Introduction to Java

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Introduction to Java

• COM379 (Part-Time)
• University of Sunderland
• Harry R Erwin, PhD

2
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

3
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.

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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
  1.4.0.

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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.

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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

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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.
• Booleanboolean
• Characterchar
• Bytebyte
• Shortshort
• Integerint
• Longlong
• Floatfloat
• Doubledouble

8
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.

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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.

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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
  .

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Garbage Collection

• Java manages the heap. When a reference type
  object goes out of scope, it gets marked for
  later clean-up.
• You dont 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.

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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).

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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

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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.

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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!