how do you duplicate a list in java

A simple way to copy a List is by using the constructor that takes a collection as its argument: List<Plant> copy = new ArrayList

<> (list); Since we're copying references here, and not cloning the objects, every amends made in one element will affect both lists. As such, it's good to use the constructor for copying immutable objects:

Full Answer

How to remove duplicates from ArrayList in Java?

• Using Iterator Approach: Get the ArrayList with duplicate values. ...
• Using LinkedHashSet A better way (both time complexity and ease of implementation wise) is to remove duplicates from an ArrayList is to convert it into a Set that does not ...
• Using Java 8 Stream.distinct () You can use the distinct () method from the Stream API. ...

How to search in a list of Java object?

Java itself provides several ways of finding an item in a list: List exposes a method called contains: As the name suggests, this method returns true if the list contains the specified element, and returns false otherwise. So when we need to check if a specific item exists in our list, we can: Customer james = new Customer ( 2, "James" ); if ...

What is list in Java with example?

Java List interface is a member of the Java Collections Framework. List allows you to add duplicate elements. List allows you to have ‘null’ elements. List interface got many default methods in Java 8, for example replaceAll, sort and spliterator. List indexes start from 0, just like arrays.

How to find an element in a list with Java?

Using Java API

• 3.1. contains () As the name suggests, this method returns true if the list contains the specified element, and returns false otherwise.
• 3.2. indexOf () This method returns the index of the first occurrence of the specified element in the given list, or -1 if the list doesn't contain the element.
• 3.3. Basic Looping. ...
• 3.4. Looping With an Iterator. ...
• 3.5. ...

How do I copy one list to another?

Python List copy()copy() Syntax. The syntax of the copy() method is: new_list = list.copy()copy() Parameters. The copy() method doesn't take any parameters.copy() Return Value. The copy() method returns a new list. ... Example: Copying a List. # mixed list my_list = ['cat', 0, 6.7] ... List copy using =

How do you copy or clone an ArrayList?

Example 1: Make a Copy of ArrayList Here, number. clone() - returns a copy of the object number. (ArrayList) - converts value returned by clone() into an arraylist of Integer type (To learn more, visit Java Typecasting)

Is duplicates allowed in list Java?

List allows duplicates while Set doesn't allow duplicate elements . All the elements of a Set should be unique if you try to insert the duplicate element in Set it would replace the existing value. List permits any number of null values in its collection while Set permits only one null value in its collection.

How do you copy an ArrayList?

The clone() method of the ArrayList class is used to clone an ArrayList to another ArrayList in Java as it returns a shallow copy of its caller ArrayList. Syntax: public Object clone(); Return Value: This function returns a copy of the instance of Object.

What is clone in Java?

The Java Object clone() method creates a shallow copy of the object. Here, the shallow copy means it creates a new object and copies all the fields and methods associated with the object. The syntax of the clone() method is: object.clone()

How do you shallow copy in Java?

Note: The clone() method of the Object class creates a shallow copy, by default.public class ShallowCopy.{private int[] data;//creates a shallow copy of values.public ShallowCopy(int[] values){data = values;}More items...

Can a list contain a duplicate?

If size of list & set are different then it means yes, there are duplicates in list.

Can lists have duplicates?

What are duplicates in a list? If an integer or string or any items in a list are repeated more than one time, they are duplicates.

Which allows duplicate values?

ArrayList allows duplicate values while HashSet doesn't allow duplicates values. Ordering : ArrayList maintains the order of the object in which they are inserted while HashSet is an unordered collection and doesn't maintain any order.

Can we copy ArrayList in Java?

The clone() method of the java. util. ArrayList class returns a shallow copy of this ArrayList instance (i.e the elements themselves are not copied). Using this method, you can copy the contents of one array list to other.

How do I copy an ArrayList to another array?

In order to copy elements of ArrayList to another ArrayList, we use the Collections. copy() method. It is used to copy all elements of a collection into another. where src is the source list object and dest is the destination list object.

How do I add a list to an existing list in Java?

Add elements of a Stream into an existing List in JavaUsing Collectors. toCollection() method. We can use a Collector to add elements to an existing list, as shown below: ... Using Stream. forEachOrdered() with List. add() method. ... Using List. addAll() method + Collectors. ... Using Stream. concat() method + Collectors.

1. Overview

In this quick tutorial, we'll explore different ways to copy a List to another List, and a common error produced in the process.

2. Constructor

A simple way to copy a List is by using the constructor that takes a collection as its argument:

3. List ConcurrentAccessException

A common problem working with lists is the ConcurrentAccessException. This usually means that we're modifying the list while we're trying to copy it, most likely in another thread.

5. Collections.copy

The Collections class consists exclusively of static methods that operate on, or return collections.

6. Using Java 8

This version of Java expands our possibilities by adding new tools. The one we'll explore in the following examples is Stream:

7. Using Java 10

Finally, one of the last Java versions allows us to create an immutable List containing the elements of the given Collection:

8. Conclusion

In this article, we learned various ways to copy a List to another List with different Java versions. We also examined a common error produced in the process.

Java Find duplicate objects in list using Stream Group by

In Java Stream perform group by operation based on that we can find duplicate object from collection or list.

Find duplicate objects in a list using a hash map

Now we are going to find duplicate objects in the list using hashmap/hashtable. This solution is useful when we also want to find the occurrences of duplicate elements.

Find duplicate objects in a list using loops

This solution is very straight forward. Here we have used loops instead of any Java collections. Here we have used two loops. Iterate over all the elements and compare with each other.

Find duplicate user-defined objects in a list

In real life use cases, we always come to a situation where we want to deal with user-defined objects. Two methods equals and hashcode we need to override when we worked with user-defined objects and want to store in the collection such as HashMap, HashSet etc..

Using LinkedHashSet

A better way (both time complexity and ease of implementation wise) is to remove duplicates from an ArrayList is to convert it into a Set that does not allow duplicates. Hence LinkedHashSet is the best option available as this do not allows duplicates as well it preserves the insertion order.

Using Java 8 Stream.distinct ()

You can use the distinct () method from the Stream API. The distinct () method return a new Stream without duplicates elements based on the result returned by equals () method, which can be used for further processing. The actual processing of Stream pipeline starts only after calling terminal methods like forEach () or collect ().