much best example with best explanation.

  6. Mike Zang says:

    Thanks for your explanation! How can I avoid this error or change it to warning?

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