IIFE/Automatic Method in Java?

Question

I know that there is a way to have a method to run automatically in java?. This is known as an IIFE in javascript, but is this possible in java?

Javascript IIFE:

(function() {
    console.log('Hello!');    
})();

Thank You! (I'm also just curious)

Answer 1

Here's an IIFE in Java:

((Function<String, String>) s -> {
    String z = "'" + s + "'";
    return z;
}).apply("aaa");

Answer 2

All of the following print "Hello world!"

JavaScript:

console.log((function() {
  const x = "Hello world!";
  return x;
})());

Java:

System.out.println(((Supplier<String>) () -> {
  String x = "Hello world!";
  return x;
}).get());

In Java it may feel more ergonomic to create a helper function to infer the type and execute the function for you:

public static <T> T iife(Supplier<? extends T> supplier) {
  return supplier.get();
}
...
System.out.println(iife(() -> {
  String x = "Hello world!";
  return x;
}));

In general you might want to consider factoring out the function. But if the function is relatively small, and especially if it captures several variables, an IIFE may be more readable. I liken an IIFE to a block expression (which Java does not have).

Answer 3

Stumbled upon this question when looking for this idea myself. I think the closest thing Java has to JavaScript IIFEs would be an instance of an abstract class, whose only method, an execute method, is overriden during the instance creation and then executed immediately after the object's instantiation. You can even get the closure aspect of it too. However, you won't be able to change what the variable refers to inside the overriden method.

JavaScript:

let subject = 'World';

(() => {
    console.log(`Hello, ${subject}!`);
})();

Java:

public abstract class Iife {
    public abstract void execute();
}

public class Main {

    public static void main(String[] args) {
        String subject = "World";

        new Iife() {
            @Override
            public void execute() {
                System.out.println("Hello, " + subject + "!");
            }
        }.execute();
    }
}

Answer 4

There is no direct way as mentioned by other people above.

Anonymous Inner Class with init() Initializer

I feel that this could be used like IIFEs but the problem is that it needs to be inside another class

Thread T  = new Thread() {
    private int num;
    Thread init(int num){
        this.num = num;
        return this;
    }
    @Override
    public void run() {
        // computes and outputs the factorial of num
        int res = 1;
        for (int i = 1; i <= num; i++) {
            res *= i;
        }
        System.out.println(res);
    }
}.init(3);

The init() can be used to pass parameters to be used for working

Answer 5

Java will automatically run the "public static void main(String[] args)" method in the class specified.

Answer 6

In a static context, you can define code wrapped within brackets using the static modifier:

public class MyClass{
    static{
        System.out.println("Running static");
    }
}

In the context of Objects, you can wrap the code in the same manner without the static modifier:

public class MyClass{

    {
        System.out.println("Initializing");
    }

}

Answer 7

There is no IIFE in java.

Java is statically typed & compiled as opposed to javascript which is dynamically typed and interpreted.

In java there is only one entry point to a program which is the method main which is static and public.

In Groovy (JVM base language) you can use repl where defined method (functions are method in java terminology) can be invoked later which may be the nearest thing to IIFE.