Java programs carry with them substantial amounts of run-time type information that
is used to verify and resolve accesses to objects at run time. This makes it possible to
dynamically link code in a safe and expedient manner. This is crucial to the robustness of
the Java environment, in which small fragments of bytecode may be dynamically updated
on a running system.
The Evolution of Java
The initial release of Java was nothing short of revolutionary, but it did not mark the end of
Java's era of rapid innovation. Unlike most other software systems that usually settle into a
pattern of small, incremental improvements, Java continued to evolve at an explosive pace.
Soon after the release of Java 1.0, the designers of Java had already created Java 1.1. The
features added by Java 1.1 were more significant and substantial than the increase in the
minor revision number would have you think. Java 1.1 added many new library elements,
redefined the way events are handled, and reconfigured many features of the 1.0 library. It
also deprecated (rendered obsolete) several features originally defined by Java 1.0. Thus,
Java 1.1 both added to and subtracted from attributes of its original specification.
The next major release of Java was Java 2, where the "2" indicates "second generation."
The creation of Java 2 was a watershed event, marking the beginning of Java's "modern
age." The first release of Java 2 carried the version number 1.2. It may seem odd that the
first release of Java 2 used the 1.2 version number. The reason is that it originally referred
to the internal version number of the Java libraries, but then was generalized to refer to
the entire release. With Java 2, Sun repackaged the Java product as J2SE (Java 2 Platform
Standard Edition), and the version numbers began to be applied to that product.
Java 2 added support for a number of new features, such as Swing and the Collections
Framework, and it enhanced the Java Virtual Machine and various programming tools. Java
2 also contained a few deprecations. The most important affected the Thread class in which
the methods suspend( ), resume( ), and stop( ) were deprecated.
J2SE 1.3 was the first major upgrade to the original Java 2 release. For the most part,
it added to existing functionality and "tightened up" the development environment. In
general, programs written for version 1.2 and those written for version 1.3 are source-code
compatible. Although version 1.3 contained a smaller set of changes than the preceding
three major releases, it was nevertheless important.
The release of J2SE 1.4 further enhanced Java. This release contained several important
upgrades, enhancements, and additions. For example, it added the new keyword assert,
chained exceptions, and a channel-based I/O subsystem. It also made changes to the
Collections Framework and the networking classes. In addition, numerous small changes
were made throughout. Despite the significant number of new features, version 1.4
maintained nearly 100 percent source-code compatibility with prior versions.
The next release of Java was J2SE 5, and it was revolutionary. Unlike most of the previous
Java upgrades, which offered important, but measured improvements, J2SE 5 fundamentally
expanded the scope, power, and range of the language. To grasp the magnitude of the
changes that J2SE 5 made to Java, consider the following list of its major new features:
Autoboxing and auto-unboxing
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