Lambda Expression example not working in java 8

I know this answer is coming long after the question was asked, but may help others wondering about the same problem as I did recently, while learning about lambdas, and because your question is valid for a novice misguided by plenty of poorly presented examples on the web.

The reason your compiler is complaining is - the example is incomplete. One way to understand the incomplete code, is that it is only the functioning portion of a context (that the example is missing) which typically can be an interface of your own design or one of the simple ones predefined in Java 8 - such as Function, BiFunction, Predicate, etc. Also, supplying the value "hello" to your String s inside the lambda {working section} somewhat beats the purpose of the s variable. Here is a more complete example of similar functionality demonstrating use of 3 String variables in lambda expression:

public class LambdaTest {
                             // define your interface
    interface Use_3_Strings {
        void apply(String s1, String s2, String s3);
    }
                             // define a test method
    public void lambdaTest1(String str1, String str2, String str3) {
                             // your lambda expression
                             // defining what to do inside { }
                             // - like a method having 3 String parameters (s1, s2, s3)
        Use_3_Strings use3strings = (s1, s2, s3) -> { System.out.println("Working with 3 strings:"); 
                                                  System.out.println("   String 1: " + s1); 
                                                  System.out.println("   String 2: " + s2);
                                                  System.out.println("   String 3: " + s3);  
                                                  StringBuilder sb = new StringBuilder();
                                                  sb.append("CSV of 3 strings:\n");
                                                  sb.append(s1);
                                                  sb.append(", ");
                                                  sb.append(s2);
                                                  sb.append(", ");
                                                  sb.append(s3);
                                                  System.out.println(sb); }; 

                             // your lambda expression in action
                             // executing what you coded inside { } above
        use3strings.apply(str1, str2, str3);

    }

}

In your main() method then do:

LambdaTest lambdaTst = new LambdaTest();
lambdaTst .lambdaTest1("Hello", "beautiful", "world");

and you should get:

Working with 3 strings:
   String 1: Hello
   String 2: beautiful
   String 3: world
CSV of 3 strings:
Hello, beautiful, world

The lambda expressions always expects a reference to be assigned.

Java 7 Code:

Valid:

new Hahaha() {
    @Override
    public void howtolaugh(int number, String sound) {
        System.out.println("say "+sound + number + " number of times");
    }
};

Java 8:

Invalid:

(int i, String sound) -> System.out.println("say "+sound + i + " number of times");

Though these two serve the same purpose, the first one is an inner class. Inner classes have a default reference to the enclosing class. When they are decompiled, a different class file is generated for them in the format <Enclosing_Class_Name>$<a_number>.class or <Enclosing_Class_Name>$<a_number>.class. Ex: TestClass$1.class.

This is not the case in lambda expressions. They do not have an implicit reference to the enclosing class neither do they have a different class generated. They need an explicit reference either as a method parameter name, or a reference object on the left hand side or a return reference.