Shulou(Shulou.com)06/02 Report--

This article mainly introduces "Java access principle and usage analysis". In daily operation, I believe many people have doubts about the principle and usage analysis of Java access rights. The editor consulted all kinds of materials and sorted out simple and easy-to-use operation methods. I hope it will be helpful to answer the doubts about "Java access principle and usage analysis". Next, please follow the editor to study!

Constructor mode thought

Initialization solves the problem that multiple constructors are overloaded and too many constructor parameters can not be remembered.

Package day7;// declares a package class Employee {private String name; private int no; private int age; private String sex; private String address; / / alt + shift + s public int getNo () {return no;} / * public Employee () {} * / public Employee setNo (int no) {this.no = no; return this;} public String getName () {return name;} public Employee setName (String name) {this.name = name; return this;} public int getAge () {return age } public Employee setAge (int age) {this.age = age; return this;} public String getSex () {return sex;} public Employee setSex (String sex) {this.sex = sex; return this;} public String getAddress () {return address;} public Employee setAddress (String address) {this.address = address; return this;} / * public Employee (String name, int no, int age, String sex, String address) {this.name = name; this.no = no; this.age = age; this.sex = sex This.address = address;} * / public void show () {System.out.println (no+ "," + name+ "," + age+ "," + sex+ "," + address);}} public class TestEmployee {public static void main (String [] args) {/ * Employee tom = new Employee ("Tom", 12args 33, "male", "Shanghai"); tom.show (); * / / * Constructor pattern thought: initialize. Solved the situation that multiple constructors are overloaded and cannot remember too many constructor parameters * / Employee tom = new Employee (). SetNo (11). SetAddress ("Beijing"). SetAge (33). SetSex ("male"). SetName ("Tom"); tom.show ();}}

Bag

Package day7;// declares a package that must be placed on the first line of the file

Hierarchical structure

Package parent package [. Subpackage.]; package name: lowercase letters; usually domain name reversal. Name of the department. Project name

The benefits of a bag

1. Management classes and interfaces 2. Prevent naming conflicts 3. Encapsulation, through the control of permissions, better

Method access methods of classes under different packages

1. Import package

Import b.Exam2ramp / types under import b package Exam2import b.Exam3Exam3Exam3Exam3Exam3Exam3After type under import subpackage

two。 In the form of fully qualified naming

B.Exam2 e2 = new b.Exam2 ()

Note: both packages have the same type and must be fully qualified and named.

Access modifier

Java divides the visibility of class members into four categories:

There are only two types of classes that are created: public and default

Static

Static is a modifier application: can be used to modify attributes, methods, blocks, class static variables

Class class name {/ / static member variable, class variable public static data type variable name;}

Package day7;class Child {static int count;} public class TestChild {public static void main (String [] args) {Child a = new Child (); Child b = new Child (); / / count is shared by all objects a.count + +; b.count + +; System.out.println (a.count); System.out.println (b.count);}}

Static variables exist with the creation of the class and take precedence over objects.

Static variable (class variable)

Belonging to a class, shared by all objects, and existing before an object. Use

Class name. Static variable name object name. Static variable name / / is less used and is easy to be confused.

The difference between static variables and instance variables

1. Static: when the class is loaded, it is created, and the allocated space is initialized by default

Instance: it can only be created when the object is created; 2. Static: belonging to a class and existing in the method area. Instance: belongs to an object. Exists in the heap. 3. Static: the declaration cycle is long and will not be released until the class is destroyed. Instance: when the object is destroyed, it is released.

The application of static variables

When data is shared, use. Used when an object is not needed or cannot be created.

Static block

Define in a class

Static {function: initialize class, initialize class variable, static variable}

Local code block

Definitions: defining in method

Public void show () {{Local Code Block function: used to control the lifecycle and scope of use of local variables. }}

Static method

Only static members can be accessed in static methods. The this,super keyword cannot be used in static methods. Super may access non-static members.

The difference between static method and instance method

1. Static: belongs to the class, calls the instance directly with the class name: belongs to the object call. two。 Static: can only directly access static member (static variable, static method) instance: can directly access static and non-static 3. Static: this,super cannot be used. Example: you can use this,super.

Application situation

When you don't have to create object access, the form is simple, or you can't create an object, you need to use a static method.

Static import

Static members of the class are imported and can be used directly after import.

Format

Import static package name. Class name. Static variable name (method)

Singleton mode

Only one object can be created for a class

Hungry man model

When the class is loaded, the static variable stores an object

Package day7;class Window {private static Window win = new Window (); private Window () {} public static Window getInstance () {return win;}} public class TestWindow {public static void main (String [] args) {Window win = Window.getInstance (); Window win1 = Window.getInstance (); System.out.println (win); System.out.println (win1);}}

Output result

Day7.Window@7852e922day7.Window@7852e922

Lazy man mode

When the class is loaded, no object is specified, and the object is created only when it is applied. In a multithreaded environment, it is recommended to use Han style because it is thread safe.

Package day7;class Window {private static Window win = null; private Window () {} public static Window getInstance () {if (win = = null) {win = new Window ();} return win;}} public class TestWindow {public static void main (String [] args) {Window win = Window.getInstance (); Window win1 = Window.getInstance (); System.out.println (win); System.out.println (win1);}}

Return the result

Day7.Window@7852e922day7.Window@7852e922

Math class of API

Common methods

Package day7;public class TestMath {public static void main (String [] args) {/ / MathSystem.out.println (Math.abs (- 33.4)); / 33.4 Math.floor / the smallest integer greater than or equal to 44.6-"doubleSystem.out.println (44.6); / / 45.0 stroke / the maximum integer less than or equal to 44.6 -" doubleSystem.out.println (44.6) / / 44.0 double / rounded to a long digit System.out.println (Math.round (44.6)); / / 45bind / the value of System.out.println to the power (Math.pow (3Power2)); / / 9.0//double [0.0prime1.0) double n = Math.random (); System.out.println (n) / / 1Min 10 int / [min, max] / / (int) (Math.random () * (max-min + 1) + min) int num = (int) (Math.random () * (10m + 1) + 1); System.out.println (num);}}

Random class

Random rand1 = new Random (11); / / 11 is random seed System.out.println (rand1.nextDouble ()); Random rand2 = new Random (11); / / System.out.println (rand2.nextDouble ())

When the random seeds are the same, the output results of the same random times are the same.

Random rand3 = new Random (11); / / Integer System.out.println (rand3.nextInt ()) in the range of int; / / [0meme 5) System.out.println (rand3.nextInt (5))

At this point, the study on the principle and usage analysis of Java access rights is over. I hope to be able to solve your doubts. The collocation of theory and practice can better help you learn, go and try it! If you want to continue to learn more related knowledge, please continue to follow the website, the editor will continue to work hard to bring you more practical articles!

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