Does addAll add duplicates?

addAll(fromTagList); is excecuted, all the images in fromTagList are added to unionList. ArrayList doen’t prevent duplication. If you want to prevent duplication, you have to use Set implementation like HashSet.

Can a HashSet have duplicates?

Duplicates: HashSet doesn’t allow duplicate values. HashMap stores key, value pairs and it does not allow duplicate keys.

How does HashSet deal with duplicates?

HashSet doesn’t maintain any order, the elements would be returned in any random order. HashSet doesn’t allow duplicates. If you try to add a duplicate element in HashSet, the old value would be overwritten. HashSet allows null values however if you insert more than one nulls it would still return only one null value.

Why does my HashSet contain duplicates?

The problem is that your Element class has not overridden the equals and hashCode methods or these implementations are broken. From Object#equals method javadoc: The equals method implements an equivalence relation on non-null object references: It is reflexive: for any non-null reference value x, x.

What happens if we add duplicate to Set?

If we insert duplicate values to the Set, we don’t get any compile time or run time errors. It doesn’t add duplicate values in the set. Below is the add() method of the set interface in java collection that returns Boolean value either TRUE or FALSE when the object is already present in the set.

How do I allow duplicates in Set?

How to add Duplicate elements in Set?

  1. You can’t. That’s the point of Set.
  2. Sets, by their mathematical definition, can’t have duplicates.
  3. You can use a list if you want duplicates.
  4. I know that the set is not allowing duplicate.
  5. In that case it’s not a set any more, so why not just use a ArrayList?

What is difference between HashSet and Hashtable?

HashSet contains unique elements and HashMap, HashTable contains unique keys….Difference between HashSet, HashMap, and HashTable in java.

HashMap HashSet Hashtable
It allows one null for key and multiple null for values It can have a single null value. It does not allow null for key as well as for value.

How does set ensure that there are no duplicates?

Each and every element in the set is unique . So that there is no duplicate element in set . Now , what happens internally when you pass duplicate elements in the add() method of the Set object , It will return false and do not add to the HashSet , as the element is already present .

Why duplicates are not allowed in HashSet?

Set is not allowed to store duplicated values by definition. If you need duplicated values, use a List. As specified on the documentation of the interface, when you try to add a duplicated value, the method add returns false, not an Exception.

How duplicates are avoided in Set?

Which containers do not allow duplicates?

So, the conclusion, use std::unordered_set or std::unordered_map (if you need the key-value feature). And you don’t need to check before doing the insertion, these are unique-key containers, they don’t allow duplicates.

How to add duplicates to a HashSet in Java?

The first thing you need to know is that HashSet acts like a Set, which means you add your object directly to the HashSet and it cannot contain duplicates. You just add your value directly in HashSet. However, HashMap is a Map type.

Can a duplicate value be added to a hashmap?

However, HashMap is a Map type. That means every time you add an entry, you add a key-value pair. In HashMap you can have duplicate values, but not duplicate keys. In HashMap the new entry will replace the old one. The most recent entry will be in the HashMap.

How is HashSet used to add elements in Java?

HashSet internally uses HashMap to add elements. In HashSet, the argument passed in add (Object) method serves as key K. Java internally associates dummy value for each value passed in add (Object) method. HashMap does not have any concept of dummy value. Storing or Adding mechanism.

Can a null value be inserted into a HashSet?

As it implements the Set Interface, duplicate values are not allowed. Objects that you insert in HashSet are not guaranteed to be inserted in the same order. Objects are inserted based on their hash code. NULL elements are allowed in HashSet. HashSet also implements Serializable and Cloneable interfaces.