In the process of studying Java, master the basic concepts to our study, whether it is J2SE, J2EE, J2ME, J2SE is the foundation of Java, so it is necessary to summarize the basic concepts so that everyone Better understanding Java's essence in the future learning process, here I summarize 30 basic concepts.
Java overview:
At present, Java is primarily applied to the Middleware development (MiddleWare) --- Treatment of communication technology between clients, early practice proves that Java is not suitable for the development of PC applications, and its development has gradually become developed in the development of handheld equipment. Internet information stations, and the development of car computer. Java is different from other languages that provide the stand-aller's independence of the program, which is known to use exact same code on Windows, Solaris, Linux other operating systems. Java syntax and C syntax is similar, and the C / C programmer is easy to master, and Java is a completely thorough object-oriented, which proposes a good GARBAGE COLLEctor waste handling mechanism to prevent memory overflow.
Java's white paper proposed 11 key features in Java language.
(1) Easy: Java's grammar is relatively simple than C , and another aspect is that Java can run software on a small machine, the basic explanation of its support for the support of its support is about 40KB, increasing basic standard libraries and The memory supported by thread needs to increase 125KB.
(2) Distributed: Java has a very powerful routine library of TCP / IP protocol, and Java applications can pass the network through the URL to access the remote object, due to the appearance of the servlet mechanism, making Java programming very efficient, Now many large web server support servlet.
(3) OO: Object-oriented design is a programming technology on an interface of objects and objects. It has a lot of object-oriented and C , in relation to multiple inheritance and the original model of Java.
(4) Agricultural feature: Java takes a safety pointer model that reduces the possibility of rewriting memory and data crash.
(5) Safety: Java is used to design a network and distribution system, which brings new security issues, and Java can build antivirus and anti-attack SYSTEM. Facts prove that Java is better in this area of the Antirate.
(6) Neutral Architecture: Java compiles its generating architecture neutral target file format can be executed on many processors, and the compiler generated command bytecode (Javabytecode) implements this feature, this byte code can be on any machine Explanation execution.
(7) Portability: The size and algorithm of the basic data structure type in Java have strict regulations, so the portability is very good.
(8) Multithread: Java handles multithreaded procedures, Java is completely handed over to the bottom operating system or thread program. So multithreading is one of the popularization causes of Java as server-side development languages.
(9) Applet and servlet: The program that can be executed on the web page called Applet, which needs to support a lot of browsers in Java, and Applet supports dynamic web pages, which is what many other languages cannot be done.
basic concepts:
1. The only relationship between the Object is what the object's interface is, just like the computer's seller, she doesn't matter what the internal structure of the power supply is, he only does it provide you with electricity, but just know that CAN or Not is not HOW and why. All programs are composed of certain properties and behavior objects, and different objects access through function calls. All communication between the objects is called by way of ways, through the package object data, large. Improve the reuse rate.
2. The most important idea inoo is class, class is a template is blueprint, constructed from the class, which creates an instance of this class (Instance)
3. Package: Incoming data and behavior in a package) and the implementation process of the object user hides the data, the data in an object calls his instance field (Instance Field)
4. By expanding a class to get a new class called inheritance, and all classes are extended by Object roots, and the root super class will be introduced.
5. 3 main features of the object
Behavior --- Description this object can do.
State --- Reflection of the object when the object is applied.
Identity - distinguishes with other similar behavior objects.
Each object has a unique Indentity and the three interacts between these three.
6. The relationship between classes:
Use-a: dependency
HAS-A: polymerization relationship
IS-A: Inheritance Relationship - Example: Class A inherits Class B, at which time A category not only has a class B method, but also its own method. (Personality exists in a commonality) 7. Construction object uses constructor : Presented by the constructor, the constructor is a special method, constructing the object and initializes it.
Example: Data class constructor is called DATA
New Data () --- Constructs a new object and initializes the current time.
Data happyday = new
Data () --- assigns an object to a variable happyday, so that the object can be used multiple times, and the value to be declared to the target variable is different. NEW is a reference.
Constructor Features: The constructor can have 0, one or more parameters
The constructor and the same name have the same name
A class can have multiple constructors
The constructor does not return value
The constructor is always used with the New operator.
8. Overload: When multiple methods have the same name and contain different parameters, it is overloaded. The compiler must select which method of calling.
9. Package Java allows one or more classes to become a group, called packets to facilitate organizational tasks, standard Java libraries are many packages .java.lang java.util Java, NET, etc. It is all Java packages in the Java and Javax package.
10. Inherited Thoughts: Allow new classes on the basis of existing classes, when you inherit an existing class, then you have multiplexed the methods and fields of this class, and you can add it in the new class. New methods and fields.
11. Extended class: Extended class fully reflects the inheritance relationship of IS-A. Form is: Class (subclass) Extends.
12. Polymorphism: In Java, the object variable is polymorphically. Multiple inheritance is not supported in Java.
13. Dynamic Binding: Mechanism of calling an object method.
(1) The type and method name of the compiler check the object declaration.
(2) The compiler checks the parameter type called by the method.
(3) Static binding: If the method type is the Priavte Static Final compiler, which method will be accurately known.
(4) When a program is running and using a dynamic binding to call a method, then the virtual machine must call the method version of the actual type of object to which X points to the X.
(5) Dynamic binding: It is very important to make the program become scalable without need to recompile the filed code.
14.Final Class: In order to prevent some people from derived new categories from your class, this type is not expandable.
15. Dynamic calls have taken longer than static calls,
16. Abstract class: The class specifies that one or more abstract methods itself must be defined as Abstract: Public Abstract String getDescripition
17. Each class in Java is expanded from the Object class.
18. Equal and toString method in theobject class .Equal is used to test whether an object is equal to another object. TOSTRING returns a string representing the object, almost every class overloads the method to return the correct state. Indicates. (Tostring method is a very important method)
19. Universal Programming: All values of any class type can be replaced with the object class variable.
20. Array List: ArrayList dynamic array list, is a class library, defined in the java.uitl package, automatically adjusts the size of the array.
21. The getClass method in the Class class Object class returns an instance of the CKASS type. When the program is started, the class containing the main method will be loaded. The virtual machine should load all the classes he need, and each loaded class is loaded. the type.
22. Class class provides powerful functional reflection for programming programming that can dynamically manipulate Java code. This feature is especially useful for JavaBeans, using reflection Java to support VB programmers used to use tools. Campaign to analyze the ability to call reflectors The package of this feature in Java is very powerful.
1. The ability to analyze class at runtime.
2. Explore the object of the class at runtime.
3. Implement universal array manipulation code.
4. Provide method objects.
This mechanism is primarily targeted by tools rather than applications and procedures.
The most important part of the reflective mechanism is to allow you to check the structure of the class. The API used by:
Java.lang.Reflect.field Returns the field.
Java.Reflect.Method Return to the method.
Java.lang.Reflect.constructor Returns the parameters.
Method pointer: Java has no method pointer, transmitting the address of a method to another, you can call it later, and the interface is a better solution. 23. Interface (interface) indicates what the class does not specify how to go Do it, a class can implement one or more interface.
24. The interface is not a class, but a set of specifications for classes that meet the interface requirements. If an interface requires 2 steps:
1. The specified interface that is declared needs to be implemented.
2. Provide definitions of all methods in the interface.
Declare a class to implement an interface. You need to use the imports keyword Class Actionb IMPLEMENTS Comparable. It is necessary to provide a CompareTo method. The interface is not a class, and an interface cannot be instantiated with a New.
25. A class has only one superclass, but a class can implement multiple interfaces. An important interface in Java Cloneable
26. Interface and callback. Programming a common mode is a callback mode. In this mode you can specify a method on the callback object when a specific time occurs. Example: ActionListener interface monitors.
Similar APIs are: java.swing.joptionpane
Java.swing.timer
Java.awt.tookit
27. Object Clone: Clone method is Object a protection method, which means that your code cannot be simply call it.
28. Internal Class: Definition of an internal class is a class defined in another internal
The reason is: 1. An internal class object can access the implementation of the object that creates it, including private data
2. For other classes in the same package, the internal classes can hide.
3. Anonymous internal classes can be very convenient to define callbacks.
4. Use the internal class to write event drivers very convenient.
29. Proxy (Proxy): 1. Specify all the methods defined by all code 2.Object class (TSTRING Equals)
30. Data Type: Java is emphasized the type of language, each variable must first declare that it is type, there are 8 basic types in Java. 4 is integer, 2 is a floating point, one is a character type , Which is used in the Unicode encoding characters, Boolean.
