如何从ArrayList中删除重复值<CustomObject>在 Java 中

Question

I have a ArrayList<CustomObject> as ArrayList<Names> in my project. The Names pojo contains name and image fields as follows:

Names.java

public class Names {


    private String name;

    private String image;

    public String getName() {
        return name;
    }

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

    public String getImage() {
        return image;
    }

    public void setImage(String image) {
        this.image = image;
    }

    public Names(String name, String image) {
        this.name = name;
        this.image = image;
    }

    @Override
    public String toString() {
        // TODO Auto-generated method stub
        return name;
    }

}

I am adding values for the fields as follows:

         ArrayList<Names> menu = new ArrayList<Names>();

         menu.add(new Names("chandru","image1"));
         menu.add(new Names("vikki","image2"));
         menu.add(new Names("karthick","image3"));
         menu.add(new Names("chandru","image4"));
         menu.add(new Names("karthick","image5"));
         menu.add(new Names("chandru","image6"));
         menu.add(new Names("karthick","image7"));
         menu.add(new Names("vikki","image8"));
         menu.add(new Names("karthick","image9"));
         menu.add(new Names("harish","image10"));
         menu.add(new Names("vivek","image11"));
         menu.add(new Names("harish","image12"));

My requirement:

Now all my requirement is to remove the repeated names contains in the ArrayList. I tried several methods like to remove duplicates as follows:

Method I: Using HashSet

Adding values into the HashSet and assigning back those values into new ArrayList<Names> named al .

          ArrayList<Names> al = new ArrayList<Names>();

          Set<Names> hs = new HashSet<Names>();
          hs.addAll(menu);

          al.clear();
          al.addAll(hs);

          System.out.println(al);

Output of Method I:

[karthick, vikki, karthick, karthick, chandru, vivek, vikki, chandru, harish, harish, karthick, chandru]

Expected Output to be:

Values after removing duplicates:

[karthick, vikki,chandru, vivek, harish]

I am also posting my entire class for your reference

import java.util.ArrayList;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Scanner;
import java.util.Set;


public class Sample {

     public static void main(String[] args) {

         ArrayList<Names> menu = new ArrayList<Names>();
         menu.add(new Names("chandru","image"));
         menu.add(new Names("vikki","image"));
         menu.add(new Names("karthick","image"));
         menu.add(new Names("chandru","image"));
         menu.add(new Names("karthick","image"));
         menu.add(new Names("chandru","image"));
         menu.add(new Names("karthick","image"));
         menu.add(new Names("vikki","image"));
         menu.add(new Names("karthick","image"));
         menu.add(new Names("harish","image"));
         menu.add(new Names("vivek","image"));
         menu.add(new Names("harish","image"));

         ArrayList<Names> al = new ArrayList<Names>();

          Set<Names> hs = new HashSet<Names>();
          hs.addAll(menu);

          al.clear();
          al.addAll(hs);

          System.out.println(al);

     }
 }

Please help me to resolve by issue which I am facing to remove duplicates from the list. Any kind of suggestions and solutions would be much helpful for me. Thanks in advance.

Answer 1

You just need to override hashCode and equals and your code works well:

   public class Names {


    private String name;

    private String image;

    public String getName() {
        return name;
    }

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

    public String getImage() {
        return image;
    }

    public void setImage(String image) {
        this.image = image;
    }

    public Names(String name, String image) {
        this.name = name;
        this.image = image;
    }

    @Override
    public String toString() {
        return name;
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Names other = (Names) obj;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        return true;
    }


}

Test Class

import java.util.ArrayList;
import java.util.HashSet;
import java.util.Set;

public class Test {


    public static void main(String[] args) {

        ArrayList<Names> menu = new ArrayList<Names>();
        menu.add(new Names("chandru","image"));
        menu.add(new Names("vikki","image"));
        menu.add(new Names("karthick","image"));
        menu.add(new Names("chandru","image"));
        menu.add(new Names("karthick","image"));
        menu.add(new Names("chandru","image2"));
        menu.add(new Names("karthick","image"));
        menu.add(new Names("vikki","image"));
        menu.add(new Names("karthick","image"));
        menu.add(new Names("harish","image"));
        menu.add(new Names("vivek","image"));
        menu.add(new Names("harish","image"));

        ArrayList<Names> al = new ArrayList<Names>();

         Set<Names> hs = new HashSet<Names>();
         hs.addAll(menu);

         al.clear();
         al.addAll(hs);

         System.out.println(al);

    }
}

Hope this help!

Answer 2

You need to override equals and hashcode method for Names class.

The hashcode method is use to calculate the bucket in HashSet and equals is used to identify the duplicate and will not add if any found already in the set.

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

  Names names = (Names) o;

  if (image != null ? !image.equals(names.image) : names.image != null) return false;
  if (name != null ? !name.equals(names.name) : names.name != null) return false;

  return true;
}

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

Answer 3

Why don't use use a Set for everything, instead of using the ArrayList. In order for the Set to work properly you will have to overwrite the the equals(E object), and hashcode() methods in your Names.class accordingly. Please see Java Generics for more hints:

Java Overriding equals and hashCode with Generics