find duplicate element in array list

Question

import java.util.ArrayList;

public class list {

protected ArrayList<String> a = new ArrayList<String>();

public boolean ad(String aa)
{ 
    boolean t=true;
    a.add(aa);

     for(String value : courses)
     {
            if(a.contains(value))
            {
             a=false;
            }
            else
            { 
                a=true;

            }
     }
    return a;

}

}

this program should return false if arraylist course contains duplicate elements.else if we are inserting new element return true.

expected output for above code is

true

but it only returns false for any condition.

Quick question about your requirement - what should be returned if the list contains a duplicate that's different from the element you're inserting? — Dawood ibn Kareem
– Dawood ibn Kareem, Commented Apr 11, 2017 at 2:18
So all you need to do is use contains to check whether the list contains the element you're adding, right? No need for a loop! But it has to be before you add the element, otherwise you'll just find the element itself. Jacob G. has posted the solution you're looking for. — Dawood ibn Kareem
– Dawood ibn Kareem, Commented Apr 11, 2017 at 2:47
@DavidWallace He said that 'it's not working with our code' which I don't believe. — Jacob G.
– Jacob G., Commented Apr 11, 2017 at 3:06
Of course, OP's code mentions a class called Instructor (in main) but they've shown us a class called list. I suspect OP is confused about which class is which, @JacobG. — Dawood ibn Kareem
– Dawood ibn Kareem, Commented Apr 11, 2017 at 3:19

Jacob G. · Accepted Answer · 2017-04-11 02:20:11Z

6

You can simply utilize ArrayList#contains to verify if an element already exists within the List.

public boolean addCourse(String course) {
    if (courses.contains(course)) {
        return false;
    }

    return courses.add(course);
}

answered Apr 11, 2017 at 2:20

Jacob G.

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2 Comments

user7799918 Over a year ago

it should return true if we are inserting new element in arraylist.but its not working with our code @jacob G

Jacob G. Over a year ago

@user7799918 That's exactly what this does.

Jaydeep Rajput · Accepted Answer · 2017-04-11 02:19:06Z

1

You are adding course in the list and then iterating thr the list, so it always gives you true. ArrayList allows duplicates.

 if(courses.contains(value))

will always return true as you are adding the course before this in arraylist.

Suggestion: You should use Set than list if you want to avoid duplicates.

answered Apr 11, 2017 at 2:19

Jaydeep Rajput

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Comments

shanwu · Accepted Answer · 2017-04-11 02:19:53Z

1

Instead of using ArrayList, how about using HashSet to keep your courses ?

http://beginnersbook.com/2013/12/hashset-class-in-java-with-example/

answered Apr 11, 2017 at 2:19

shanwu

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Comments

Mr.Tuấn · Accepted Answer · 2017-04-11 03:33:27Z

Try the code below:

import java.util.ArrayList;
public class list {
  protected ArrayList<String> courses = new ArrayList<String>();
  protected String temp = "";

  public list(String str, String str2) {

  }

  public boolean addCourse(String course) {
    boolean a = true;
    if (courses.isEmpty()) {
        courses.add(course);
        temp = course;
    } else {
        if (temp.equalsIgnoreCase(course)) {
            a = false;
            temp = "";
        } else {
            a = true;
            courses.add(course);
            temp = course;
        }
    }
    return a;
  }
  public static void main(String[] args) {
    list inst = new list("John", "WIU");
    System.out.println(inst.addCourse("CS560"));
    System.out.println(inst.addCourse("CS500"));
  }
}

Py-Coder · Accepted Answer · 2017-04-11 02:33:03Z

0

Simple way you can do:

For Each time executing else block so. remove else block it will work.

for(String value : courses)
 {
        if(courses.contains(value))
        {
         a=false;
         break;
        }

            a=true;

 }

answered Apr 11, 2017 at 2:33

Py-Coder

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