2. Difference between a Class and an
Object
A class
A class is a template
Definition of a class is only given during design/coding
time
A class can be compared to a struct in c
An object
An object is an instance of a class
An object is made only during run time
An object is made from the definition of a class
4. Object/instance
So far we have defined a class and written
code to instantiate it. But we still have not
created the object.
Compile: javac A.java
Run: java A.class
Only now an object or instance of class A is
created.
JVM creates the instance or object based
on class definition found in the byte-code
5. Some rules for creating class in
java
A class can be defined in any file. However,
a public class must be in a file name that
matches the class name.
Example of a public class:
public class Student{
String code;
String name;
}
file name: Student.java
6. Some rules for creating class in
java
Example of non-public class:
class A{
String name;
String code;
}
Class B{
int age;
String program;
}
Fliename could be anything, such as
Example.java
7. Some rules for creating class in
java
Please note that many non-public classes
may reside in one file.
But a file can have only one public class.
A file can have one public class and many
non-public classes.
8. Packages
Packages are folders, roughly speaking.
Typically, “package” is the first line in any
java program.
Many java files can be placed under a
package.
Package is a way to organize codes.
11. Compiling packaged classes
Suppoer A.java is under com folder.
The first line of A.java is:
package com;
To compile issue command
javac com/A.java
The output A.class file will be produced
under com folder
To run issue command:
java com.A
12. Compiling non-packaged classes
Suppose B.java has no package definition
To compile issue command:
javac B.java
To run issue command
java B
13. Compiling a package
To compile all files under package issue
command:
javac com/*.java
To compile multiple packages issue
command:
javac com/*.java com/abc/*.java
