Do Lambda expressions have to be inside a method? Can they exist outside a method as a member of a class

Question

I am trying to see, the difference in passing functions as parameters between Scala and Java. I have moved to Scala a few years ago, not in touch with Java much. Can I have a Lambda expression outside of a method, like below?. It compiles and works fine, but Is there anything wrong with it. So far all the examples I have seen have Lamdas inside a method. The below code is just an example, I am aware you can achieve the same thing with Supplier Interface.

@FunctionalInterface
public interface IFunc {

    String printMessage();
}


public class JavaFunc {

    public String message() {

        return functionA(functionB);
    }

    IFunc functionB = () ->  "Hi, I am functionB from Java, I am passed to functionA as a parameter.";

    public String functionA(IFunc funcB) {
        return funcB.printMessage();
    }

}

Answer 1

You can declare lambda expression any where in class

If you declare inside a method (which is similar to local variables, scope inside of method)

public String message() {

  IFunc functionB = () ->  "Hi, I am functionB from Java, I am passed to functionA as a parameter.";

    return functionA(functionB);
}

If you declare in the class (Which is similar to instance variables that can be accessed in all instance methods directly and with object reference in static area )

public class JavaFunc {

 IFunc functionB = () ->  "Hi, I am functionB from Java, I am passed to functionA as a parameter.";

public String message() {

    // use here functionB
    return functionA(functionB);
}

Example : lambda expression for Predicate and anonymous inner class

p1,p2 are at instance level and p3,p4 are local

public class DemoMain {

Predicate<Integer> p1 = i -> i > 5;

Predicate<Integer> p2 = new Predicate<Integer>() {

    @Override
    public boolean test(Integer t) {
        // TODO Auto-generated method stub
        return false;
    }
};

public static void main(String args[]) {

    DemoMain m = new DemoMain();
    System.out.println(m.p1.test(10));
    System.out.println(m.p2.test(10));
}

public void m1() {

    System.out.println(p1.test(10));
    System.out.println(p2.test(10));

    Predicate<Integer> p3 = i -> i > 5;

    Predicate<Integer> p4 = new Predicate<Integer>() {

        @Override
        public boolean test(Integer t) {
            // TODO Auto-generated method stub
            return false;
            }
        };

    }

 }

Answer 2

To put it in another way, lambda expressions are also known as function objects, so wherever you can use a normal objects, you can also use lambdas: you can define them as attributes, you can pass them as method parameters, etc.

public ExampleClass {
    //Initializing the consumer with our method reference (subset of lambdas)
    static Consumer<Integer> printer = System.out::println;

    public static void useLambda(Consumer<Integer> myIntegerPrinter, Integer value) {
         myIntegerPrinter.accept(value);
    }

    public static void main(String[] args) {
         // passing the lambda as parameter to useLambda
         useLambda(printer, 100);
    }
}