[course02] 00 Package and Class

[course02] 00 Package and Class

Two packages used in this course

print statements

System.out.println("Rather a lot to type");

import java.util.*;
import static net.mindview.util.Print.*;

public class HelloDate {
    //: operators/HelloDate.java

    public static void main(String[] args) {
        print("Hello, it’s: ");
        print(new Date());
    }
}

net.mindview.util.jar

add the root directory of that code tree to your computer’s CLASSPATH environment variable.

TextIO

import group.collapsar.java.basic.TextIO;

/**
 * A program that reads an integer that is typed in by the
 * user and computes and prints the square of that integer.
 */
public class PrintSquare {

    public static void main(String[] args) {

        int userInput;  // The number input by the user.
        int square;     // The userInput, multiplied by itself.

        System.out.print("Please type a number: ");
        userInput = TextIO.getlnInt();
        square = userInput * userInput;

        System.out.println();
        System.out.println("The number that you entered was " + userInput);
        System.out.println("The square of that number is " + square);
        System.out.println();

    } // end of main()

} //end of class PrintSquare

where is textio.TextIO

TextIO

add the file to package and import the class auto.

packages

A typical Java program has user-defined classes whose objects interact with those from Java class libraries.

In Java, related classes are grouped into packages, many of which are provided with the compiler.

import packagename.*;

import packagename.ClassName;

import packagename.subpackagename.ClassName;


import java.util Array;

Basic class

A Java program must have at least one class, the one that contains the main method. The Java files that comprise your program are called source files.

A compiler converts source code into machine-readable form called bytecode.

typical java code:

/*
Program FirstProg.java Start with a comment, giving the program name and a brief description of that the program does.
*/ 

import package1.*; 
import package2.subpackage.ClassName;

public class FirstProg //note that the file name is FirstProg.java 
{
    public static type1 method1(parameter list) {
        < code for method 1 > 
    } 
    public static type2 method2(parameter list) {
        < code for method 2 > 
    }
    ...

    public static void main(String[] args) { 
        < your code > 
    }
}

1. All Java methods must be contained in a class.

2. The words class, public, static, and void are reserved words, alsocalled keywords. (This means they have specific uses in Java and may not be used as identifiers.)

3. The keyword public signals that the class or method is usable outside of the class, whereas private data members or methods are not.

4. The keyword static is used for methods that will not access any objects of a class, such as the methods in the FirstProg class in the example above. This is typically true for all methods in a source file that contains no instance variables. Most methods in Java do operate on objects and are not static. The main method, however, must always be static.

5. The program shown above is a Java application.