Скачать или смотреть How to Pass LinkedList as Value in Java

Learn how to pass a `LinkedList` as a value in Java instead of by reference, ensuring your original list remains intact during operations.
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This video is based on the question https://stackoverflow.com/q/62337226/ asked by the user 'AlanS2000' ( https://stackoverflow.com/u/13283825/ ) and on the answer https://stackoverflow.com/a/62337977/ provided by the user 'Higigig' ( https://stackoverflow.com/u/13173633/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Introduction

If you're coming from languages like C+ + or C, the concept of passing variables can be quite different in Java. You might find yourself running into a common issue: when you pass a LinkedList (or any object, for that matter) to a method, it might seem like you're passing it by value, but Java actually passes it by reference. This can lead to unintended modifications of your list, which might not be what you intended. In this guide, we'll discuss how to effectively pass a LinkedList as a value, ensuring its integrity during method calls.



Understanding the Problem

In a recent scenario, a developer encountered a problem where the shortestJobFirst method did not print anything as expected. The issue was traced back to passing the LinkedList jobOrder to the firstComeFirstServe method. Instead of passing the list in a way that allowed it to remain unchanged, it was being altered by reference, leading to the removal of all nodes after processing.

Here’s a simplified example of what the code looked like:

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To solve this, the developer wondered whether creating multiple linked lists or using a method like subList() would be the solution. However, this would not provide the necessary functionality since subList() only offers a view of the list, not a copy that allows modifications.



How to Properly Pass a LinkedList as Value

The Solution: Using the clone() Method

In Java, while primitive types are always passed by value, objects (including LinkedList) are passed by reference. To work around this, we can utilize the clone() method that the LinkedList class implements. By creating a clone of the LinkedList, we can pass this clone to our method without modifying the original list.

Step-by-Step Implementation

Clone the LinkedList: Create a cloned copy of your LinkedList when calling the method.

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Use the Cloned List: The method can operate on this cloned list freely, allowing you to implement your logic without impacting the original list.

Call the Shortest Job First Method: After calling firstComeFirstServe, you can still call shortestJobFirst on the original list without fear of it being empty.

Revised Code Example

[[See Video to Reveal this Text or Code Snippet]]

Important Considerations

Performance: Cloning a large LinkedList may come with a performance overhead as it creates a new instance of the list.

Deep Copy vs. Shallow Copy: The clone() method creates a shallow copy, meaning that the objects in the list are not cloned but referenced. If the Job object changes, those changes will reflect in both lists. For a deep copy, you'd need additional logic.



Conclusion

Passing a LinkedList as a value in Java is straightforward once you understand how references work. By using the clone() method, you can ensure that your original list remains unchanged while still being able to manipulate a copy for your method operations. This technique allows you to maintain data integrity across different algorithms and functions, ultimately leading to cleaner, more reliable code.

For any developer transitioning into Java, mastering these concepts will prove invaluable for effective coding practices. Happy coding!