Streams

Overview

A Stream is a pipeline through which data elements flow, letting you perform operations like filtering, mapping, sorting, and aggregation without writing explicit loops. It was introduced in Java 8 (package java.util.stream).

In simple words:

Stream = Data Source → Intermediate Operations → Terminal Operation

Real-Life Example: Factory Conveyor Belt

Factory	Stream concept
Nothing moves until the packing station is on	Lazy evaluation
Bottles pass through stations one by one	Elements flow one at a time
A packed batch cannot be run through again	A Stream cannot be reused
The original crate of bottles is unchanged	Source collection is not modified

Stream vs Collection

Feature	Collection	Stream
Purpose	Stores data	Processes data
Data structure	Yes	No (it is a pipeline)
Modifies the source	Yes (add/remove)	No
Reusable	Yes	No (one terminal operation, then closed)
Evaluation	Eager	Lazy
Iteration	External (you write the loop)	Internal (Stream does it)
Can be infinite	No	Yes (`iterate`, `generate`)

Stream Pipeline

Step	Rules
Create	From a collection, array, values, builder, or generator
Intermediate	Zero or more, return another Stream, lazy, can be chained
Terminal	Exactly one, triggers execution, produces the result, closes the Stream

Example: Without Stream vs With Stream

Count salaries greater than 3000.

List<Integer> salaries = List.of(3000, 4000, 1000, 9000, 1000, 3500);

// Without Stream
int count = 0;
for (int salary : salaries) {
    if (salary > 3000) {
        count++;
    }
}
System.out.println(count);   // 3

// With Stream
long total = salaries.stream()
.filter(salary -> salary > 3000)
.count();
System.out.println(total);   // 3

The loop says how to do it. The Stream says what you want. This is the declarative style.

Important Properties

Property	Meaning
Does not store data	It carries elements from a source
Does not modify the source	Results go into a new collection or value
Lazy	Intermediate operations wait for a terminal operation
Single use	Closed after the terminal operation
Can be sequential or parallel	`stream()` or `parallelStream()`

Interview Definition

A Java Stream is a lazily evaluated, single-use pipeline of operations that processes elements from a data source in a declarative, functional style without modifying the source.

Next TopicStream Creation