How to Correctly Copy a `java.util.List` into Another `java.util.List` in Java?

Question

What is the best method to copy the contents of one `java.util.List` to another without causing exceptions?

List<SomeBean> originalList = ...; // Original list to copy from
List<SomeBean> copiedList = new ArrayList<>(originalList); // Efficient way to copy

Answer

Copying a `java.util.List` in Java can be tricky, especially when dealing with scenarios such as copying from a web service populated list to another empty list. This guide will provide strategies for safely and effectively copying lists without iterations or causing exceptions.

// Example of using ArrayList's constructor
List<SomeBean> wsList = app.allInOne(template);
List<SomeBean> wsListCopy = new ArrayList<>(wsList); // This will create a copy without IndexOutOfBoundsException

// Example using Streams (Java 8 and above)
List<SomeBean> wsListCopyStream = wsList.stream().collect(Collectors.toList());

Causes

  • Using `Collections.copy()` incorrectly, as the destination list must be initialized to the same size as the source list.
  • Attempting to access sublists or manipulate concurrent access without proper synchronization can lead to exceptions.

Solutions

  • Use the constructor of `ArrayList` that accepts a Collection to create a copy of the original list directly.
  • Consider using the Java 8 Stream API for a more elegant approach if you want a new list derived from an existing one.

Common Mistakes

Mistake: Trying to use `Collections.copy()` without pre-initializing the destination list to the same size as the source list.

Solution: Always initialize the destination list to the same size or use the constructor of ArrayList as shown.

Mistake: Using sublists while the original list is being accessed concurrently, causing `ConcurrentModificationException`.

Solution: Ensure that you do not modify the original list while accessing its sublists or duplicate data.

Helpers

  • copy java list
  • java.util.List
  • Collections.copy
  • ArrayList
  • Java Stream API
  • cloning lists in Java
  • IndexOutOfBoundsException
  • ConcurrentModificationException

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