Java, how do I search a specific variable of Objects saved in a array list

Question

I have a Personnel class, where I store all of Objects of type student, professor, tutor...

class Personnel {
     ArrayList<Student> activeStudentsList = new ArrayList<Student>();
}

and I have a class Student

class Student extends Person {
    public Student (String studentID, String firstName, String lastName) {
        super(firstName, lastName);
        this.studentID = studentID;
    }
}

Now, what I want to do before adding a student into array list, is to check if he's already there. I was using this:

private boolean isStudentActive(String studentID) {
    if (activeStudentsList.contains(studentID)) {
        System.out.println("The student " + studentID + " is already on the list.");
        return true;
    } else
        return false; 
}

The problem is, my array list is ArrayList<Student>, so I can't search for a specific String (studentID). How do I do this? How do I only search String studentID of each object on the list? EDIT: (Is there somethin like activeStudentsList.studentID.contains(studentID) ?)

Bhesh Gurung · Accepted Answer · 2011-10-28 15:43:08Z

10

Your Student needs to implement the equals and hashCode properly and you can just use List.contains.

answered Oct 28, 2011 at 15:43

Bhesh Gurung

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Comments

JB Nizet · Accepted Answer · 2011-10-28 15:41:03Z

4

Iterate through the list, test if the current student's ID is equal to the one you're looking for. If yes, return true, else continue. At the end of the iteration, return false.

Or, if you want something easier and faster, use a Map<String, Student> instead of the list, storing the students indexed by their ID. You may use a HashMap, or LinkedHashMap if you need to preserve the insertion order like a List does.

answered Oct 28, 2011 at 15:41

JB Nizet

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1 Comment

vedran Over a year ago

I used HashMap instead and it's even more efficient. Thank you

Java Drinker · Accepted Answer · 2011-10-28 15:49:58Z

2

A very simplistic implementation based on Bhesh's mention of making equals and hashcode use the student id:

class Student extends Person {

    private String studentID;

    public Student (String studentID, String firstName, String lastName) {
        super(firstName, lastName);
        this.studentID = studentID;
    }

    @Override
    public boolean equals(Object object) {
        if(object instanceof Student) {
            Student s = (Student) object;
            return this.studentID.equals(s.studentID);
        }
        return false;
    }

    @Override
    public int hashCode() {
        return studentID.hashCode();
    }
}

answered Oct 28, 2011 at 15:49

Java Drinker

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Comments

Wizetux · Accepted Answer · 2011-10-28 15:52:03Z

1

Assuming that the studentID string is public or that you have a public Get method for it, you can use the Java For-each loop to check each element in the list:

for (Student s : activeStudentsList)
{
   if (s.studentID == studentID)
      return true;
}
return false;

answered Oct 28, 2011 at 15:52

Wizetux

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Comments

charly's · Accepted Answer · 2011-10-28 16:02:04Z

1

Enjoy your time and take the time to use Guava libraries : http://code.google.com/p/guava-libraries/.

try
        {
            Person item = Iterables.find(this.personList,
                    new Predicate<Person>() {
                        public boolean apply(Person q)
                        {
                            return itemId.equals(q.getId());
                        }
                    });
            return item;
        } catch (NoSuchElementException exception)
        {
            return null;
        }

answered Oct 28, 2011 at 16:02

charly's

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Comments

Steve J · Accepted Answer · 2011-10-28 16:11:54Z

OK, here's my version. Avoid loops and terminating conditions and whatnot, and just program functionally using Google's Guava methods:

import java.util.ArrayList;
import java.util.Collection;
import java.util.Comparator;
import java.util.HashSet;

import com.google.common.base.Predicate;
import com.google.common.collect.Iterables;
import com.google.common.collect.TreeMultimap;

public class TestClass {

    class Person {
        private String firtName = null;
        private String lastName = null;

        public Person(String firtName, String lastName) {
            super();
            this.firtName = firtName;
            this.lastName = lastName;
        }
    };

    class Student extends Person {
        private String studentID = null;

        public Student(String studentID, String firstName, String lastName) {
            super(firstName, lastName);
            this.studentID = studentID;
        }

        public String getStudentID() {
            return studentID;
        }
    }

    class Personnel {
        ArrayList<Student> activeStudentsList = new ArrayList<Student>();

        public boolean isStudentActive(final String studentID) {
            return Iterables.size(Iterables.filter(activeStudentsList,
                    new Predicate<Student>() {
                        @Override
                        public boolean apply(Student arg0) {
                            return arg0.getStudentID().equals(studentID);
                        }
                    })) == 1;
        }
    }

}

It might seem a bit top-heavy for this sort of search, but I find using Filters and Transforms very useful in more complicated scenarios.

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