Why does Apple.applePrice = applePrice + applePrice; equal to 200 but Apple.applePrice + applePrice equals 150?

Apple.applePrice is class variable, applePrice is parameter which is send into class method addPrice.

Apple.applePrice = applePrice + applePrice;

in first case

apple.addPrice(50);

do 50+50, in second case

apple2.addPrice(100);

do 100+100, so the result is 200. The right solution of addPrice method is

Apple.applePrice += applePrice;

which take value of Apple.applePrice and add applePrice, so in first case 0+50=50 and then 50+100=150. It could be like this

Apple.addPrice(50);
Apple.addPrice(100);

and result would be same, because method addPrice and variable Apple.applePrice are static.

I cannot understand Why the 'Apple.applePrice' (which equivalent to the 1st value in the main method, which is stored at the 1st call) and 'applePrice' (which is equivalent to the 2nd value in the main method, which is stored at the 2nd call)? I mean why do we have to use Apple. to the 1st and not to the 2nd

As an experiment, think about a different way of writing the main method and everything becomes clearer. Please replace 'apple1' and 'apple2' with the class reference, just for playing around and understanding the code better:

public static void main(String[] args) {
        Apple.addPrice(50);
        Apple.addPrice(100);
        System.out.println("The cost of apples is " + Apple.applePrice);
    }

Here a call is made to the 'Apple' class's 'addPrice' method. You do not need the 2 variables 'apple1' and 'apple2' for getting the desired value '150' (replace them again when you have your code evaluated by codegym). These variables just throw sand into your eyes, like a smokescreen they make understanding it difficult what is going on. The implementation of the 2 variables 'apple1' and 'apple2' is totally redundant (= bad code style !!). In the method 'addPrice' (inside the 'Apple' class) you now just need to invoke the 'applePrice' variable and refer to the class 'Apple' with the dot operator. Do this on the left side of the statement. Then add together 'Apple.applePrice' (which equivalent to the 1st value in the main method, which is stored at the 1st call) and 'applePrice' (which is equivalent to the 2nd value in the main method, which is stored at the 2nd call). This adds up to a value of '150'. Voilà! Now do not forget to implement the objects 'apple1' and 'apple2' again and have your code evaluated!

The addPrice is adding up the entered price twice and doesn't use anything from before.

Apple.applePrice = applePrice + applePrice;

The applePrice is static, so it doesn't matter if you call the addPrice from apple or apple2, it still belongs to Apple (you can also call it by Apple.addPrice). At the first call:

apple.addPrice(50);

It adds up 50+50, with the result 100 and stores into the applePrice variable. At the second call:

apple2.addPrice(100);

It adds up 100+100, so 200 is the result what stored to the applePrice variable (and the 100 before is disappeared). I guess the misunderstanding comes from the variable naming and scope. When you are using the "applePrice" by itself in the method it is the passed applePrice, and when you are using the Apple.applePrice it is the variable from the Apple class.