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The Java Interview Handbook: 300+ Interview Questions

The Stream API

Explore how the Java Stream API simplifies processing collections using functional programming techniques. Understand intermediate and terminal operations like filter, map, and flatMap, and learn to use collectors for grouping data. This lesson helps you write clean, efficient, and readable data pipelines in modern Java.

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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.

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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?

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);

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