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# Improvements to Program Execution Speed

### Enhancements in JDK 6

> Descriptions of the many performance enhancements in JDK 6 can be found on the performance web site in the Java SE 6 Performance Whitepaper .

### Enhancements in JDK 5.0

> The following are some of the enhancements made for improved program execution speed in JDK 5.0.
> - Garbage collection ergonomics - Provides for the automatic detection and choice of the client or server runtime compiler, which enhances performance on server-class machines. See "Server-Class Machine Detection" and "Garbage Collection Ergonomics" at Java Virtual Machines for more information.

> - StringBuilder class - The addition of a new class `StringBuilder` that works essentially as an unsynchronized StringBuffer for performance enhancement. You should replace all StringBuffer uses with StringBuilder unless you need the synchronization (which you almost certainly don't). StringBuilder is almost always faster than StringBuffer.

> - Java 2DTM technology - These Java 2D performance enhancements have spin-off performance benefits in other areas of functionality such as Swing/JFC. Examples include improved acceleration for BufferedImage objects, support for hardware-accelerated rendering using OpenGL, and improved text-rendering performance. See New Java 2D Features for more information.

> - Image I/O - Performance and memory usage improvements have been made when reading and writing JPEG images. See "Improved Performance" at Image I/O Features.

### Enhancements in J2SDK 1.4

> The following are some of the enhancements made for improved program execution speed in J2SDK 1.4.
> - Java 2DTM technology - These Java 2D performance enhancements have spin-off performance benefits in other areas of functionality such as Swing/JFC. One example of performance improvement in J2SDK 1.4 is that remote X display on Solaris is greatly improved. See New Java 2D Features for more information on Java 2D enhancements, including performance enhancements.

> - java.nio - The New I/O API provides for improved performance in the areas of buffer management, scalable network and file I/O, character-set support, and regular-expression matching. The New I/O APIs supplement the I/O facilities in the `java.io` package. For more information, see I/O APIs.

> - Reflection - Beginning with J2SDK 1.4.0, certain reflective operations, specifically java.lang.reflect.Field, java.lang.reflect.Method.invoke(), java.lang.reflect.Constructor.newInstance(), and Class.newInstance(), have been rewritten for higher performance. Reflective invocations and instantiations are several times faster than in previous releases.

> - Networking - The networking functionality in J2SDK 1.4 has improved performance for Http streaming.

> - java.math - New static method `probablePrime` has been added to class `BigInteger` for generating prime numbers. Previously, the sole means of generating a prime `BigInteger` required the caller to provide a 'certainty' (the log of the probability of compositeness that the caller was willing to tolerate). However, providing an unreasonable value for the certainty could lead to poor performance (if the certainty was too high) or serious bugs (if it was too low). The new prime-generation method provides an efficient algorithm for generating primes using an automatically determined, appropriate value for the certainty based on a draft ANSI specification (X9-80).

> - Hotspot - The Java HotSpot[TM] virtual machine now uses "full-speed debugging". In the previous version of HotSpot, when debugging was enabled, the program executed using only the interpreter. Now, the full performance advantage of HotSpot Technology is available to programs running with debugging enabled. The improved performance allows long running programs to be more easily debugged. It also allows testing to proceed at full speed and the launch of a debugger to occur on an exception.

### Enhancements in J2SDK 1.3

> The following are some of the changes made to improve the program execution speed and general runtime performance in previous release J2SDK 1.3. These enhancements have been carried forward into J2SDK 1.4.

> Links to relevant bug reports on the BugParade web site are provided were applicable for further information.

> - Improved Performance for readUTF and writeUTF - The implementations of the `readUTF` and `writeUTF` methods of `DataInputStream` and `DataInputStream` have been modified to improve performance. This is a fix for bug number 4219771 on the BugParade web site.

> - Improved Performance of JScrollPane Painting - The implementation of JScrollPane has been improved so that it does not cause excessive painting and screen flashing when double buffering is not used.

> - Improved JTable Performance - The performance of the JTable's column-handling functionality has been improved. Previously, the performance of the operations of adding columns or recalculating column widths did not scale well as the number of columns became large. The implementation has been changed to improve scalability. This is a fix for bug number 4223642 on the BugParade web site.

> - Paint Coalescing - Painting performance has been enhanced through the implementation of an improved paint coalescing algorithm. Areas that require repainting are coalesced into a single, non-rectangular repaint area as they are posted to the event queue. This has the effect batching repaints, executing multiple repaints simultaneously in a manner more efficient than in previous releases.

> - Frame Resizing - Resizing of top-level frames has been improved.

> - Internal Frame Blitting - By using `Graphics.copyArea` calls, Swing's DefaultDesktopManager now needs to redraw far less when frames are dragged.

> - Performance Enhancements in `BigIntenger` - Class `java.math.BigInteger` has been reimplemented in pure Java programming-language code. Previously, `BigInteger`'s implementation was based on the Plumb C library. The new implementation performs all standard operations much faster than the old implementation. The speed-up realized in the new implemenation can be as much as 5x or more, depending on the operation being performed.

> - Provision for Better-Performing Numeric Operations - The Java 2 Platform now contains two classes which provide API for performing general numeric operations.
>   - `java.lang.StrictMath` - Class `java.lang.StrictMath` is a renaming of class `java.lang.Math` from previous versions of the Java platform, but otherwise retains the previous specification of `Math`. In particular, it is defined to return bit-for-bit reproducible results in all implementations.
>   - `java.lang.Math` - Class `java.lang.Math` provides the same set of API as does class `StrictMath`. However, unlike some of the numeric functions of class `StrictMath`, all implementations of the equivalent functions of class `Math` are not defined to return the bit-for-bit same results, but can vary within specified constraints. This relaxation permits better-performing implementations where strict reproducibility is not required.

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