Java Programing

simply ignore objects you no longer use, with confidence that the interpreter will clean them up at an appropriate time. Sun’s current implementation of Java uses a conservative mark-and-sweep garbage collector that runs intermittently in the background, which means that most garbage collecting takes place between I/O pauses, mouse clicks, and keyboard hits. Next generation runtime systems like HotSpot have more advanced garbage collection that can even differentiate the usage patterns of objects (such as short-lived versus long-lived) and optimize their collection. Once you get used to garbage collection, you won’t go back. Being able to write air-tight C code that juggles memory without dropping any on the floor is an important skill, but once you become addicted to Java you can “realloc” some of those brain cells to new tasks. You may hear people say that Java doesn’t have pointers. Strictly speaking, this statement is true, but it’s also misleading. What Java provides are references a safe kind of pointer and Java is rife with them. A reference is a strongly typed handle for an object. All objects in Java, with the exception of primitive numeric types, are accessed through references. If necessary, you can use references to build all the normal kinds of data structures you’re accustomed to building with pointers, such as linked lists, trees, and so forth. The only difference is that with references you have to do so in a type-safe way. Another important difference between a reference and a pointer is that you can’t do pointer arithmetic with references (they can only point to specific objects or elements of an array). A reference is an atomic thing; you can’t manipulate the value of a reference except by assigning it to an object. References are passed by value, and you can’t reference an object through more than a single level of indirection. The protection of references is one of the most fundamental aspects of Java security. It means that Java code has to play by the rules; it can’t peek into places it shouldn’t. Unlike C or C++ pointers, Java references can point only to class types. There are no pointers to methods. People often complain about this missing feature, but you will find that most tasks that call for pointers to methods, such as callbacks, can be accomplished using interfaces and anonymous adapter classes instead.[2] (We will discuss these in Chapter 6, and in the Swing-related chapters; they are heavily used in tying together graphical user interface components). [2] As of Java 1.1, there is a Method class, which lets you have a reference to a method. This is part of the Java reflection API. You can use a Method object to construct a callback, but it’s not the normal way of doing things. Finally, arrays in Java are true, first-class objects. They can be dynamically allocated and assigned like other objects. Arrays know their own size and type, and although you can’t directly define or subclass array classes, they do have a well-defined inheritance relationship based on the relationship of their base types. Having true arrays in the language alleviates much of the need for pointer arithmetic like that in C or C++. 1.4.5 Error Handling Java’s roots are in networked devices and embedded systems. For these applications, it’s important to have robust and intelligent error management. Java has a powerful exception-handling mechanism, somewhat like that in newer implementations of C++. Exceptions provide a more natural and elegant way to handle errors. Exceptions allow you to separate error-handling code from normal code, which makes for cleaner, more readable applications. When an exception occurs, it causes the flow of program execution to be transferred to a predesignated “catcher” block of code. The exception carries with it an object that contains information about the situation that caused the exception. The Java compiler requires that a method - 13

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