Choosing good abstractions

Coupling and cohesion The efficiency of the programs you design depends on how well you choose the classes they use.Well-abstracted classes lead to programs that are easy to maintain and extend. However, if you choose bad abstractions, you could end up with an error-prone application that doesn't model the problem domain effectively. Simplicity is the key to choosing good classes.The classes you choose should capture only one key abstraction. If a class performs two functions, then you should split it into two separate classes, each with its...

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Objects, classes, and UML

Classes in UML A class is represented in UML by a rectangle with three compartments. These are Class attributes operations() Class The first compartment in the UML diagram contains the class name, which UML recommends is centered and in bold. It also contains general properties like package owner or stereotype. attributes The second compartment contains the class's attributes. Attributes are features within a classifier that describe a range of values that instances of the classifier may hold. operations() The third compartment contains the...

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State, behavior, and identity

Objects Much of the power and flexibility of modern software analysis and design derives from its use of objects. The use of objects allows designers to create programs that are more easily maintained and extended. And it makes it easier to design very complex or large-scale programs. The advantage of using objects in programming is that they allow us to model the real world more accurately. Procedural programming represents the world as a series of processes acting on data. However, object-oriented programming represents it the way we really...

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Runtime and checked exceptions

Java packages contain Exception subclasses that describe exceptions that are specific to each package. Each subclass of Exception represents a particular exception type. The RuntimeException class - a subclass of Exception - and its subclasses describe exception objects that are thrown automatically by the Java Virtual Machine (JVM) at runtime.Runtime exceptions are generally caused by bugs in the source code. For instance, you need to ensure that a divisor is not equal to zero before dividing by it. The RuntimeException subclass contains various...

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The Throwable class

All Java exceptions and errors are subclasses of a class in the java.lang package called Throwable. Only an object of type Throwable can be thrown in code, including exceptions and system errors. Methods of the Throwable class include getMessage toString initCause printStackTrace getMessage The getMessage method returns an appropriate String error message from a Throwable object.The getMessage method returns null if the object was not created with an error message. You should provide descriptive messages for all exceptions handled in code. toString The...

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Using try, catch, and finally blocks

Java's exception-handling code is specified within a try...catch...finally block. The try block encloses the code that might cause an exception to occur. The code in the try block is called protected code.// basic try statement syntax try {   // protected code } catch (ExceptionType1 Identifier1) {   // exception-handling code } catch (ExceptionType2 Identifier2) {   // exception-handling code } finally {   // 0 or 1 finally clause }You use zero or more...

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Introducing exception handling

In programming terms, an exception is an event that stops the normal execution of a program. For example, a program might try to access an element outside the bounds of an array, or some file input and output operation might fail. You need to ensure that your Java programs can deal with exceptions without simply crashing.When an exception occurs at run time, the method in which it occurs creates an exception object. Execution is aborted unless the exception is handled somewhere along the call...

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New methods of the File class

In Java SE 6.0, the File class has been enhanced to include new methods that provide information about disk usage: getTotalSpace() getFreeSpace() getUsableSpace() getTotalSpace() The getTotalSpace method provides information on a particular File's partition size in bytes. getFreeSpace() The getFreeSpace method provides information about the available space still unused in a particular File partition in bytes. getUsableSpace() Similar to getFreeSpace, the getUsableSpace method further checks...

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Console Class

The Console class is a new feature of Java 6.0. It provides an alternative to the standard streams currently being used. The Console is commonly used as a support in securing password entries. It contains methods that can suppress the characters being displayed on the user's screen, and remove them from memory when they are no longer needed. To use the Console, you must retrieve the Console object using the System.console method. If the Console object is available, the Console object is...

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Reader and Writer classes

Reader classes are similar to input streams, and writer classes are similar to output streams. Reader classes descend from the abstract Reader class, whereas the Writer classes descend from the abstract Writer class. Both readers and writers are divided into low-level and high-level classes. Low-level classes communicate with I/O devices, and high-level classes communicate with the low-level ones. Readers and writers are designed specifically for Unicode characters. Low-level readers and...

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High-level streams

High-level input and output streams communicate with low-level streams rather than with I/O devices. You can use them for high-level input and output. Most of Java's high-level input and output classes inherit attributes from the FilterInputStream and FilterOutputStream superclasses. In turn, these classes inherit from the abstract InputStream and OutputStream classes. Suppose you are using a DataInputStream constructor for one of these classes. You need to pass an InputStream to the constructor...

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Low-level streams

When Java reads or writes data, it opens a data stream, reads or writes the information, and closes the stream. Java uses the stream, reader, and writer classes for streamed data. Stream classes deal with general data input and output, whereas the reader and writer classes deal specifically with Unicode and Unicode Transformation Format (UTF) string input and output. Data received from or sent to general I/O devices consists of bytes. However, Java can support higher-level I/O by piecing...

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The java.io.File class

Programs are required to write and read data to and from external sources - such as files, other programs, or network resources. The Java Input/Output (I/O) package - java.io - enables Java programs to read and write data in various formats. Text, sound, graphics, and video files can be processed by appropriate classes from the java.io package. The java.io package contains classes that enable you to access data both sequentially and at random. Note In sequential data access, data is read...

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Painting Swing components

Painting GUI components In Java, components are rendered on screen in a process known as painting. Although this is usually handled automatically, there are occasions when you need to trigger repainting, or modify the default painting for a component. The core painting mechanism is based on the Abstract Windowing Toolkit (AWT). In newer versions of Java, Swing painting mechanisms are based on and extend the functionality of AWT mechanisms. Components can be heavyweight or lightweight. A...

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