# GS Collections

GS Collections is a collections framework for Java. It has JDK-compatible List, Set and Map implementations with a rich API and set of utility classes that work with any JDK compatible Collections, Arrays, Maps or Strings. The iteration protocol was inspired by the Smalltalk collection framework.

## Download
* [Version 5.0.0](http://www.goldmansachs.com/gs-collections/binaries/gs-collections-5.0.0.zip)
* [Older versions](https://github.com/goldmansachs/gs-collections/releases)

## Quick Example
GS Collections puts iteration methods on the container types. Lambdas are simulated using anonymous inner classes. Here's a code example that demonstrates the usual style of programming with GS Collections.

```java
MutableList<Person> people = FastList.newListWith(person1, person2, person3);
MutableList<String> sortedLastNames = people.collect(Person.TO_LAST_NAME).sortThis();
System.out.println("Comma separated, sorted last names: " + sortedLastNames.makeString());
```

Person.TO_LAST_NAME is defined as a constant Function in the Person class.

```java
public static final Function<Person, String> TO_LAST_NAME = new Function<Person, String>()
{
    public String valueOf(Person person)
    {
        return person.lastName;
    }
};

```
In Java 8, the Function can be replaced with a lambda:

```java
MutableList<String> sortedLastNames = people.collect(person -> person.getLastName()).sortThis();
```

Or, a method reference:

```java
MutableList<String> sortedLastNames = people.collect(Person::getLastName).sortThis();
```

## Why GS Collections?
* Improves readability and reduces duplication of iteration code (enforces DRY/OAOO)
* Implements several, high-level iteration patterns (select, reject, collect, inject into, etc.) on "humane" container interfaces which are extensions of the JDK interfaces
* Provides a consistent mechanism for iterating over Collections, Arrays, Maps, and Strings
* Provides replacements for ArrayList, HashSet, and HashMap optimized for performance and memory usage
* Performs more "behind-the-scene" optimizations in utility classes
* Encapsulates a lot of the structural complexity of parallel iteration and lazy evaluation
* Adds new containers including Bag, Interval, Multimap, and immutable versions of all types
* Has been under active development since 2005 and is a mature library

## Documentation
The best way to learn about GS Collections is to dive into the [code kata](https://github.com/goldmansachs/gs-collections-kata). The kata is a fun way to learn idiomatic GS Collections usage and hone your skills through practice.

For more comprehensive documentation, take a look at the [Reference Guide](https://github.com/goldmansachs/gs-collections/wiki).

## Contributions
We currently do all development in an internal Subversion repository and are not prepared to take external contributions. However, we watch the [issue tracker](https://github.com/goldmansachs/gs-collections/issues) for bug reports and feature requests.

## FAQ
### Why is Goldman Sachs open-sourcing GS Collections?

* We believe that GS Collections offers a significant advantage over existing solutions. We hope others will benefit from it.
* We believe in the power of the technical community to help improve GS Collections.
* Technology is a huge part of what we do at Goldman Sachs. GS Collections exemplifies our commitment to technology.
* We use open source software in many of our operations. We have benefited from the work of others and we'd like to give something back.

### Does Goldman Sachs use GS Collections?
Yes, we use GS Collections in many of our internal applications.