Java NullPointerException在构造函数的类中

Question

I have made a Java class where I have defined a constructor and some methods but I get a NullPointer Exception, and I don't know how I could fix It.

public class Job {

    String idJob;
    int time;
    int timeRun;
    Job j1;

    List<Job> startBeforeStart;
    List<Job> restricted;

    Job(String idJob, int time){
        this.idJob=idJob;
        this.time=time;

    }


    public boolean isRestricted() {
        return restricted.size() != 0;
    }

    public void startsBeforeStartOf(Job job){
            startBeforeStart.add(job);
            job.restricted.add(this);
    }

    public void startsAfterStartOf(Job job){
            job.startsBeforeStartOf(this);
    }

    public void checkRestrictions(){

        if (!isRestricted()){
            System.out.println("+\n");
            }
        else{
            Iterator<Job> itR = restricted.iterator();
            while(itR.hasNext()){
                 Job j1 = itR.next();
                 if(time>timeRun){
                     System.out.println("-\n");
                     time--;
                 }
                 else {
                     restricted.remove(j1);
                 }
            }
        }
    }






    @Override
    public boolean equals(Object obj) {
        return obj instanceof Job && ((Job) obj).idJob.equals(idJob);
    }



     public void run() {
         timeRun++;
     }



}

PS Looking in a forum a user says that to fix the error I should make an ArrayList inside the constructor (without modify the received parameters that should remain String id and int time ), but I haven't understand what He mean.

Answer 1

You are not creating an instrance of List<Job> for both the lists startBeforeStart and restricted - you only declare a variable, which is assigned with a null pointer.

Thus, whenever you try to access this List [for example: return restricted.size() != 0; ] - you are trying to dereference a null pointer - which causes your NPE.

You should create an instance of the List - using the new operator [probably in the constructor].

Have a look at ArrayList and LinkedList and chose which is better for you.

For example, if you use to use an ArrayList for both, your c'tor should be something like:

Job(String idJob, int time){
    this.idJob=idJob;
    this.time=time;
    startBeforeStart = new ArrayList<Job>();
    restricted= new ArrayList<Job>();

}