Lambda with Collections

Module: Generics and Collections
Topic: Lambda with Collections

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📋 Table of Contents

A stream is an infinite sequence of consumable elements (i.e a data structure) for the consumption of an operation or iteration. Any Collection can be exposed as a stream. It looks complex, but once you get it, it is very simple. The operations you perform on a stream can either be

1. Intermediate operations like map, filter, sorted, limit, skip, concat, substream, distinct, peek, etc producing another java.util.stream.Stream or a
2. Terminal operations like forEach, reduce, collect, sum, max, count, matchAny, findFirst, findAny, etc producing an object that is not a stream . Streams: intermediate(blue) & terminal (red) Basically, you are building a pipeline as in Unix. In Unix we “pipe” operations, and in Java 8, we stream them. The stream( ) is a default method added to the Collection interface in Java 8. The stream( ) returns a java.util.starem. Stream interface with multiple abstract methods like filter, map, sorted, collect, etc. The DelegatingStr eam is the implementing class. Intermediate operations are lazy operations, which will be executed only after a terminal operation was executed. So when you call .filter(i -> i % 3 == 0) the lambda body isn’ t being executed at the moment. It will only be executed after a terminal operation was called ( collect, in the example shown below). This is essential to understand from the viewpoint of adding break points in your IDE for debugging purpose. Go through these examples to get a good handle on the stream concepts. 11 numbers 1 to 10 and an extra 6 are a) filtered first for multiples of 3 b) filtered for values less than 7 c) remove duplicates by adding to a Set d) print the result.s -l | grep “Dec” | Sort +4n | more

i -> i % 3 == 0 is a lambda expr ession used as a predicate to filter only multiples of 3. So, Q. what is this “lambda expression”? A. In OOP or imperative programming, x = x+ 5 makes sense, but in mathematics or functional pr ogramming, you can’ t say x = x + 5 because if x were to be 2, you can’ t say that 2 = 2 + 5. In functional programming you need to say f(x) -> x + 5. Java 8 Stream Example 1: mport java.util.Arrays ; mport java.util.List; mport java.util.stream .Collectors ;

In the above example, filter and peek are intermediate operations that return a “Stream” object. The “peek” is used for debugging. The “collect(…)” is a terminal operation that returns a “Collection” object, which extends “Iterable”interface which has the “forEach(…)” method. Don’ t confuse this with the “forEach()” method in the “java.util.stream.Stream”. Output: Example 2: Same as above, let’ s introduce another terminal operation sum() .public class Java8LambdaDebug {

public static void main (String [] args) { List list = Arrays .asList (1,2,3,4,5,6,7,8,9,10, 6);// 6 is repeated list.stream () .filter (i -> i % 3 == 0) //multiples of 3 .peek (i -> System.out.println ("Debug pt1: " + i)) .filter (i -> i < 7) .peek (i -> System.out.println ("Debug pt2: " + i)) .collect (Collectors .toSet ()) // remove duplicates i.e 6 .forEach (i -> System.out.println ("result: " + i)); } Debug pt1: 3 Debug pt2: 3 Debug pt1: 6 Debug pt2: 6 Debug pt1: 9 Debug pt1: 6 Debug pt2: 6 esult : 3 esult : 6

In the above example, filter, peek, and mapT oInt are intermediate operations that return a “Stream” object. “sum” is terminal operation that returns a result. Output:mport java.util.Arrays ; mport java.util.List; public class Java8LambdaDebug {

public static void main (String [] args) { List list = Arrays .asList (1,2,3,4,5,6,7,8,9,10, 6);//6 is repeated

final int sum = list.stream () .filter (i -> i % 3 == 0) //multiples of 3 .peek (i -> System.out.println ("Debug pt1: " + i)) .filter (i -> i < 7) .peek (i -> System.out.println ("Debug pt2: " + i)) .mapT oInt(Integer ::intValue) .sum(); //duplicate 6 is included 3+6+6=15

System.out.println ("sum=" + sum); } Debug pt1: 3 Debug pt2: 3 Debug pt1: 6 Debug pt2: 6 Debug pt1: 9 Debug pt1: 6 Debug pt2: 6

Example 3: Let’s mix “intermediate” and “terminal” operations up. In the above example, filter(..), peek(..), stream(..), and mapT oInt(..) are intermediate operations that return a “Stream” object. “collect(…)” and “sum()” are terminal operations. Since, “collect” returns a “Collection&lt’;T>” terminal object after removing the duplicate value of 6 with the help of toSet(), we need to call the stream() again to get the “Stream” object back. Finally, “sum()” is a terminal operation.sum=15 mport java.util.Arrays ; mport java.util.List; mport java.util.stream .Collectors ; public class Java8LambdaDebug {

public static void main (String [] args) { List list = Arrays .asList (1,2,3,4,5,6,7,8,9,10, 6);//6 is repeated

final int sum = list.stream () .filter (i -> i % 3 == 0) //multiples of 3 .peek (i -> System.out.println ("Debug pt1: " + i)) .filter (i -> i < 7) .peek (i -> System.out.println ("Debug pt2: " + i)) .collect (Collectors .toSet ()) //remove duplicate i.e. 6 .stream () .mapT oInt(Integer ::intValue) .sum(); //duplicate is removed 3+6=12

System.out.println ("sum=" + sum); }

Output: So, if still having trouble grasping this, have a look at the Java 8 API docos for Interfaces Stream, Iterable and Interface Collection. Pay attention to default methods and return objects. So, now with a little bit of help from the Java 8 API docs, you can perform different combination of operations on a collection of data. You can also debug by placing break points in your IDE like eclipse by keeping in mind that intermediate ops are lazily evaluated after a terminal operation. The peek() intermediate operation is very handy for debugging as well. Have you completed this unit? Then mark this unit as completed. Mark as Completed « Previous Unit Next Unit »Debug pt1: 3 Debug pt2: 3 Debug pt1: 6 Debug pt2: 6 Debug pt1: 9 Debug pt1: 6 Debug pt2: 6 sum=9

Source: Extracted from PDF Last Updated: 2026-06-03

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📚 Related Topics

• Collections Overview
• Generics Introduction
• OOP Introduction

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💡 Key Takeaways

Review the questions above and ensure you understand:

• Core concepts and their practical applications
• Real-world scenarios and use cases
• Best practices and common pitfalls

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