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The Java Interview Handbook: 300+ Interview Questions

The Stream API

Explore the core concepts of Java's Stream API to process collections efficiently and declaratively. Learn how to use intermediate and terminal operations such as filter, map, flatMap, and grouping with collectors. This lesson helps you understand lazy evaluation and how to build concise Java data-processing pipelines without mutable state or complex loops.

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Processing collections is a common task in Java. Before streams, this often required loops and mutable state. The Stream API supports declarative data processing, which can improve readability and maintainability.

Let’s look at the core concepts of Streams that frequently appear in technical interviews:

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What is the Stream API, and how does it use Lambda expressions?


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What is the difference between intermediate and terminal operations?

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Let us see how intermediate and terminal operations chain together.

Java
List<String> names = List.of("Alice", "Bob", "Charlie", "David");
long count = names.stream()
    .filter(name -> name.startsWith("C"))
    .count();
System.out.println("Names starting with C: " + count);

  • Line 3: We create a stream from the names list.

  • Line 4: We apply the filter() ...