How to Use Java 8 Stream API to Filter Instances and Perform Casting?

How can I filter instances in Java 8 Stream API and perform casting?

List<Object> mixedList = Arrays.asList("String1", 2, "String2", 4);
List<String> stringList = mixedList.stream()
                                     .filter(item -> item instanceof String)
                                     .map(item -> (String) item)
                                     .collect(Collectors.toList());

Java 8 Stream API provides a powerful toolkit to manipulate collections in a functional style. Filtering instances and casting them is a common requirement when dealing with heterogeneous collections. In this guide, we'll explore how to effectively filter out instances of a specific type and cast them appropriately for further processing.

List<Object> mixedList = Arrays.asList("String1", 2, "String2", 4);
List<String> stringList = mixedList.stream()
                                     .filter(item -> item instanceof String)
                                     .map(item -> (String) item)
                                     .collect(Collectors.toList());
System.out.println(stringList); // Output: [String1, String2]

Causes

• Working with collections that contain mixed data types.
• Need to process only specific object types from a heterogeneous list.

Solutions

• Use the `filter` method to specify the condition for filtering.
• Utilize the `map` method to cast the filtered objects to the desired type.
• Collect the results using `Collectors.toList()` to create a new list of the filtered and casted objects.

Mistake: Forgetting to check the instance type before casting, which can lead to ClassCastException.

Solution: Always use `instanceof` check before casting to ensure type safety.

Mistake: Not collecting the results into a list after filtering and mapping.

Solution: Use `Collectors.toList()` to gather the output from the stream operations.