Course: Java Programming (Coursera)
Instructor: Dashi Tang, Peking University

Java History

• Born in 1995 by James Gosling, SUN.
• Platforms:
  • Java SE: J2SE, Standard Edition.
  • Java EE: J2EE, Enterprise Edition.
  • Java ME: J2ME, Micro Edition.

What is Java

• Simple, OOP, cross-platform, secure, muti-threads.
• "C++--":
  • No pointers, automatical memory control, stable data types, no header files, no macros, no multiple inheritance, no global variables other than classes...

Java Mechanism

• JVM (Java Virtual Machine)
  • source.java --compile (javac)--> source.class (bytecode) --run--> JVM for different platforms
• JRE = JVM + API (lib)
  • Load code (by class loader).
  • Check code (by bytecode verifier).
  • Run code (by runtime interpreter).
• Code Security
  
• Garbage Collection
  
• JDK (Java Development Kit) = JRE + Tools
  

Object-Oriented Programming

• Class is an abstract of objects, Object is an instance of class.
  • Class = Field + Method

• Features:

  • Encapsulation
    • Pack data and functions into a class.
    • Allow selective hiding of properties and methods in an object.
  • Inheritance
    • Superclass and subclass could share data and methods.
    • e.g.
      

   //Superclass
   class Person{
       int age;
       String name;
       void sayHello(){...}
   }
  
   //Subclass
   class Student extends Person{
       String school;
       double score;
       void meetTeacher(){...}
   }  
  

  • Polymorphism
    • Implementing the same fuction on different objects may generate different results.
    • e.g.

   foo(Person p){p.sayHello();}
   foo(new Student());
   foo(new Teacher());
   //Different results!