Java Language Keywords.doc

Java Language Keywords abstract Java Keyword The abstract keyword may modify a class or a method. An abstract class can be extended (subclassed) but cannot be instantiated directly. An abstract method is not implemented in the class in which it is declared, but must be overridden in some subclass. Examples public abstract class MyClass { } public abstract String myMethod(); Remarks A class with an abstract method is inherently abstract and must be declared abstract. An abstract class cannot be instantiated. ? A subclass of an abstract class can only be instantiated if it implements all of the abstract methods of its superclass. Such classes are called concrete classes to differentiate them from abstract classes. ?? If a subclass of an abstract class does not implement all of the abstract methods of its superclass, the subclass is also abstract. ?? The abstract keyword cannot be applied to methods that are static, private or final, since such methods cannot be overridden and therefore cannot be implemented in subclasses. ?? No methods of a final class may be abstract since a final class cannot be subclassed. boolean Java Keyword boolean is a Java primitive type. A boolean variable may take on one of the values true or false. Examples boolean valid = true; if (valid) { statement } Remarks A boolean variable may only take on the values true or false. A boolean may not be converted to or from any numeric type. ?? Expressions containing boolean operands can contain only boolean operands. ?? The Boolean class is a wrapper class for the boolean primitive type. break Java Keyword The break keyword is used to prematurely exit a for, while, or do loop or to mark the end of a case block in a switch statement. Examples for (i=0; imax; i++) { if (loop finished early) { break; } } int type = some value; switch (type) { case 1: statement break; case 2: statement break; default: statement } Remarks break always exits the innermost enclosing while, for, do or switch statement. ?? by