How to find a duplicate object of arraylist in java

You can use

Collections.frequency(collection, object);

So if frequency method return a number > 1 it means that you have more same object...

Collections is get by java.util.Collections

and that metod Returns the number of elements in the specified collection equal to the specified

Here you are the api:

frequency(c, o)

Default implementation of equals() method provided by java.lang.Object compares memory location and only return true if two reference variable are pointing to same memory location. equals() method used to avoid duplicates on HashSet.

The problem is that you didn't override the equals() and hash() methods.

A common source of bugs is the failure to override the hashCode method. You must override hashCode in every class that overrides equals. Failure to do so will result in a violation of the general contract for Object.hashCode, which will prevent your class from functioning properly in conjunction with all hash-based collections, including HashMap, HashSet, and Hashtable. [Effective Java]

import java.util.*;

public class HashSetTest {

    public static class Dummy {
        private int age;
        private String name;

        public int getAge() {
            return age;
        }

        public void setAge(int age) {
            this.age = age;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (!(o instanceof Dummy)) return false;

            Dummy dummy = (Dummy) o;

            if (age != dummy.age) return false;
            if (name != null ? !name.equals(dummy.name) : dummy.name != null) return false;

            return true;
        }

        @Override
        public int hashCode() {
            int result = age;
            result = 31 * result + (name != null ? name.hashCode() : 0);
            return result;
        }

        @Override
        public String toString() {
            return String.format("age:%d, name:%s", age, name);
        }
    }

    public static void main(String[] args) {
        Dummy dummy01 = new Dummy();
        dummy01.setAge(14);
        dummy01.setName("XXXX");

        Dummy dummy02 = new Dummy();
        dummy02.setAge(15);
        dummy02.setName("YYYY");

        Dummy dummy03 = new Dummy();
        dummy03.setAge(14);
        dummy03.setName("XXXX");

        List<Dummy> dummies = new ArrayList<Dummy>();
        dummies.add(dummy01);
        dummies.add(dummy02);
        dummies.add(dummy03);

        Set<Dummy> uniqueDummies = new HashSet<Dummy>();
        for (Dummy dummy : dummies) {
            uniqueDummies.add(dummy);
        }

        System.out.println(uniqueDummies);
    }
}

You Can Visit this Link

Sample Code of this link

//ArrayList with duplicates String
List<String> duplicateList = (List<String>) Arrays.asList("Android" , "Android", "iOS", "Windows mobile");
System.out.println("size of Arraylist with duplicates: " + duplicateList.size()); //should print 4 becaues of duplicates Android

System.out.println(duplicateList);

//Converting ArrayList to HashSet to remove duplicates
HashSet<String> listToSet = new HashSet<String>(duplicateList);

//Creating Arraylist without duplicate values
List<String> listWithoutDuplicates = new ArrayList<String>(listToSet);
System.out.println("size of ArrayList without duplicates: " + listToSet.size()); //should print 3 becaues of duplicates Android removed

System.out.println(listWithoutDuplicates);

Output:
size of Arraylist with duplicates: 4
[Android, Android, iOS, Windows mobile]
size of ArrayList without duplicates: 3
[Android, Windows mobile, iOS]