Creating a static block and putting inside elements of an future created map. How is this possible?

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Hello, so we have the program below. The static block is last. Upon creation of the class, from my knowledge the first elements that will be created are static variables and static blocks of code. My question is, considering that the static block of code will be created before the map element, how can we put elements in the map is this will be created later on? It's like being in the present, going to the future to take some elements with you and coming back to the present to implement them even though they do not exist yet.

public class Solution {
    public static Map<Double, String> labels = new HashMap<>();

    public static void main(String[] args) {
        System.out.println(labels);
    }

    static {
        labels.put(2.33, "Caca");
        labels.put(3.20, "Pipi");
        labels.put(1.2, "dasa");
        labels.put(4.44, "patru");
        labels.put(5.778, "slas");
    }
}

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Nouser

Level 36 , Germany

7 December 2020, 10:55

solution

As far as I understand this, you can replace a statement with a block. Nothing else happens here. So when it is the turn to analyze the static context, then the compiler will go through to code from top to bottom. First static content is the Map labels, then follows the static block. At that time the map is known and the code in the block is able to execute and add data to the map. If you move the map declaration below the static block it should fail with an illegal forward reference. You use a variable before it has been declared.

Andrei

Level 41

7 December 2020, 11:02

Ah, yes, Map is STATIC therefore loaded in the beginning as well. What if map wasn't static? Would it have happened like I mentioned ?

Nouser

Level 36 , Germany

7 December 2020, 11:06

useful

from a static context you can't access objects cause the object reference (this) is unknown

Andrei

Level 41

7 December 2020, 11:19

So basically it wouldn't even create the map because the program would not compile therefore not go further. Got it, thank you!

Nouser

Level 36 , Germany

7 December 2020, 11:37

The map would get created. But as it is a field, you can just access it having the this reference. Inside of the static block the this reference is missing and you'll get an error, meaning the data doesn't get added. In addition to that instance blocks get initialized later Parent : static block Child : static block Parent : instance block Parent : constructor Child : instance block Child : constructor

Andrei

Level 41

7 December 2020, 11:53

But how would be the map created if it doesn't even compile when the map is not static. "java: non-static variable labels cannot be referenced from a static context"

public class Solution {
    public Map<Double, String> labels = new HashMap<>();

    public static void main(String[] args) {
        System.out.println(labels);
    }

    static {
        labels.put(2.33, "Caca");
        labels.put(3.20, "Pipi");
        labels.put(1.2, "dasa");
        labels.put(4.44, "patru");
        labels.put(5.778, "slas");
    }
}

Isn't the object map created at compile time?

Nouser

Level 36 , Germany

7 December 2020, 12:07

Of course you're right. This are just assumptions if the code would compile and what happens then.

Andrei

Level 41

7 December 2020, 12:11

Ah, ok, I understand. Thank you! 😁🤝