Introduction to Java lesson 92 -- wrapper classes

Monkeys learn to program 2020-11-13 02:42:15
introduction java lesson wrapper classes


1.1 Overview of packaging

1.1.1 Overview of packaging

    ·  In the context of type conversion , There is a special transformation , Need to put int Such basic data types are converted to objects ;

    ·  All basic types have a corresponding class , Packaging (wrapper).

    image.png

Overview of packaging ( To continue 1)

    ·  Packaging class is immutable class , After constructing the wrapper class object , Changing the value of the package in it is not allowed ;

    ·  A wrapper class is final Of , Can't define their subclasses .

Basic types Basic types Parent class
int java.lang.Integer java.lang.Number
long java.lang.Long java.lang.Number
double java.lang.Double java.lang.Number
short java.lang.Short java.lang.Number
float java.lang.Float java.lang.Number
byte java.lang.Byte java.lang.Number
char java.lang.Character java.lang.Object
boolean  java.lang.Boolean java.lang.Object

1.2 8 There are three basic data types: packaging

1.2.1 Number And its main methods

    ·  abstract class Number yes Byte、Double、Float、Integer、Long and Short The parent of a class ;

    · Number Subclass of must provide to convert the value represented to byte,double,float,int,long and short Methods ;

        doubleValue() With double Form returns the specified value

        intValue()     With int Form returns the specified value

        floatValue()     With float Form returns the specified value

Number And its main methods ( To continue 1)

 /**  test Number Of intValue Methods and doubleValue Method  */
    public void testIntValueAndDoubleValue(){
        Number d=123.45;
        Number n=123;
        // Output d and n The type of object
        System.out.println(d.getClass().getName());//java.lang.Double
        System.out.println(n.getClass().getName());//java.lang.Integer
        
        int intValue=d.intValue();
        double doubleValue=d.doubleValue();
        System.out.println(intValue+","+doubleValue);//123,123.45
        
        intValue=n.intValue();
        doubleValue=n.doubleValue();
        System.out.println(intValue+","+doubleValue);//123,123.0
    }

1.2.2 Integer Common functions

    ·  This class provides several methods , Can be in int The type and String Type to type , There are also constants such as :

        static int MAX_VALUE The value is 231  -1 The constant , Express int The maximum value that the type can represent

        static int MIN_VALUE The value is -231 The constant , Express int The minimum value that a type can represent

Integer Common functions ( To continue 1)

    · Integer Static method of parseInt Used to convert a string to int.

 /** String Convert to int */
    public void testParseInt(){
        String str="123";
        int value=Integer.parseInt(str);
        System.out.println(value);//123
        str=" Water and electricity ";
        // Will throw out NumberFormatException
        //value=Integer.parseInt(str);
    }

     If the format of a string is not an integer , Will not be able to convert .

1.2.3 Double Common functions

    ·  stay Double Class to wrap a basic type in the object double Value

    ·  Construction method

        - Double(double value)

        - Double(String s)

    ·  Method

        - double doubleValue() Back here Double Object's double value

        - static double parseDouble(String s) Back to a new double value , The value is initialized with the specified String Value represented

Double Common functions ( To continue 1)

 /** String Convert to double */
    public void testParseDouble(){
        String str="12345.00";
        double value=Double.parseDouble(str);
        //12345.0
        System.out.println(value);
        
        str="¥12345.00";
        
        // Will throw out NumberFormatException
        //value=Double.parseDouble(str);
    }

1.2.4 Automatic packing and unpacking operation

    ·  from Java5.0 Version later added to autoboxing function

    ·  Automatically " Unpacking " and " Packing " Depend on JDK5 The compiler of is at compile time " Preprocessing " Work

    ·  The falling code is written in the correct form :

        Integer a=100;    // Packing

        Integer b=200;    // Packing

        Integer c=a+b;    // Unpack and then pack

        double d=c;    // Unpacking

Automatic packing and unpacking operation ( To continue 1)

    ·  Packing and unpacking are " compiler " Approved , Not virtual machines . The compiler inserts the necessary method calls when generating the bytecode of the class

        - Integer a=100 => Integer a=Integer.valueOf(100)

        - Integer b=200 => Integer b=Integer.valueOf(200)

        - Integer c=a+b=>Integer c=Integer.valueOf(a.intValue()+b.intValue())

        -int d=c =>int d=c.intValue()

Automatic packing and unpacking operation ( To continue 2)

    image.png

Official account , Get a learning video

版权声明
本文为[Monkeys learn to program]所创,转载请带上原文链接,感谢

  1. [front end -- JavaScript] knowledge point (IV) -- memory leakage in the project (I)
  2. This mechanism in JS
  3. Vue 3.0 source code learning 1 --- rendering process of components
  4. Learning the realization of canvas and simple drawing
  5. gin里获取http请求过来的参数
  6. vue3的新特性
  7. Get the parameters from HTTP request in gin
  8. New features of vue3
  9. vue-cli 引入腾讯地图(最新 api,rocketmq原理面试
  10. Vue 学习笔记(3,免费Java高级工程师学习资源
  11. Vue 学习笔记(2,Java编程视频教程
  12. Vue cli introduces Tencent maps (the latest API, rocketmq)
  13. Vue learning notes (3, free Java senior engineer learning resources)
  14. Vue learning notes (2, Java programming video tutorial)
  15. 【Vue】—props属性
  16. 【Vue】—创建组件
  17. [Vue] - props attribute
  18. [Vue] - create component
  19. 浅谈vue响应式原理及发布订阅模式和观察者模式
  20. On Vue responsive principle, publish subscribe mode and observer mode
  21. 浅谈vue响应式原理及发布订阅模式和观察者模式
  22. On Vue responsive principle, publish subscribe mode and observer mode
  23. Xiaobai can understand it. It only takes 4 steps to solve the problem of Vue keep alive cache component
  24. Publish, subscribe and observer of design patterns
  25. Summary of common content added in ES6 + (II)
  26. No.8 Vue element admin learning (III) vuex learning and login method analysis
  27. Write a mini webpack project construction tool
  28. Shopping cart (front-end static page preparation)
  29. Introduction to the fluent platform
  30. Webpack5 cache
  31. The difference between drop-down box select option and datalist
  32. CSS review (III)
  33. Node.js学习笔记【七】
  34. Node.js learning notes [VII]
  35. Vue Router根据后台数据加载不同的组件(思考->实现->不止于实现)
  36. Vue router loads different components according to background data (thinking - & gt; Implementation - & gt; (more than implementation)
  37. 【JQuery框架,Java编程教程视频下载
  38. [jQuery framework, Java programming tutorial video download
  39. Vue Router根据后台数据加载不同的组件(思考->实现->不止于实现)
  40. Vue router loads different components according to background data (thinking - & gt; Implementation - & gt; (more than implementation)
  41. 【Vue,阿里P8大佬亲自教你
  42. 【Vue基础知识总结 5,字节跳动算法工程师面试经验
  43. [Vue, Ali P8 teaches you personally
  44. [Vue basic knowledge summary 5. Interview experience of byte beating Algorithm Engineer
  45. 【问题记录】- 谷歌浏览器 Html生成PDF
  46. [problem record] - PDF generated by Google browser HTML
  47. 【问题记录】- 谷歌浏览器 Html生成PDF
  48. [problem record] - PDF generated by Google browser HTML
  49. 【JavaScript】查漏补缺 —数组中reduce()方法
  50. [JavaScript] leak checking and defect filling - reduce() method in array
  51. 【重识 HTML (3),350道Java面试真题分享
  52. 【重识 HTML (2),Java并发编程必会的多线程你竟然还不会
  53. 【重识 HTML (1),二本Java小菜鸟4面字节跳动被秒成渣渣
  54. [re recognize HTML (3) and share 350 real Java interview questions
  55. [re recognize HTML (2). Multithreading is a must for Java Concurrent Programming. How dare you not
  56. [re recognize HTML (1), two Java rookies' 4-sided bytes beat and become slag in seconds
  57. 【重识 HTML ,nginx面试题阿里
  58. 【重识 HTML (4),ELK原来这么简单
  59. [re recognize HTML, nginx interview questions]
  60. [re recognize HTML (4). Elk is so simple