Java 8: Reference to [method] is ambiguous

Question

Does anybody understand why the following code will compile fine in Java 7 and below, but fails with Java 8.

public static void main(String[] args) throws Exception {
    put(get("hello"));
}

public static <R> R get(String d) {
    return (R)d;
}

public static void put(Object o) {
    System.err.println("Object " + o);
}

public static void put(CharSequence c) {
    System.err.println("CharSequence " + c);
}

public static void put(char[] c) {
    System.err.println("char[] " + c);
}

The get method has a generic return type. In JDK 7 and below this compiles fine and the put method with the Object parameter is chosen. In JDK 8 this cannot be compiled, indicating the put method is ambiguous.

Apparently JDK 8 is skipping over the Object-parameter method and finding the last two sub-Object-parameter methods and complaining about them (i.e. if you add another put method with some other parameter type, the compiler will switch and complain about the new last two methods)

This seems like a bug.

Answer 1

Your problem is a side-effect of Generalized Target-type Inference, an improvement in Java 8.

What is Target-type Inference

Let's take your example method,

public static <R> R get(String d) {
    return (R)d;
}

Now, in the method above, the generic parameter R cannot be resolved by the compiler because there's no parameter with R.

So, they introduced a concept called Target-type Inference, which allows the parameter to be inferred based on the assignment parameter.

So, if you do,

 String str = get("something"); // R is inferred as String here
 Number num = get("something"); // R is inferred as Number here

This works well in Java 7. But the following does not,

put(get("something");
static void Put(String str) {} //put method

Because type inference worked only for direct assignments.

If there's no direct assignment, then the generic type was inferred as Object.

So, when you compiled the code with Java 7, your put(Object) method was called without any problems.

What they did in Java 8

They improved the type inference to infer the type from method calls and chained method calls

More details about them here and here

So now, you can directly call put(get("something")) and the generic type will be inferred based on the parameter of the put() method.

But as you know, the methods, put(Charsequence) and put(char[]) match the arguments. So there's the ambiguity.

Fix?

Just tell the compiler exactly what you want,

put(TestClass.<CharSequence>get("hello")); // This will call the put(CharSequence) method.

Answer 2

Looks like this is a known incompatibility.

See "Area: Tools / javac" section of this article. And this bug.

Synopsis

The following code which compiled, with warnings, in JDK 7 will not compile in JDK 8:

import java.util.List;
 
class SampleClass {
 
    static class Baz<T> {
        public static List<Baz<Object>> sampleMethod(Baz<Object> param) {
            return null;
        }
    }
 
    private static void bar(Baz arg) {
        Baz element = Baz.sampleMethod(arg).get(0);
    }
} 

Compiling this code in JDK 8 produces the following error:

SampleClass.java:12: error:incompatible types: Object cannot be converted to Baz
    Baz element = Baz.sampleMethod(arg).get(0);
                                          
Note: SampleClass.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
1 error 

In this example, a raw type is being passed to the sampleMethod(Baz<Object>) method which is applicable by subtyping (see the JLS, Java SE 7 Edition, section 15.12.2.2).

An unchecked conversion is necessary for the method to be applicable, so its return type is erased (see the JLS, Java SE 7 Edition, section 15.12.2.6). In this case the return type of sampleMethod(Baz<Object>) is java.util.List instead of java.util.List<Baz<Object>> and thus the return type of get(int) is Object, which is not assignment-compatible with Baz.