Скачать или смотреть Efficiently Filter and Subtract from Two Lists in Java 8 Using Streams

Learn how to filter objects from two lists and efficiently subtract their attributes using Java 8 streams with this straightforward guide and example.
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Filtering and Subtracting Objects from Two Lists in Java 8

When working with collections in Java, especially when using Java 8 streams, it’s common to encounter scenarios where you need to manipulate data from different lists. One such scenario could involve filtering objects based on certain attributes and then performing operations like subtraction. In this guide, we'll delve into how to filter objects from two lists and subtract their attributes, ultimately storing the result in a third list.

The Problem

Suppose we have two lists of objects (let's call them Manipulate) where each object has an id and a quantity. The objective is to filter the objects from these two lists by their id, subtract the quantity from one list to another, and store the modified objects in a new list.

Consider the following setup:

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Here, list a contains objects with id 1 and 2 and their respective quantities. List b also contains objects with the same ids but with lesser quantities. The goal is to subtract the quantities in list b from those in list a based on matching ids and store the results in a new list.

The Initial Attempt and Issue

The initial approach to solve this problem is to use the Java 8 streams feature. Unfortunately, the attempt led to compilation errors because the result of the stream operation wasn't being collected into a list. Here's the incorrect code snippet:

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The error message indicated that a Stream<Object> was provided instead of a List<Manipulate>. This typically indicates that the stream processing results were not being properly collected into a list format which the program expects.

The Solution

To resolve this issue, we need to ensure that the stream is collected back into a list after the processing is completed. This can be accomplished by adding a .collect(Collectors.toList()) at the end of the stream operation. Here’s the corrected code:

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Explanation of the Solution

Stream Creation: We begin by creating a stream from list a.

Mapping: For each object k in list a, we filter through list b to find matching ids.

For Each Operation: Within the forEach, we compute the new quantity by subtracting the quantity from the filtered object in list b.

Updating Quantity: The quantity in the object from list a is updated in place.

Collecting Results: Finally, we collect the modified objects back into a list of type Manipulate using .collect(Collectors.toList()).

Conclusion

Using Java 8 streams makes it easier to handle collections and perform operations seamlessly. Always remember to collect your stream results back into the desired collection type to avoid compilation errors. With this straightforward solution, you can now efficiently filter objects and perform operations like subtraction across two lists in Java.

By mastering these techniques, you can enhance your Java programming skills and work more effectively with data collections in your applications.