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First-class function in JavaScript and Java

12 Feb 2017
JavaScript Java

What is first-class function?

  • Can be named by variables.
  • Can be passed as arguments to procedures.
  • Can be returned as the results of procedures.
  • Can be included in data structures.

Functions are first class objects in JavaScript

With reference from this post, functions are first class objects in JavaScript:

  • A function is an instance of the Object type
  • A function can have properties and has a link back to its constructor method
  • You can store the function in a variable
  • You can pass the function as a parameter to another function
  • You can return the function from a function
function hello() {
  return 'hello'
}

// Function is instance of the Object type
hello instanceof Object // true

// Function can has properties and has a link back to its constructor method
hello.personName = 'Bob'

console.log(hello.personName)
console.log(hello.constructor)

// You can store the function in a variable
var storedFunction = hello
storedFunction() // same as hello()

// You can pass the function as a parameter to another function
function saySomething(functionAsParameter) {
  console.log('I say: ' + functionAsParameter());
}

saySomething(hello);  // will print 'I say: hello'

// You can return the function from a function
function getSecretFunction(){
  return hello
}

var secretFunction = getSecretFunction()
secretFunction()

First class objects in Java

In Java these are first class objects:

  • Primitive values: int, double, long, short, byte, char…
  • Objects: new Integer(1), new ArrayList(), new Person()…

Classes and methods are second class objects…until when Java 8 was introduced:

public class FilterTool {
    public List<String> filter(List<String> list, Predicate<String> predicate) {
        List<String> result = new ArrayList<>();
        for (String string : list) {
            if (predicate.test(string)) {
                result.add(string);
            }
        }
        return result;
    }

    public static boolean moreThan2Characters(String word) {
        return word.length() > 2;
    }
}

We will try to pass a function as parameter to other function

@Test
public void test() {
    FilterTool filterTool = new FilterTool();
    List<String> originalList = new ArrayList<>();
    originalList.add("a");
    originalList.add("aa");
    originalList.add("aaa");
    originalList.add("aaaa");

    // Pass moreThan2Characters as parameter
    Assert.assertEquals(2, filterTool.filter(originalList, FilterTool::moreThan2Characters).size());
}

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