Android Training (Java article) 4th day-Object oriented

2. Class concept: Class is an abstract concept, it is "object Service", and the class and object are interdependent;
   

   The concept of objects: "People" who really do things;

   Encapsulation Concept: Explicit and implicit refers to methods or constants (fields);

4. How to create an object:

   Syntax: Class name Object name (variable name) =new class name (whether with parameters);

   Meaning: Call the member variables and methods inside this class;

   Precautions: null pointer exception;

6. Construction Method:

   Syntax: Permission + class name (with parameter) {}//method name must match the class name;

   When to invoke: The method is called when the object is created;

   Note: Any class that has a non-parametric constructor is defaulted, unless overridden by a new constructor method.

8. Set (), Get () method:

   Note: There must be a member variable before the use of these two methods is meaningful;

10. Overloads of the method:

    Syntax: Permissions + (Static) + return value type + method name (with parameter) {}

    Premise: Must be in the same class;

    Function: Can reduce the trouble of defining method name;

    Requirement: A method with no return value that belongs to the same method in the same class;

    The overloads of the method are only related to the number and type of formal parameters, and are irrelevant to others;

    The construction method also exists overloaded;

12. This keyword:

    Three major features:

    Can represent the properties of a class;

    You can use this to call the construction method of this class;

    An object that can represent the current class;

    Precautions: cannot be placed in the static method;

14. Static keyword:

    Features: Static is only related to the class, and is not related to the object;

    Static itself is an object of an "inner" class (the object is the class itself);

    Note: The method can no longer be modified with static;

16. Package, Import

    The package declares a name for itself; (com/org begins with public/private)

    The import is placed inside the package, bringing in the packages that are referenced in this class (JDK, custom classes), and the packages are required;

    How to look at the API:

    First: First look at the class where the package is wanted, look at the introduction;

    Second: Look at the construction method inside there are several overloaded methods, the role of what is;

    Third: Look at the member method inside whether there are necessary methods, directly with the object to call;

    The package name to be remembered for the API documentation:

    java.lang.*: No guide package is required in Java;

    Java.awt.*: Interface (GUI) class; graphical user interface

    java.net.*: network communication;

    java.io.*: input, output stream;

    Java.util.*: Common tool class;

18. Eclipse shortcut keys

    CTRL + Left: Enter Java underlying code (class for API documentation)

    Ctrl+o: Find the method of relative class;

    Ctrl+1: Reverse writing code, from right to left;

20. Inherited:

    Concept: If an inheritance relationship exists for two classes, the subclass can have all the member variables and methods of the parent class;

    Syntax: Permissions +class+ Child class +extends+ parent class {}

    Note: Java can only single inheritance, but multiple inheritance;

22. Permission modifiers

    
    

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    Note: Protected if it is in a different package, there must be an inheritance relationship to call, otherwise it cannot be called;

    There can be only one public class in multiple classes, and permissions only modify classes, methods, member variables;

24. Rewrite:

    Condition: Only in inheritance, method head must be identical;

    Note: The normal method of the parent class overrides the overridden method of the quilt class;

    The overridden method within the parent class cannot be higher than the override method permission in the subclass; (except private)

26. Static issues:

    Class loading Order: (static block, static variable) > static method > Construction method > member variable > member method;

After-school exercises:

1, there is known to have a game games, there is a play (); It has four sub-categories, namely C9, CF, CS, LOL

Requirements: Four subclasses must rewrite the play () method;

public class Game {

public void Play () {

System.out.println ("Game");

}

public static void Main (string[] args) {

Game[] g = new game[4];

G[0] = new C9 ();

G[1] = new CF ();

G[2] = new CS ();

G[3] = new LOL ();

for (int i=0;i<g.length;i++) {

G[i].play ();

}

}

}

Class C9 extends Game {

public void Play () {

System.out.println ("C9");

}

}

Class CF extends Game {

public void Play () {

System.out.println ("CF");

}

}

Class CS extends Game {

public void Play () {

System.out.println ("CS");

}

}

Class LOL extends Game {

public void Play () {

System.out.println ("LOL");

}

}

This article is from the "Kun" blog, please be sure to keep this source http://linyingkun.blog.51cto.com/2393912/1574787

Android Training (Java article) 4th day-Object oriented