In Chapter 11, you will see several exception handling strategies. The Class.forName method is an example of a method that throws a checked exception. If an exception can occur despite your best efforts, then the compiler insists that you provide a handler. The compiler does not check whether you provide a handler for these errors-after all, you should spend your mental energy on avoiding these mistakes rather than coding handlers for them.īut not all errors are avoidable. However, many common exceptions, such as accessing a null reference, are unchecked. With checked exceptions, the compiler checks that you provide a handler. There are two kinds of exceptions: unchecked exceptions and checked exceptions. You may already have seen exception reports when you accidentally used a null reference or overstepped the bounds of an array. If you don't provide a handler, the program still terminates and prints a message to the console, giving the type of the exception. When an error occurs at runtime, a program can "throw an exception." Throwing an exception is more flexible than terminating the program because you can provide a handler that "catches" the exception and deals with it. We cover exception handling fully in Chapter 11, but in the meantime you will occasionally encounter methods that threaten to throw exceptions. The C++ type_info can only reveal a string with the name of the type, not create new objects of that type. The Java Class is quite a bit more versatile than type_info, though. The Class class is similar to the type_info class in C++, and the getClass method is equivalent to the typeid operator.
However, virtual constructors in C++ are not a language feature but just an idiom that needs to be supported by a specialized library. The newInstance method corresponds to the idiom of a virtual constructor in C++.
#Java reflection method how to#
See the section "A Primer on Catching Exceptions" on page 219 to see how to supply an exception handler whenever you use this method. Otherwise, the forName method throws a checked exception. This works if className is the name of a class or interface. You would use this method if the class name is stored in a string that varies at runtimeruntime. You can obtain a Class object corresponding to a class name by using the static forName method. String name = cl.getName() // name is set to "" If the class is in a package, the package name is part of the class name: Date d = new Date() If e is an employee, or Manager Harry Hacker For example, the statement (e.getClass().getName() + " " + e.getName()) Probably the most commonly used method of Class is getName. Just like an Employee object describes the properties of a particular employee, a Class object describes the properties of a particular class. The getClass() method in the Object class returns an instance of Class type. The class that holds this information is called, somewhat confusingly, Class. However, you can also access this information by working with a special Java class. Runtime type information is used by the virtual machine to select the correct methods to execute. This information keeps track of the class to which each object belongs. While your program is running, the Java runtime system always maintains what is called runtime type identification on all objects. If you are interested in programming applications rather than tools for other Java programmers, you can safely skip the remainder of this chapter and return to it later. Reflection is a powerful and complex mechanism however, it is of interest mainly to tool builders, not application programmers. Take advantage of Method objects that work just like function pointers in languages such as C++.
#Java reflection method code#
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