OCA / OCP Java SE 8 Programmer Practice Tests: Chapter 13 Question 40 Errata

[Tamas Szekeres]

Question is:

Why can’t String::charAt be used as a method reference?

Offered answers:

A. Method references can only be used on static methods.
B. Method references can pass either the instance or the parameter from the lambda, but not both.
C. The charAt() method takes an int rather than Integer parameter.
D. There is no charAt() method in the String class.

Answer, according to Answers to Review Questions:

B. ... With method references, only one item can be supplied at runtime. Here, we need either a String instance with no parameters in the method or a static method with a single parameter. The charAt() method is an instance method with a single parameter so does not meet this requirement.

Correct answer: it can be used as a method reference:

import java.util.function.BiFunction;

public class MethodRef {
    public static void main(final String[] args) {
        final MethodRef playground = new MethodRef();
        playground.opFunc(String::charAt, "abcd", 1);
    }

    private void opFunc(final BiFunction<String, Integer, Character> fn, final String str, final int pos) {
        System.out.println(fn.apply(str, pos)); // this will print "b"
    }

}

[Jeanne Boyarsky]

You're right! It never occurred to me to do this! Let's pretend the question says "with a Function" because that's the only way to save it .

I've given you a cow for this great find and added it to the errata