How does compiler ensure that equivalent lambda for below statement
BinaryOperator<String> concatOperator = String::concat;
is
BinaryOperator<String> concatOperator = (resultString, inputString) -> resultString.concat(inputString);
and not
BinaryOperator<String> concatOperator = (resultString, inputString) -> inputString.concat(resultString);
This behaviour is well-documented in the JLS
15.13.3. Run-Time Evaluation of Method References
If the compile-time declaration is an instance method, then the target reference is the first formal parameter of the invocation method. Otherwise, there is no target reference.
If the compile-time declaration is an instance method, then the arguments to the method invocation expression (if any) are the second and subsequent formal parameters of the invocation method. Otherwise, the arguments to the method invocation expression are the formal parameters of the invocation method.
and it seems reasonable and intuitive. If you take a method with arity n (n > 2), it becomes obvious that the target reference should be the first parameter, not the last, not the one in the middle.