Question 102

Explanation

UKOCP38454:

Target type of lambda expression must be a Functional interface.

Functional interface must have only one non-overriding abstract method but Functional interface can have constant variables, static methods, default methods, private methods and overriding abstract methods [equals(Object) method, toString() method etc. from Object class].

Let's check which of the given options are Functional interfaces:

interface MyInterface {
    String toStr();
}

✓  MyInterface has only one non-overriding abstract method [toStr()] and hence a Functional interface.

Given lambda expression () -> "" means method accepts no argument and returns a String object. It is a correct implementation of toStr() method.

interface MyInterface {
    String toStr();
    String toString();
}

✓  MyInterface has only one non-overriding abstract method [toStr()] and toString() method is overriding abstract method, therefore it is also a valid Functional interface.

() -> ""; is the correct implementation of toStr() method.

interface MyInterface {
    Object toStr();
    String toString();
    static void print(String str) {
        System.out.println(str);
    }
}

✓  MyInterface has only one non-overriding abstract method [toStr()] and toString() method is overriding abstract method & print(String) is the static method, therefore it is also a valid Functional interface.

() -> ""; is the correct implementation of toStr() method.

interface MyInterface {
    Object toStr();
    boolean equals(MyInterface);
}

✗  My interface has two non-overriding abstract methods. Please note that boolean equals(Object); is overriding abstract method but boolean equals(MyInterface); is non-overriding abstract method.

interface MyInterface {
    String toString();
}

✗  String toString(); is overriding abstract method and hence in this case, MyInterface doesn't contain non-overriding abstract method. Therefore it is not a Functional interface.

interface MyInterface {
    CharSequence test();
}

✓  MyInterface has only one non-overriding abstract method [test()] and hence a Functional interface.

As String class implements CharSequence, therefore value returned by Lambda expression [""] (String type) can easily be assigned to CharSequence reference.

() -> ""; is the correct implementation of CharSequence test();

interface MyInterface {
    StringBuilder m();
}

✗  MyInterface has only one non-overriding abstract method [m()] and hence a Functional interface.

Value returned by Lambda expression [""] (String type) can not be assigned to StringBuilder reference.

() -> ""; is NOT the correct implementation of StringBuilder m();