elastic/elasticsearch · error · IllegalArgumentException

cannot use a static method as a function reference [%s::%s/%

Error message

cannot use a static method as a function reference [%s::%s/%d] with a non-static captured variable

What it means

FunctionRef.create throws this IllegalArgumentException at compile time when a static method is used as a function reference with a non-static captured variable. The compiler first resolves the method as an augmented (instance) method; if that succeeds but the reference also requires capturing an instance (numberOfCaptures == 1), the method is static and cannot accept a receiver capture. Painless rejects this because a static method has no 'this' to bind.

Source

Thrown at modules/lang-painless/src/main/java/org/elasticsearch/painless/FunctionRef.java:204

                        methodName,
                        captured ? interfaceTypeParametersSize : interfaceTypeParametersSize - 1
                    );

                    if (painlessMethod == null) {
                        throw new IllegalArgumentException(
                            Strings.format(
                                "function reference [%s::%s/%d] matching [%s, %s/%d] not found",
                                typeName,
                                methodName,
                                interfaceTypeParametersSize,
                                targetClassName,
                                interfaceMethodName,
                                interfaceTypeParametersSize
                            )
                        );
                    }
                } else if (captured) {
                    throw new IllegalArgumentException(
                        Strings.format(
                            "cannot use a static method as a function reference [%s::%s/%d] with a non-static captured variable",
                            typeName,
                            methodName,
                            interfaceTypeParametersSize
                        )
                    );
                }

                delegateClassName = painlessMethod.javaMethod().getDeclaringClass().getName();
                isDelegateInterface = painlessMethod.javaMethod().getDeclaringClass().isInterface();
                isDelegateAugmented = painlessMethod.javaMethod().getDeclaringClass() != painlessMethod.targetClass();

                if (Modifier.isStatic(painlessMethod.javaMethod().getModifiers())) {
                    delegateInvokeType = H_INVOKESTATIC;
                } else if (isDelegateInterface) {
                    delegateInvokeType = H_INVOKEINTERFACE;
                } else {

View on GitHub (pinned to db6a809a66)

Solutions

  1. If the method is static, use the class-qualified form (Type::method) without an instance capture.
  2. If you need an instance binding, ensure the referenced method is an instance (non-static) method.
  3. Rewrite as an explicit lambda to make the binding explicit: '() -> Type.method()' or '() -> instance.method()'.

Example fix

// before
String s = '42';
Supplier<Integer> f = s::parseInt;  // parseInt is static
// after
Supplier<Integer> f = () -> Integer.parseInt(s);
Defensive patterns

Strategy: validation

Validate before calling

// Use class-qualified form for static methods; instance form only for instance methods:
// // Static:  Function<String,Integer> f = Integer::parseInt;
// // Instance: Function<String,String> f = String::toUpperCase;

Prevention

When it happens

Trigger: Writing a method reference that captures an instance variable but resolves to a static method: 'String s = 'abc'; Supplier<Integer> len = s::valueOf;' where valueOf is static but the reference tries to capture s as a receiver. The augmented lookup finds the static method, but the capture flag indicates an instance binding is expected.

Common situations: Confusion between static and instance method references (e.g. Type::method vs instance::method). Refactoring a method to static without updating the reference syntax. Capturing an instance for a method that is actually static.

Related errors

AI-assisted analysis of elastic/elasticsearch@db6a809a66 (2026-08-12). Data as JSON: /api/errors/b627b5bb39cf951d. Report an issue: GitHub.