Java Collectors
In Java, the `Collectors` class is a part of the Java Stream API introduced in Java 8, which provides a set of static utility methods for transforming and accumulating elements of a stream into collections, aggregating them, or summarizing their contents. It serves as a powerful framework for advanced data manipulation and collection, allowing developers to easily perform tasks such as grouping elements, partitioning them, counting occurrences, and creating lists, sets, or maps from the output of stream operations. Commonly used collectors include `toList()`, `toSet()`, `toMap()`, `joining()`, `groupingBy()`, and `partitioningBy()`, which enable concise and expressive data processing in functional programming style. The `Collector` interface enables the customization of collection strategies, thereby making it a versatile tool for efficient data handling and transformation in Java applications.
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1 - Definition: Java Collectors are part of the Java Stream API introduced in Java 8, enabling the accumulation of elements from a stream into a collection.
2) Packages: Collectors are found in the `java.util.stream` package, providing various static methods to facilitate different kinds of data gathering.
3) toList(): This collector collects the elements of a stream into a `List`. It's a straightforward way to create a List from a stream.
4) toSet(): Similar to `toList()`, this collector gathers elements into a `Set`, which ensures that no duplicates are present.
5) toMap(): This collector helps in creating a `Map` from a stream of key value pairs, allowing you to specify how to extract keys and values from the stream elements.
6) joining(): This collector concatenates the elements of a stream into a single `String`. It provides options for delimiters, prefix, and suffix.
7) groupingBy(): This collector groups elements by a classifier function, resulting in a `Map` where keys are the result of applying the classifier, and values are Lists of items.
8) partitioningBy(): It’s a specialized form of `groupingBy`, which divides elements into two groups; it returns a Map with Boolean keys.
9) counting(): This collector counts the number of elements in a stream. It returns a `Long` representing the count.
10) summarizingInt/Long/Double(): These collectors provide statistical summaries of numeric stream elements, returning an instance of `IntSummaryStatistics`, `LongSummaryStatistics`, or `DoubleSummaryStatistics`.
11) reducing(): This collector performs a reduction on the elements of a stream using an associative accumulation function and returns an `Optional`.
12) collect() Method: This is the terminal operation of a stream that uses a collector. It allows for the transformation of stream elements into different forms, typically collections.
13) Immutable Collections: Collectors can generate immutable collections using methods such as `toUnmodifiableList()` or `toUnmodifiableSet()`.
14) Custom Collectors: You can create your own custom collectors by implementing the `Collector` interface, allowing you to specify how the accumulation and combination should be performed.
15) Parallel Operations: Many collectors support parallel stream operations, allowing for improved performance by processing data concurrently.
16) Combining Collectors: Java provides methods to combine collectors, allowing advanced aggregation operations by utilizing existing collectors together.
17) Use Cases: Common use cases for collectors include data aggregation (like total sales), grouping results (like categorizing customers), and data transformations.
These points provide a solid foundation for understanding Java Collectors, their functionalities, and their applications. They can be tailored for deeper discussions or practical coding exercises during training sessions.
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