Java-实用类

比眉伴天荒 2022-05-18 05:49 177阅读 0赞

## Java  API ##

Java应用程序编程接口（Java  Application  Programming  Interface,Java API）是运行库的集合，预先定义了一些接口和类，程序员可以直接使用这些已经被打包的接口和类来开发具体的应用，节约了程序员大量的时间和精力。API除了有“应用程序编程接口”的意思外，还特指API的说明文档，也称帮助文档。

Java API提供了如下常用的包。

1.java.lang:编写Java程序时最广泛使用的包，自动导入到所有的程序中，包含了Java程序的基础类和接口。包装类、Math类、String类等常用的类都包含在此包中，java.lang包还提供了用于管理类的动态加载、外部进程创建、主机环境查询和安全策略实施等“系统操作”的类。

2.java.util:包含了系统辅助类，特别是Collection、List和Map等集合类。

3.java.io:包含了与输入/输出有关的类，如文件操作等类。

4.java.net:包含了与网络有关的类，如Socket、ServerSocket等类。

5.java.sql:包含了与数据库有关的类，如Connection、Statement等类。

## 枚举 ##

枚举是指由一组固定的常量组成的类型。使用关键字enum定义。

示例：定义表示性别的枚举。

关键代码：public enum Genders\{

Male,Female

## 包装类 ##

Java语言是面向对象的，但是Java中的基本数据类型却不是面向对象的，这在实际开发中存在很多不便。为了解决这个不足，在设计类时为每个基本数据类型设计了一个对应的类，称为包装类。如图：

包装类和基本数据类型的对应表

<table style="width:656px;"> 
 <tbody> 
  <tr> 
   <td style="width:332px;">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;基本数据类型</td> 
   <td style="width:323px;">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 包装类</td> 
  </tr> 
  <tr> 
   <td style="width:332px;">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; byte</td> 
   <td style="width:323px;">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Byte</td> 
  </tr> 
  <tr> 
   <td style="width:332px;">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; boolean</td> 
   <td style="width:323px;">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Boolean</td> 
  </tr> 
  <tr> 
   <td style="width:332px;">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; short</td> 
   <td style="width:323px;">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;Short</td> 
  </tr> 
  <tr> 
   <td style="width:332px;">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;char</td> 
   <td style="width:323px;">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;Character</td> 
  </tr> 
  <tr> 
   <td style="width:332px;">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; int</td> 
   <td style="width:323px;">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;Integer</td> 
  </tr> 
  <tr> 
   <td style="width:332px;">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; long</td> 
   <td style="width:323px;">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;Long</td> 
  </tr> 
  <tr> 
   <td style="width:332px;">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;float</td> 
   <td style="width:323px;">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Float</td> 
  </tr> 
  <tr> 
   <td style="width:332px;">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; double</td> 
   <td style="width:323px;">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;Double</td> 
  </tr> 
 </tbody> 
</table>

包装类的用途主要有两个：

1.包装类作为和基本数据类型对应的类存在，方便对象的操作。

2.包装类包含每种基本数据类型的相关属性，如最大值、最小值等，以及相关的操作方法。

### ·基本数据类型转换为包装类 ###

一、使用包装类的构造方法

1.public Type(type value).

2.public Type(String value).

Type表示包装类     参数type为基本数据类型

例：Integer intValue=new Integer(21);

或者Integer intValue=new Integer("21");

二、使用包装类的valueOf()方法

例：Integer intValue=Integer.valueOf("21");

### ·包装类转换成基本数据类型 ###

例：Integer integerId=new Integer(25);

int intId=integerId.intValue();

Boolean b1=Boolean.valueOf(true);

boolean bool=b1.booleanValue();

###        ·基本类型和包装类的自动转换 ###

在Java5之后，编译器会自动完成转换

例：Integer intObject=5;          //基本数据类型转换成包装类

int intValue=intObject;       //包装类转换成基本数据类型

常用的提取和搜索字符串的方法

![70][]

代码：

运行结果如下：

![70 1][]

代码：

public class StringDemo {
    	public static void main(String[] args) {
           String gushi="长亭外 古道边 芳草碧蓝天 晚风扶 柳笛声残 夕阳山外山";
    		String[] arr=gushi.split(" ");
    		for(String s:arr) {
    			System.out.println(s);
    		}
    		System.out.println("*******************************************");
    		String finalstr=String.join(" ", arr);
    		System.out.println(finalstr);
    	}
    
    }

运行结果如下：

![70 2][]

[70]: /images/20220518/132a8fdbd44c473a8d25adf5295bb930.png
[70 1]: /images/20220518/1b3d0d2f6ba44af7a855cc67de697965.png
[70 2]: /images/20220518/da8f3f36e7294101a0d98044da6ae490.png