<P> During this time, the lower layers of the operating system (the Mach kernel and the BSD layers on top of it) were re-packaged and released under the Apple Public Source License . They became known as Darwin . The Darwin kernel provides a stable and flexible operating system, which takes advantage of the contributions of programmers and independent open - source projects outside Apple; however, it sees little use outside the Macintosh community . During this period, the Java programming language had increased in popularity, and an effort was started to improve Mac Java support . This consisted of porting a high - speed Java virtual machine to the platform, and exposing macOS - specific "Cocoa" APIs to the Java language . </P> <P> The first release of the new OS--Mac OS X Server 1.0--used a modified version of the Mac OS GUI, but all client versions starting with Mac OS X Developer Preview 3 used a new theme known as Aqua . Aqua was a substantial departure from the Mac OS 9 interface, which had evolved with little change from that of the original Macintosh operating system: it incorporated full color scalable graphics, anti-aliasing of text and graphics, simulated shading and highlights, transparency and shadows, and animation . A key new feature was the Dock, an application launcher which took advantage of these capabilities . Despite this, OS X maintained a substantial degree of consistency with the traditional Mac OS interface and Apple's own Apple Human Interface Guidelines, with its pull - down menu at the top of the screen, familiar keyboard shortcuts, and support for a single - button mouse . The development of Aqua was delayed somewhat by the switch from OpenStep's Display PostScript engine to one developed in - house that was free of any license restrictions, known as Quartz . </P> <P> With the exception of Mac OS X Server 1.0 and the original public beta, the first several macOS versions were named after big cats . Prior to its release, version 10.0 was code named "Cheetah" internally at Apple, and version 10.1 was code named internally as "Puma". After the code name "Jaguar" for version 10.2 received publicity in the media, Apple began openly using the names to promote the operating system: 10.3 was marketed as "Panther", 10.4 as "Tiger", 10.5 as "Leopard", 10.6 as "Snow Leopard", 10.7 as "Lion", and 10.8 as "Mountain Lion". "Panther", "Tiger", and "Leopard" were registered as trademarks, and Apple registered "Lynx" and "Cougar", but these were allowed to lapse, with Apple instead using the name of iconic locations in California for subsequent releases: 10.9 was called "Mavericks", 10.10 was called "Yosemite", 10.11 was called "El Capitan", and 10.12 was called "Sierra". </P> <P> On September 13, 2000 Apple released a $29.95 "preview" version of Mac OS X (internally codenamed Kodiak) in order to gain feedback from users . It marked the first public availability of the Aqua interface, and Apple made many changes to the UI based on customer feedback . Mac OS X Public Beta expired and ceased to function in spring 2001 . </P>

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