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Question about the task

Horse racing

Java Core, Level 6, Lesson 5

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Hi, although my code works, but it is not meeting the requirements if someone could help

Figure out what the program does.
Implement the calculateHorsesFinished method.
It must:
1. Calculate and return the number of horses that have finished. Use the isFinished() method.
2. If a horse has not yet crossed the finish line (!IsFinished()), then:
2.1. Display "Waiting for " + horse.getName().
2.2. Wait until it finishes the race. Think about what method you need to use to do this.
2.3. Not treat such a horse as finished.

Requirements:

The calculateHorsesFinished method must return the number of horses that have finished.
The calculateHorsesFinished method must call the isFinished method on each horse in the passed list.
If any of the horses in the passed list has not yet finished, then the calculateHorsesFinished method should display "Waiting for " + horse.getName(). Example output for the first horse: "Waiting for Horse_01".
If any of the horses in the passed list has not yet finished, then the calculateHorsesFinished method must wait until it finishes. Use the correct method for waiting.
After the program is finished, the console must indicate that all the horses have finished. Example output for the first horse: "Horse_01 has finished the race!".

package com.codegym.task.task16.task1607; /* Horse racing Figure out what the program does. Implement the calculateHorsesFinished method. It must: 1. Calculate and return the number of horses that have finished. Use the isFinished() method. 2. If a horse has not yet crossed the finish line (!IsFinished()), then: 2.1. Display "Waiting for " + horse.getName(). 2.2. Wait until it finishes the race. Think about what method you need to use to do this. 2.3. Not treat such a horse as finished. Requirements: 1. The calculateHorsesFinished method must return the number of horses that have finished. 2. The calculateHorsesFinished method must call the isFinished method on each horse in the passed list. 3. If any of the horses in the passed list has not yet finished, then the calculateHorsesFinished method should display "Waiting for " + horse.getName(). Example output for the first horse: "Waiting for Horse_01". 4. If any of the horses in the passed list has not yet finished, then the calculateHorsesFinished method must wait until it finishes. Use the correct method for waiting. 5. After the program is finished, the console must indicate that all the horses have finished. Example output for the first horse: "Horse_01 has finished the race!". */ import java.util.ArrayList; import java.util.List; public class Solution { public static void main(String[] args) throws InterruptedException { List<Horse> horses = prepareHorsesAndStart(10); while (calculateHorsesFinished(horses) != horses.size()) { calculateHorsesFinished(horses); break; } } static int finishedCount =0; public static int calculateHorsesFinished(List<Horse> horses) throws InterruptedException { for (Horse horse: horses) { if (horse.isFinished == true){ finishedCount++; break;} else { System.out.println("Waiting for " + horse.getName()); horse.join(); } } return finishedCount; } public static List<Horse> prepareHorsesAndStart(int horseCount) { List<Horse> horses = new ArrayList<>(horseCount); String number; for (int i = 1; i < horseCount + 1; i++) { number = i < 10 ? ("0" + i) : "" + i; horses.add(new Horse("Horse_" + number)); } //Start each horse in the arrayList, then return the horses. for (int i = 0; i < horseCount; i++) { horses.get(i).start(); } return horses; } public static class Horse extends Thread { // private boolean isFinished; public Horse(String name) { super(name); } public boolean isFinished() { return isFinished; } public void run() { String s = ""; for (int i = 0; i < 1001; i++) { // Delay s += "" + i; if (i == 1000) { s = " has finished the race!"; System.out.println(getName() + s); isFinished = true; } } } } }

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Nouser

Level 36 , Germany

13 March 2021, 07:31

The calculateHorsesFinished should loop through the entirs horses list and check, if a horse already has finished. If so, increase a counter and return that count. Now assume horse0 has finished and all the rest not. Then for horse0 you increase the count, then you break and return 1 to the caller. With the next call of the method horse0 still has finished, and cause you decided to make the count a static method, you increase the count to 2 but after horse0 your loop breaks again. You will never check the other horses. After you checked horse0 10 times you break the loop in the main and say all have finished. Means you have to declare and initialize your counter inside calculateHorsesFinished and you need to remove the break. And delete lines 31, 32. They are not necessary at all. Calling calculateHorsesFinished is already done inside the condition. More a problem is the break cause it'll stop your loop from checing if all horses have finished. The main ends and after that the horse threads tell you the have finished. In the end you have a result that looks like all would work but nothing at all did.

Mina Nabil

Level 17 , Sydney, Australia

13 March 2021, 09:28

Thanks Nouser. I just did not understand why Calling calculateHorsesFinished again is not necessary if you could explain more

Nouser

Level 36 , Germany

13 March 2021, 11:59

solution

while (calculateHorsesFinished(horses) != horses.size()) {
    //calculateHorsesFinished(horses);
    //break;
}

inside the while condition you already call calculateHorsesFinished, it returns the count of finished horses so a some state of the programm it may (for the jvm) look similar to while (7 != horses.size()). In the next loop iteration it may already be while (8 != horses.size())... so all relevant calculation is already done in the condition. When you call the method again in the body, then it calculates again and returns back the result, eg. 5. Then you have a method body with

{
    5;
}

this does nothing at all (except maybe a join call inside calculateHorsesFinished , but that we ignore atm). More of a problem is the break. It ends the loop and calculateHorsesFinished won't get called again. To be precise it just gets invoked exactly two times. One time inside of the while condition, a second time inside the body and then we reach the break and leave the loop no matter if really all horses reached the finish line. What you now see are just the remaining threads coming to an end and printing the result.

Mina Nabil

Level 17 , Sydney, Australia

13 March 2021, 13:17

thanks Nouser, just in your opinion, don't you think that this while (calculateHorsesFinished(horses) != horses.size()) loop is a bit strange, like normally I would write while(true){ calculateHorsesFinished(horses); if(calculateHorsesFinished(horses) == horses.size()) break; } What do you think ?

Nouser

Level 36 , Germany

13 March 2021, 13:36

useful

You again invoke calculateHorsesFinished two times and one time you do not save its result. If you want to do it that way, and in my opinion that is just personal preference, then I'd save the result in a variable or drop the first call

while(true){
    if(calculateHorsesFinished(horses) == horses.size())
        break;
}

while(true){
    int horsesFinished = calculateHorsesFinished(horses);
    if(horsesFinished == horses.size())
        break;
}

I use both approaches but infinite loops that you need to stop using a break statement aren't that pretty for me. On the other hand they often make it more clear what you want to do. To sum it up, it depends ;) eg. constructs like that one are easy to understand I and use them all the time. And I do not need to worry about an abort condition inside the while block. And you know at the first view when the loop breaks without the need to search for it.

while (!(input = reader.readLine()).equals("exit")) {

Mina Nabil

Level 17 , Sydney, Australia

13 March 2021, 13:51

Thanks Nouser so much for the explanation, but the code is not working, first 3 conditions not met public class Solution { public static void main(String[] args) throws InterruptedException { List<Horse> horses = prepareHorsesAndStart(10); while (calculateHorsesFinished(horses) != horses.size()) { } } public static int calculateHorsesFinished(List<Horse> horses) throws InterruptedException { int finishedCount =0; for (Horse horse: horses) { if (horse.isFinished == true){ finishedCount++; } else { System.out.println("Waiting for " + horse.getName()); horse.join(); } } return finishedCount; } public static List<Horse> prepareHorsesAndStart(int horseCount) { List<Horse> horses = new ArrayList<>(horseCount); String number; for (int i = 1; i < horseCount + 1; i++) { number = i < 10 ? ("0" + i) : "" + i; horses.add(new Horse("Horse_" + number)); } //Start each horse in the arrayList, then return the horses. for (int i = 0; i < horseCount; i++) { horses.get(i).start(); } return horses; } public static class Horse extends Thread { private boolean isFinished; public Horse(String name) { super(name); } public boolean isFinished() { return isFinished; } public void run() { String s = ""; for (int i = 0; i < 1001; i++) { // Delay s += "" + i; if (i == 1000) { s = " has finished the race!"; System.out.println(getName() + s); isFinished = true; } } } } }

Nouser

Level 36 , Germany

13 March 2021, 14:05

Maybe try to reset the task. I just checked and my code that validated looks pretty much the same as yours.

public static void main(String[] args) throws InterruptedException {
    List<Horse> horses = prepareHorsesAndStart(10);
    while (calculateHorsesFinished(horses) != horses.size()) {
    }
}

public static int calculateHorsesFinished(List<Horse> horses) throws InterruptedException {
    int finishedCount = 0;
    for (Horse horse : horses) {
        if (horse.isFinished()) {
            finishedCount++;
        } else {
            System.out.println("Waiting for " + horse.getName());
            horse.join();
        }
    }
    return finishedCount;
}