Using Classes

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Chapter 10
Using Classes

   10.1 Getting Access
   10.2 Constructors
   10.3 Using Instance Methods
   10.4 Using Class Methods
   10.5 Accessing Fields
   10.6 Assignment #11: Referencing Constructors and Methods

10.1 Getting Access

The Java distributions is provided with a programmed set of classes, referred to as fundamental classes.
The programmers may add classes of their own.
To use a specific feature of a class, we need to specify the name of the feature, the class owning the feature, and the package where the class belongs.
class prg{
public static void main(String[] args){
java.lang.System.out.println( "Hello world!" );
} }
A class can be made available with the command
   import <package name>.<class name>
In such a case, if no ambiguity arises, the name of the package is not required in the referenced feature. The same holds for the class name.
import java.lang.System;
class prg{
  public static void main(String[] args){
     System.out.println( "Hello world!" );
} }
The following variant of the ‘import’ command loads all the classes from the specified package.
   import <package name>.*

import java.lang.*;
class prg{
  public static void main(String[] args){
     System.out.println( "Hello world!" );
} }
The classes of ‘java.lang’ are automatically loaded, so there is no need to request them.
class prg{
public static void main(String[] args){
System.out.println( "Hello world!" );
} }

10.2 Constructors

The values of the primitive data types can be expressed in programs through literals.
The values of the class data types can be expressed in programs using constructors, through instructions of the following form.
new <constructor> ( <arguments> )
The values available for a class data type are said to be objects, and they are cosidered to be instances of the class.
class example{
  public static void main(String[] args){
      System.out.println( new java.util.Date() );
      System.out.println( new java.util.Date( 123456789L ) );
} }
A variable of a primitive data type holds a value of the given type.
A variable of a class data type holds a reference, or a pointer, to an instance of the given data type.
class example{
  public static void main(String[] args){
     java.util.Date a, b, c = new java.util.Date();
     a = new java.util.Date();
     b = a;
     System.out.println( b );
} }

10.3 Using Instance Methods

An instance method is a message requeting an action from a class instance
A reference to an instance method should be quantified to a class instance (possibly, idirecxtly, through a variable)
<instance>.<method> ( <arguments> )
A non-void instance method produces a return value to be assigned to a variable of compatible type.

import java.util.Date;
class prg{
  public static void main( String [] args ){
    Date a, b;
    int c;
    a = new Date ( 1L );
    b = new Date ( 2L );
    c = a.compareTo( b );
} }

10.4 Using Class Methods

A class method is a message submitted to the class environment,
A reference to a class method should be quantified to a class name
<class>.<method> ( <arguments> )
A non-void instance method produces a return value to be assigned to a variable of compatible type.

        // java.lang.Math: static double abs(double a)
class prg{
  public static void main( String [] args ){
    double a, b;
    a = -1.2;
    b = Math.abs( a );
} }

10.5 Accessing Fields

An instance field holds infomation about the specific instance.
A reference to an instance field should be quatified to the instance.
<instance>.<field>
A class field holds infomation regarding the class as a group.
A reference to a calss field should be quatified to the class name.
<class name>.<field>

Exercises

10.6 Assignment #11: Referencing Constructors and Methods

Due: We, Apr 30, midnight

Consider the following incomplete program.

class lab11{
   public static void main(String[] args){
      Integer in;
      in = new Integer (  );
      System.out.println( in );
      Long ;
      ln = new Long (  );
      System.out.println( ln );
      Boolean bo;
      bo = new  ( false );
      System.out.println( bo );
      Double db;
       = new Double (  );
      println(  );
      java.util.Date da;
      da = new Date (  );
      System.out.println( da );
      String st;
      st = new String("A string. ");
      st = concat ( st );
      System.out.println( st );
} }

Add the missing code to the program, so that it will produce the following output.

1234
123456789012345
false
1.2E-33
Sat Jan 01 00:00:00 EST 2000
A string. A string.

Notes:

You are not allowed to remove any code from the given program
Assume lab ‘lab11’ for the submit program, and file name ‘lab11.java’.

Note: Files that fail to compile, and execute when applicable, will not be examined. They will be awarded a grade of 0 points.

Q&A

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Chapter 10Using Classes