1. Overview of the Java language

Java Memorabilia

1. 1990, Sun company James Gosling led the "Green" plan to develop new language--oak;
2. 1994, the first Java Web browser: Webrunner,oak renamed Java;
3. 1995, Sun Company officially released the Java language;
4. In 1996, Sun released JDK1.0, including the JDK (development environment) and JRE (operating environment);
5. 1997, Sun company released JDK1.1;
6. In 1998, Sun released JDK1.2, releasing Jsp/servlet and dividing Java into three versions of EE, J2SE and J2ME.
   • J2ME: Mainly used for control of mobile devices;
   • J2se:java Technology Core, is the other two versions of the core;
   • Java EE: Enterprise-level application development;
7. Oracle acquired Sun in 2009;
8. 2011, Oracle releases JAVASE7;
9. 2014, Oracle releases JAVASE8;
10. 2017, Oracle releases javaSE9;

Java is an object-oriented programming language, not only absorbs the various advantages of the C + + language, but also rejects the difficult to understand C + + inheritance, pointers and other concepts, so the Java language has a powerful and easy to use two features. As the representative of static object-oriented programming language, the Java language implements the object-oriented theory, which allows programmers to do complex programming in an elegant way.

--Quote from "Baidu Encyclopedia"

The above is from the Baidu Encyclopedia on the introduction of Java a reference, because there is not much contact C + +, for C + + multiple inheritance and pointers are only limited to hear, so we only say Java, explain this paragraph: I personally think that the Java language is the most important and the most need to master the first is four words: object-oriented.

Process oriented:

Different from object-oriented, the development of software development is called process-oriented, the development method according to the functional subdivision system software, from top to bottom execution, the smallest unit is a function, the execution of the main function calls the ordinary function, the normal function calls in turn, complete the software function. Software in all the modules are refined into a function, such a program, the need to break the objective world model into a function, and the human thinking habits inconsistent, and when the requirements change, may need to modify the structure of the module from the top, its own limitations;

Object-oriented:

The object-oriented design approach is based on the reality of the real World (object) as the center to think, according to the characteristics of the specific transaction, it is abstracted as the smallest design unit in Java program-class. In the system construction, we also adopt the natural way of thinking of mankind. A behavior or function exists as a method in a program and is called by an object.

For example:

Scenario 1: A want to drink water

Process-oriented Description: Drink (A, water);

Object-oriented Description: A drink (water);

The obvious object-oriented description is more in line with our own thinking habits.

Object:

Specifically, the object: Take a display example, I now use a Dell23-inch rotating screen black display, it can be used to display code and documents, your mind is not the first time you will imagine this monitor the general appearance, we say:

{  properties:  {   brand: Dell,  Size: 23 inch,  color: black,  can rotate: yes};  function: Display}

In this case, we abstract the smallest unit of the object, that is, the class, the composition of a basic class: the attribute + method; In the Java language, we also call properties a global variable that describes an object's properties, called methods, to describe what an object can do, This kind of thing with common properties and methods forms the abstraction of the most central concept in Java-the class () object.

Basic features of object-oriented languages:

1. Encapsulation: To hide the details of the object, exposing some common methods to access, such as display code, we do not need to know how to display the details, only need to invoke the display method can be;
2. Inheritance: Inheritance is an important concept in object-oriented programming language, which is mainly used for reusing code, inheriting another class through one class, and obtaining the properties and methods of the parent class.
3. Polymorphism: From inheritance, the simple point is that the parent class refers to the object of the subclass, the runtime still shows the behavior characteristics of the subclass, that is, the same type of object execution of the same method, may show a variety of behavior characteristics;
4. In addition, abstraction is also an important feature, abstract is that we only need to focus on the current topic related aspects and ignore irrelevant aspects, such as the display, we said that the properties are just a part, there are many properties can be described, but not necessarily, we only care about the parts of the system.

1. Overview of the Java language