CODEWITHSUNDEEP
java
eleon
eleon

Reputation: 183

Java - How can I add an object to a list of another object

I have an Class called Process and a Class Called Event. Process has a list of Events as one of its attributes. I am reading a file and populating the Process attributes and the Event attributes but when I try to add the Event to the events list of the Process object I get NullPointerException. I have more experience in C# but I have to do this in Java. This is what a line in the file looks like: 

0 1 CPU 10 IO 3 CPU 50 IO 3 CPU 10

These are the classes:

public class Process {

int arrivalTime;
int priority;
LinkedList<Event> events;
}

Event.java,

public class Event {
String type;
int duration;
}

FCFS.java,

public class FCFS{

public static void main(String[] args) {
    List<Process> readyQueue = new LinkedList<Process>();

    BufferedReader reader;
    try {
        reader = new BufferedReader(new FileReader("path.txt"));
        String line = reader.readLine();
        while (line != null) {
            String[] arr = line.split(" ");

            Process process = new Process();

            process.arrivalTime = Integer.parseInt(arr[0]);
            process.priority = Integer.parseInt(arr[1]);

            for(int i = 2; i < arr.length; i = i + 2) {

                Event event = new Event();
                event.type = arr[i];
                event.duration = Integer.parseInt(arr[i+1]);
                process.events.add(event);

            }

            readyQueue.add(process);
            // read next line
            line = reader.readLine();

        }
        reader.close();
    } catch (IOException e) {
        e.printStackTrace();
    }   

}
}

Upvotes: 1

Views: 85

Answers (2)

Mohit
Mohit

Reputation: 134

You have not initialized the events list. Either initialize it in the Process class itself to an empty list or after creating Process object.

In Java, if you do not initialize a Class type, it defaults to null.

Upvotes: 1

Sam
Sam

Reputation: 153

I guess it is due to not initializing the events. can you try doing the below

public class Process {

int arrivalTime;
int priority;
LinkedList<Event> events = new LinkedList();

}

Upvotes: 1

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