Introduction to Java COM379 (Part-Time) University of Sunderland Harry R Erwin, PhD

Java Java is just one of a number of languages designed to occupy the intersection between object-orientation and C. Other such languages include: –C++ –C# –Objective C

Assumptions It is assumed here that you know C++ or C, or some Algol-based language. This lecture discusses the differences between C/C++ and Java based on Flanagan, 2002, Java in a Nutshell, 4th edition. For more detail, see the Sun introductory tutorials and Flanagan.

Preprocessor Java has no preprocessor: No macros. No analogs of #define, #include, or #ifdef. No header files No conditional compilation assert was added as a language statement in Java

Global Variables None, nada. Packages contain classes. Classes contain fields and methods. Methods contain local variables. To simulate a global variable, use a public static member field of some class. For examples of how to do this, look at the Arrays, Math, or Collections classes.

Primitive Types All the primitive types in Java have well-defined, machine- independent sizes and properties. Learn them. These include: –boolean –char –byte –short –int –long –float –double

‘Horses for Courses’ Primitive types lack methods and cannot be stored in collections that expect some sort of object. Each primitive type has a corresponding class (with useful methods) that provides instances that can be stored in a collection. Boolean—boolean Character—char Byte—byte Short—short Integer—int Long—long Float—float Double—double

Pointers There are no programmer-accessible pointers in Java. Classes and arrays in Java are reference types. Java manages the underlying pointers. There is no way to convert from a reference to a primitive object like you can treat a C pointer as an integer type. You cannot use a reference to access a private member attribute.

Objects Reference types inherit from the class Object. Object provides a number of methods, including: –Class getClass(); –String toString(); –boolean equals(Object o); // by value –int hashcode(); // also by value –Object clone() …; toString(), equals(), hashcode(), and clone() should usually be overridden if a class uses them.

Strings A String is an object like a C++ string, not an array like a C string. A String is constant once it is created. If you want to change a String, give the name a new value. Among other ways, Strings can be created by the toString operator applied to an object, by setting the object equal to a literal (String name =“data”;), and by concatenation using + and +=.

Garbage Collection Java manages the heap. When a reference type object goes out of scope, it gets marked for later clean-up. You don’t need to delete or return any storage. Cleanup happens at the convenience of the Java runtime environment. You can suggest that the time is right by calling System.gc(), but that is only a suggestion. This is why Java is unsuitable for real-time applications, even though it was designed for embedded systems. To create an object of a reference type (array or class instance), you usually use the new operator.

Syntax Variables may be declared anywhere. The variable name is in scope in the local block from the point of declaration. Reference types are set to null (non-existent) until they are given a value. Primitive types have a default value. Forward references within a class definition are usually OK, but not within method code. Within a method, local variables must be in scope before they are used. Method overloading is allowed. The argument type list is part of the method signature. No operator overloading (except for the String class, which has + and += defined).

None of the Following are Available: No goto statement No structs (use class) No unions No enums (use object constants) No bitfields No typedefs No method pointers (use functors) No variable-length argument lists

Write Once/Run Anywhere Java is designed to be architecture-independent. The compiler will convert your codefiles into class files that can be executed anywhere. I run it under MacOS X; the Suns run it under Solaris, and Windows also runs it. That makes it slower than native code, but faster than interpreted scripts like PHP or Perl. This scares M$.

Summary Java is based on C and C++, but is not an extension of either. Assuming Java is C with classes will lead you into problems. Know the similarities and the differences!