MT311 Java Application Development and Programming Languages

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MT311 Java Application Development and
Programming Languages

• Li Tak Sing(???)

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Constructor

• A constructor is like a method except that
• it should have the same name as the class
• it should have no return type.
• A constructor will be called when you use the
  'new' keyword to create an instance of a class.

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Default Constructors

• If you do not provide any constructor, the Java
  will create a default constructor for it.

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Default Constructors

• When the default constructor is executed, all its
  attributes will be initialized according to the
  following rules
• if the attribute is numerical, like int, byte,
  float etc., the attribute is assigned the value
  of 0.
• if the attribute is a class variable, then it is
  initialized to "null" which represent that the
  variable is not referring to any object yet.
• if the attribute is boolean, then it will be
  assigned the value of false.

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Default Constructors

• public class ABC
• int a
• float b
• boolean c
• ABC d
• public static void main(String st)
• ABC abcnew ABC() //default constructor
• System.out.println(abc.a" "abc.b"
  "abc.c" "abc.d)

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Default Constructors

• The output of the above program is
• 0 0.0 false null

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Default Constructors

• If you want to initialize the attributes to other
  value, you can do it like thispublic class ABC
  
• int a1
• float b2
• boolean ctrue
• ABC dnull
• public static void main(String st)
• ABC abcnew ABC()
• System.out.println(abc.a" "abc.b"
  "abc.c" "abc.d)

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Default Constructors

• The output of the above program is
• 1 2.0 true null

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Constructors

• The above method has a problem in that all
  instance of ABC would be the same when they are
  just created, i.e., the attribute a will
  definitely be 1, the attribute b will be 2.0 etc.
• If we want to have the attributes initialized to
  different values depending on different
  situations, we need to write our own constructors.

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Constructor

• A constructor is a piece of code that will be
  executed when a new instance of class is created.
• Just like methods, a constructor can have a
  number of parameters.
• Just like methods, you overload a constructor
  provided that these constructors have different
  signatures.
• Unlike methods, a constructor cannot have return
  value.

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Constructors

• public class ABC
• int a1
• float b2
• boolean ctrue
• public ABC(int aa)
• aaa
• public ABC(float bb)
• bbb
• public ABC(boolean cc)
• ccc

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Constructors

• ...
• ABC a1new ABC() //error, now ABC does not have
  a default
• // constructors
• ABC a2new ABC(10) //a2.a10, a2.b 2.0, a2.c
  true
• ABC a3new ABC(10.0) //a3.a1, a3.b10.0, a3.c
  true
• ABC a4new ABC(false) //a3.a1, a3.b2.0,
  a3.cfalse.

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Constructors

• Consider a change in ABC
• public class ABC
• int a1
• float b2
• boolean ctrue
• public ABC(float bb)
• bbb
• public ABC(boolean cc)
• ccc

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Constructor

• ...
• ABC a1new ABC() //error, now ABC does not have
  a default
• // constructors
• ABC a2new ABC(10) //a2.a1, a2.b 10.0, a2.c
  true
• ABC a3new ABC(10.0) //a3.a1, a3.b10.0, a3.c
  true
• ABC a4new ABC(false) //a3.a1, a3.b2.0,
  a3.cfalse.

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Garbage collection

• When you create a new object, it takes up the
  computer's resources. So as you create more and
  more objects, it is possible that all the
  computer's resources are used up.
• So it is necessary to release those resources
  that have been taken up by objects that will
  never be used again.

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Garbage Collection

• Consider the following code
• for (int i0ilt10i)
• ABC abcnew ABC()
• ....
• In each loop, an ABC is created. Assume that
  once the loop is finished, these ABCs are no
  longer used and referenced, then the resources
  taken up by these ABCs can be released to other
  use.

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Garbage collection

• In Java, this is done by a process called garbage
  collection.
• From time to time, the process will check whether
  there are resources that are taken up by objects
  that are no long used. It will then release the
  resources back to the system.

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Finalize

• When the garbage collection process is releasing
  the resource taken up by an object, it is
  possible that this object is holding some
  important information. For example, this object
  may have an opened file with some contents that
  have not been written to the file properly. So
  by releasing the resources held by the object may
  result in loosing this contents.

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Finalize

• In order to prevent this from happening, we need
  to have some code to close the file before the
  resources are released.
• We need to use the finalize method for this
  purpose.

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Finalize

• The finalize method must have no parameters and
  no return type. So it is always declared
  aspublic void finalize()
• When the garbage collector is going to release
  the resource of an object, it will invoke the
  finalize method of the object first.

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Finalize

• So it is the programmer's responsibility to put
  the code in the finalize method to do whatever it
  need to do before the resources are released, for
  example, close a file or close a network
  connection.
• However, in real Java applications, you rarely
  use the finalize method because it is usually
  good habit to close a file when you are done with
  the file. Not to wait until the object is
  garbage collected.

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Packages

• A package is a collection of classes.
• It is very similar to directories in a file
  system.
• In a file system, a directory can have a number
  of other directories and files.
• So a package can contain a number of other
  packages and classes.

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Packages

• For example, you have been using the following
  command many timesSystem.out.println("....")
  Actually, System is a class defined in Package
  java.lang. So here, java is a package, lang is
  also a package that is defined in java and then
  System is a class defined in lang. Then, out is a
  static object of the class System. println is a
  method of the object out.

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Packages

• So actually, the above statement can be changed
  asjava.lang.System.out.println("...")

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Packages

• The following statement in a java file define the
  package that the class is inpackage myPackage
• When a class is defined in myPackage, then the
  class file should be put under a directory called
  myPackage.

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Packages

• Consider the following java filepackage
  abc.defpublic class MyClass public void
  method() .... static int
  aStaticVariable2

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Packages

• After compilation of the file, the MyClass.class
  file should be put in the following
  directoryabc/def
• When other programs wants to use MyClass, it can
  be referred to asabc.def.MyClass

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Packages

• If you do not want to use that long name, you can
  have the following statementimport
  abc.def.MyClassThis statement will tell the
  compiler where to find MyClass and therefore we
  can simply refer to the class by using MyClass,
  not the full name.

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Packages

• The way of using the import keyword isimport
  abc.def.This statement inform the compiler to
  look for all classes in the package abc.def, not
  just MyClass as that in the last statement. So
  all classes defined in the directory abc/def can
  be used without the need of the full name.

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Packages

• So, when you want to use a class ABC in a package
  def.ghi, you can do either of the following ways
• use the full name def.ghi.ABC
• include the statement import def.ghi.ABC and
  then you can refer to the class as ABC in the
  program.
• include the statement import def.ghi. and then
  you can refer to the class as ABC.

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Packages

• There are three situations where you do not need
  to specify the package at all
• The code that uses the class is in the same
  package of the class.
• The class belongs to the package java.lang. This
  package hold the most fundamental classes of Java
  and therefore there is no need to use either the
  import statement or the full name for classes in
  that directory.
• The class does not belong to any package.

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Packages

• That is why when you use System.out.println(".."),
  you do not need to preceed it with java.lang.

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Packages

• When you create a project in Netbeans, you will
  be asked about the package name. Then, usually,
  when you create a class, the class will be placed
  in that package.

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Where to find a package

• We know that when a class ABC is defined in a
  package def.ghi, then ABC.class should be found
  in the directory def/ghi. However, how can the
  Java system knows where is def/ghi? The is done
  by either using the environment variable called
  CLASSPATH or you can set the classpath when you
  run the java commandjava -classpath... ABC

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Where to find a package

• The other way is that Java will look for the
  current directory. ABC.class is actually
  at/jkl/mno/def/ghi/ABC.class,
• Then, in order for the Java interpreter to find
  the class, we can set the classpath to /jkl/mno
  or we run the command at the directory /jkl/mno.