Introduction to Java - University of Sunderland

Download Report

Transcript Introduction to Java - University of Sunderland

Introduction to Java
University of Sunderland
Harry R. Erwin, PhD
• We will explore basic C syntax that is
reused in Java.
• We will also have one or two lectures on the
new stuff in Java 5.0.
• Today we will examine some differences
between Java 1.4 and C++
• Java is just one of several languages
designed to occupy the intersection between
object-orientation and C.
• Other such languages include:
– C++
– C#
– Objective C
• 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 5.0 based on
Flanagan, 2005, Java in a Nutshell, 5th
• For more detail, see the Sun introductory
tutorials and Flanagan’s book.
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 1.4.
Global Variables
• None, nada.
• Packages contain classes. Classes contain
fields and methods. Methods contain local
• 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, machineindependent sizes and properties. Learn them cold. I
guarantee there will be an exam question of some sort
worth 10 marks on them.
• These include:
‘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
• There are no programmer-accessible pointers in
• Classes and arrays (and interfaces) in Java are
reference types. Java manages the underlying
• 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.
• Know the three basic reference types!
• 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() …;
These are always available for a reference type.
• toString(), equals(), hashcode(), and clone()
should usually be overridden if a class uses them.
• 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 memory (heap or free store). When a
reference type object goes out of scope, it gets marked for
later clean-up.
• You never 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.
• 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 that you
need to memorize.
• 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
No goto statement
No structs (use classes)
No unions
No enums (use object constants, changed for Java
No bitfields
No typedefs
No method pointers (use functors)
No variable-length argument lists (changed for
Java 5.0)
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$.
• Java is based on C and C++, but is not an
extension of either. Assuming Java is C
with classes will lead you into serious
problems. My exam questions are designed
to be nearly impossible to answer based
only on a knowledge of C/C++.
• Know the similarities and the differences!