javaapp接口开发实例 java api接口开发

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mob6454cc67554d 2023-07-23 23:39:41

文章标签 javaapp接口开发实例 java System 字符串 文章分类 Java 后端开发

1 前言

亲爱的小伙伴萌,目前我们看到的是Java基础部分的一个新的部分API,这是个啥,又能做啥呢?
其实可以概括成一句话:帮助我们站在巨人的肩膀上,实现更加高效的开发,那么我们来一探究竟吧~

2 什么是API

API（Application Programming Interface，应用程序接口）是一些预先定义的函数。目的是提供应用程序与开发人员基于某软件可以访问的一些功能集，但又无需访问源码或理解内部工作机制的细节.
API是一种通用功能集,有时公司会将API作为其公共开放系统,也就是公司制定自己的系统接口标准,当需要进行系统整合,自定义和程序应用等操作时,公司所有成员都可以通过该接口标准调用源代码.

Java.util包是java中的工具包,包含各种实用工具类/集合类/日期时间工具等各种常用工具包
import java.util.Scanner;
import java.util.Arrays;
java.lang包是java的核心,包含了java基础类
包括基本Object类/Class类/String类/基本数学类等最基本的类,这个包无需导入,默认会自动导入
import java.lang.Object;
import java.lang.String;
import java.lang.StringBuilder/StringBuffer;
正则表达式
包装类等等 –

那么接下来我们会学习一些常用的类,帮助大家完成"基建工程",首先第一位选手就是传说中的Object类

3 Object

3.1概念

Object类是所有Java类的祖先,也就是说我们所说的”顶级父类”

它存在于java.lang.Object,这个包不需要我们手动导包

需要注意的是:每个类都使用Object作为超类.所有对象(包括数组)都实现这个类的方法.

在不明确给出超类的情况下,Java会自动把Object类作为要定义类的超类.

javaapp接口开发实例 java api接口开发_System

3.2 常用方法介绍

toString()

本方法用于返回对应对象的字符串表示

hashCode()

本方法用于返回对应对象的哈希码值
小贴士:哈希码值的得出是通过一种算法,意在让不同的对象具有不同的哈希码值,用于区分不同的对象.
但是有时候也存在不同对象哈希码值相同的特殊情况,我们称之为”哈希碰撞”现象

equals()

本方法用于指示其他某个对象是否与当前对象”相等”

3.3 练习: Object类练习

创建包: cn.tedu.api
创建类: TestObject.java

package cn.tedu.api;

import java.util.Objects;

/*本类用于测试顶级父类Object/
//总结:
//1.如果想要查看对象的属性值,需要重写toString()
// 否则使用的就是Object的默认实现,打印的是对象的地址值
//2.如果要判断两个对象间的所有属性值相同,比较结果为true,就需要重写equals()
// 否则使用的就是Object的默认实现,通过比较的是两个对象的地址值,不是属性
public class TestObject {
public static void main(String[] args) {
//4.创建对象进行测试
Student s = new Student(“海绵宝宝”,3);
Student s2 = new Student(“海绵宝宝”,3);
/测试toString():返回的是该对象的字符串表示/
/查看源码的方式:按住Ctrl键,移到目标位置,鼠标点击即可/
//5.打印s对象
//5.1测试1打印的是cn.tedu.api.Student@1b6d3586,是地址值,因为Object默认实现:
//return getClass().getName() + “@” + Integer.toHexString(hashCode());
//5.2测试2打印的是Student{name=‘海绵宝宝’, age=3},是重写toString()以后的结果
System.out.println(s);
/测试hashcode()/
//6.测试hashCode()
System.out.println(s.hashCode());//460141958
//7.测试equals()
//7.1第一次测试执行结果是false
//底层:用了进行对象间的比较,==比较的是两个对象的地址值,s和s2是两个对象,地址值不同
//public boolean equals(Object obj) { return (this == obj);}
//需求:比较s与s2对象,如果属性值相同,就认为是同一个对象,并让equals()返回true
//7.2第二次测试执行结果是true
System.out.println(s.equals(s2));
}
}
//1.创建学生类
class Student{
//2.定义属性
String name;//姓名
int age;//年龄
//3.生成构造方法
//右键-Generate-Constructor-按住Shift可以全选属性
//3.1定义无参构造
public Student() { }
//3.2定义全参构造
public Student(String name, int age) {
this.name = name;
this.age = age;
}
//6.添加重写的toString()
@Override
public String toString() {
return “Student{” +
“name=’” + name + ‘’’ +
“, age=” + age +
‘}’;
}

<span >@Override</span>
<span >public</span> <span >boolean</span> <span >equals</span><span >(</span>Object o<span >)</span> <span >{<!-- --></span>
    <span >if</span> <span >(</span><span >this</span> <span >==</span> o<span >)</span> <span >return</span> <span class="token boolean">true</span><span >;</span>
    <span >if</span> <span >(</span>o <span >==</span> null <span >||</span> <span >getClass</span><span >(</span><span >)</span> <span >!=</span> o<span >.</span><span >getClass</span><span >(</span><span >)</span><span >)</span> <span >return</span> <span class="token boolean">false</span><span >;</span>
    Student student <span >=</span> <span >(</span>Student<span >)</span> o<span >;</span>
    <span >return</span> age <span >==</span> student<span >.</span>age <span >&&</span> Objects<span >.</span><span >equals</span><span >(</span>name<span >,</span> student<span >.</span>name<span >)</span><span >;</span>
<span >}</span>

4 String

4.1 特点

String是一个封装char[]数组的对象,字符串不可变

通过下图中的底层实现可以看出:被final修饰,是常量

String str = “abc”; 等效于：char data[] = {‘a’, ‘b’, ‘c’};

javaapp接口开发实例 java api接口开发_字符串_02

4.2 创建String对象的方式

方式一:

String(char[] value) 分配一个新的 String，使其表示字符数组参数中当前包含的字符序列。

方式二: String str = “abc”;

javaapp接口开发实例 java api接口开发_字符串_03

如果是第一次使用字符串，java会在字符串堆中常量池创建一个对象。
再次使用相同的内容时，会直接访问堆中常量池中存在的对象。

4.3 常见方法

length()-查看字符串的长度
charAt()—获取指定下标处位置上的字符
lastIndexOf()-某个字符最后一次出现的位置
substring()-截取子串,如果参数有两个左闭右开[1,5)
equals()-判断两个串是否相等,注意String重写了Object的此方法,所以内容相同就返回true
startsWith()-判断是不是以参数开头
endsWith()–判断是不是以参数结尾
split()—以指定字符分割
trim()-去掉首尾两端的空格
getBytes()-把串转换成数组
toUpperCase()-变成全大写
toLowerCase()-变成全小写
String.valueOf(10)-把int类型的10转换成String类型

4.4 练习:String常用方法练习

创建包: cn.tedu.api
创建类: TestString.java

package cn.tedu.api;
/*本类用于测试String类的用法*/
public class TestString {
    public static void main(String[] args) {
        //1.创建String对象方式一
        /*1.字符串底层维护的是char[].存放在堆中*/
        char[] value = {'a','b','c'};
        String s1 = new String(value);
        String s11 = new String(value);
        //2.创建String对象方式二
        /*2.此种方式创建,底层也会new String(),存放在堆中常量池,效率高*/
        String s2 = "abc";
        String s22 =  "abc";
        //3.测试
        /*3.==比较的是地址值
        Object中equals()的默认实现比较的也是==地址值
        但String类对此方法做了重写,比较的就是字符串的具体内容*/
        System.out.println(s1 == s2);//false
        System.out.println(s1.equals(s2));//true
        System.out.println(s1 == s11);//false,没有高效效果
        System.out.println(s2 == s22);//true,有高效的效果
    }
}

创建包: cn.tedu.api
创建类: TestString2.java

package cn.tedu.api;

import java.nio.charset.StandardCharsets;
import java.util.Arrays;
import java.util.Locale;

/本类用于测试String常用方法/
public class TestString2 {
public static void main(String[] args) {
//1.创建字符串–堆中常量池,有高效效果
String s = “abc”;
char[] value = { ‘a’,‘b’,‘c’,‘b’};
String ss = new String(value);

<span >//2.测试常用方法</span>
    System<span >.</span>out<span >.</span><span >println</span><span >(</span>s<span >.</span><span >charAt</span><span >(</span><span >1</span><span >)</span><span >)</span><span >;</span><span >//b,获取下标为1处位置上的字符</span>

    String s2 <span >=</span> s<span >.</span><span >concat</span><span >(</span><span >"cxy"</span><span >)</span><span >;</span><span >//拼接字符串,但是并不影响本串的值</span>
    System<span >.</span>out<span >.</span><span >println</span><span >(</span>s2<span >)</span><span >;</span><span >//abccxy,如果想多次使用,可以再定义一个变量保存拼接结果</span>
    System<span >.</span>out<span >.</span><span >println</span><span >(</span>s<span >)</span><span >;</span><span >//abc,串s本身的值没有被影响</span>

    System<span >.</span>out<span >.</span><span >println</span><span >(</span>s<span >.</span><span >endsWith</span><span >(</span><span >"y"</span><span >)</span><span >)</span><span >;</span><span >//false,判断是否以指定元素结尾</span>
    System<span >.</span>out<span >.</span><span >println</span><span >(</span>s<span >.</span><span >startsWith</span><span >(</span><span >"a"</span><span >)</span><span >)</span><span >;</span><span >//ture,判断是否以指定元素开头</span>

    System<span >.</span>out<span >.</span><span >println</span><span >(</span>ss<span >.</span><span >equals</span><span >(</span>s<span >)</span><span >)</span><span >;</span><span >//true,String重写了,比较的是串本身的内容</span>
    System<span >.</span>out<span >.</span><span >println</span><span >(</span>s <span >==</span> ss<span >)</span><span >;</span><span >//false,比较的是地址值,一个高效在堆中常量池,一个不是,地址值不同</span>

    System<span >.</span>out<span >.</span><span >println</span><span >(</span>s<span >.</span><span >getBytes</span><span >(</span><span >)</span><span >)</span><span >;</span><span >//[B@1b6d3586,把字符串转化成byte[],所以打印数组地址值</span>
    System<span >.</span>out<span >.</span><span >println</span><span >(</span>s<span >.</span><span >indexOf</span><span >(</span><span >"b"</span><span >)</span><span >)</span><span >;</span><span >//1,获取b字符在串中第一次出现的下标值</span>
    System<span >.</span>out<span >.</span><span >println</span><span >(</span>ss<span >.</span><span >lastIndexOf</span><span >(</span><span >"b"</span><span >)</span><span >)</span><span >;</span><span >//3,获取b字符在串中最后一次出现的位置</span>
    System<span >.</span>out<span >.</span><span >println</span><span >(</span>ss<span >.</span><span >length</span><span >(</span><span >)</span><span >)</span><span >;</span><span >//4,获取指定字符串的长度</span>

    String s3 <span >=</span> <span >"a b c d e"</span><span >;</span>
    <span >/*本方法的作用是根据指定元素分割字符串,返回值类型是String[]*/</span>
    System<span >.</span>out<span >.</span><span >println</span><span >(</span>s3<span >.</span><span >split</span><span >(</span><span >" "</span><span >)</span><span >)</span><span >;</span><span >//[Ljava.lang.String;@4554617c</span>
    System<span >.</span>out<span >.</span><span >println</span><span >(</span>Arrays<span >.</span><span >toString</span><span >(</span>s3<span >.</span><span >split</span><span >(</span><span >" "</span><span >)</span><span >)</span><span >)</span><span >;</span><span >//使用数组工具类打印数组具体元素</span>

    System<span >.</span>out<span >.</span><span >println</span><span >(</span>s3<span >.</span><span >substring</span><span >(</span><span >3</span><span >)</span><span >)</span><span >;</span><span >// c d e,从下标为3的位置处开始截取剩下的字符串</span>
    System<span >.</span>out<span >.</span><span >println</span><span >(</span>s3<span >.</span><span >substring</span><span >(</span><span >1</span><span >,</span><span >5</span><span >)</span><span >)</span><span >;</span><span >// b c,截取的[1.5)的字符串,含头不含尾</span>

    System<span >.</span>out<span >.</span><span >println</span><span >(</span>s3<span >.</span><span >toUpperCase</span><span >(</span><span >)</span><span >)</span><span >;</span><span >//A B C D E,把串变成全大写</span>
    System<span >.</span>out<span >.</span><span >println</span><span >(</span>s3<span >.</span><span >toLowerCase</span><span >(</span><span >)</span><span >)</span><span >;</span><span >//a b c d e,把串变成全小写</span>

    s3 <span >=</span> <span >"       abced     "</span><span >;</span>
    System<span >.</span>out<span >.</span><span >println</span><span >(</span>s3<span >.</span><span >trim</span><span >(</span><span >)</span><span >)</span><span >;</span><span >//abced,去除数据首尾两端的空格</span>

    System<span >.</span>out<span >.</span><span >println</span><span >(</span>String<span >.</span><span >valueOf</span><span >(</span><span >10</span><span >)</span><span >)</span><span >;</span><span >//10,把int10转成成String类型</span>
    System<span >.</span>out<span >.</span><span >println</span><span >(</span><span >"20"</span><span >+</span><span >10</span><span >)</span><span >;</span><span >//2010,String与int数据进行了拼接</span>
    System<span >.</span>out<span >.</span><span >println</span><span >(</span>String<span >.</span><span >valueOf</span><span >(</span><span >10</span><span >)</span><span >+</span><span >10</span><span >)</span><span >;</span><span >//1010,把int10转成成String类型</span>
    System<span >.</span>out<span >.</span><span >println</span><span >(</span><span >10</span><span >+</span><span >10</span><span >)</span><span >;</span><span >//20,int10+int10=int20</span>
<span >}</span>

}

5 StringBuilder/StringBuffer

5.1 特点

封装了char[]数组
是可变的字符序列
提供了一组可以对字符内容修改的方法
常用append()来代替字符串做字符串连接”+”
内部字符数组默认初始容量是16：super(str.length() + 16);
如果大于16会尝试将扩容，新数组大小原来的变成2倍+2，容量如果还不够，直接扩充到需要的容量大小。int newCapacity = value.length * 2 + 2;
StringBuffer 1.0出道线程安全，StringBuilder1.5出道线程不安全

5.2 常见方法

append()

5.3 练习3：测试字符串连接

创建包: cn.tedu. api
创建类: TestString2.java

需求: 测试+拼接字符串的效率
package cn.tedu.api;
/**本类用于测试字符串拼接的效率*/
public class TestString3 {
	public static void main(String[] args) {
		//method();//完成通过"+"进行拼接
		method2();//优化"+"拼接
	}

<span >public</span> <span >static</span> <span >void</span> <span >method2</span><span >(</span><span >)</span> <span >{<!-- --></span>
	<span >//1.定义字符串</span>
	String str <span >=</span> <span >"abcdefghijklmnopqrstuvwxyz"</span><span >;</span>
	
	<span >//2.将指定的字符串拼接10000次</span>
	<span >/**优化1:String-->StringBuffer/StringBuilder*/</span>
	StringBuffer sb <span >=</span> <span >new</span> <span >StringBuffer</span><span >(</span><span >)</span><span >;</span><span >//创建工具类对象1</span>
	StringBuilder sb2 <span >=</span> <span >new</span> <span >StringBuilder</span><span >(</span><span >)</span><span >;</span><span >//创建工具类对象2</span>
	
	<span >//2.2拼接10000次</span>
	<span >/**4.可以添加一个计时功能*/</span>
	<span >//4.1获取系统当前时间作为开始时间</span>
	<span >long</span> t1 <span >=</span> System<span >.</span><span >currentTimeMillis</span><span >(</span><span >)</span><span >;</span><span >//ms</span>
	<span >for</span><span >(</span><span >int</span> i <span >=</span> <span >0</span><span >;</span> i <span ><</span> <span >10000</span> <span >;</span> i<span >++</span><span >)</span> <span >{<!-- --></span>
		<span >/**优化2:+ -->append() */</span>
		<span >//result = result + str;</span>
		sb<span >.</span><span >append</span><span >(</span>str<span >)</span><span >;</span>
		<span >//sb2.append(str);</span>
	<span >}</span>
	<span >//4.2获取系统当前时间作为结束时间</span>
	<span >long</span> t2 <span >=</span> System<span >.</span><span >currentTimeMillis</span><span >(</span><span >)</span><span >;</span><span >//ms</span>
	<span >//3.打印拼接好的字符串</span>
	<span >//注意:eclipse控制台显示数据有限制,都打印在同一行了,所以控制台看不出来打印的内容</span>
	<span >//可以通过全选,复制拿出来看数据是否拼接成功</span>
	<span >//4.3打印拼接总共花费的总时长</span>
	System<span >.</span>out<span >.</span><span >println</span><span >(</span>sb<span >.</span><span >toString</span><span >(</span><span >)</span><span >)</span><span >;</span><span >//打印拼接的结果</span>
	System<span >.</span>out<span >.</span><span >println</span><span >(</span>t2<span >-</span>t1<span >)</span><span >;</span>
<span >}</span>

<span >public</span> <span >static</span> <span >void</span> <span >method</span><span >(</span><span >)</span> <span >{<!-- --></span>
	<span >//1.定义字符串</span>
	String str <span >=</span> <span >"abcdefghijklmnopqrstuvwxyz"</span><span >;</span>
	
	<span >//2.将指定的字符串拼接10000次</span>
	<span >//2.1定义变量,用来保存最终拼接的结果</span>
	String result <span >=</span><span >""</span><span >;</span>
	<span >//2.2拼接10000次</span>
	<span >/**4.可以添加一个计时功能*/</span>
	<span >//4.1获取系统当前时间作为开始时间</span>
	<span >long</span> t1 <span >=</span> System<span >.</span><span >currentTimeMillis</span><span >(</span><span >)</span><span >;</span><span >//ms</span>
	<span >for</span><span >(</span><span >int</span> i <span >=</span> <span >0</span><span >;</span> i <span ><</span> <span >10000</span> <span >;</span> i<span >++</span><span >)</span> <span >{<!-- --></span>
		result <span >=</span> result <span >+</span> str<span >;</span>
	<span >}</span>
	<span >//4.2获取系统当前时间作为结束时间</span>
	<span >long</span> t2 <span >=</span> System<span >.</span><span >currentTimeMillis</span><span >(</span><span >)</span><span >;</span><span >//ms</span>
	<span >//3.打印拼接好的字符串</span>
	<span >//注意:eclipse控制台显示数据有限制,都打印在同一行了,所以控制台看不出来打印的内容</span>
	<span >//可以通过全选,复制拿出来看数据是否拼接成功</span>
	System<span >.</span>out<span >.</span><span >println</span><span >(</span>result<span >)</span><span >;</span>
	
	<span >//4.3打印拼接总共花费的总时长</span>
	System<span >.</span>out<span >.</span><span >println</span><span >(</span>t2<span >-</span>t1<span >)</span><span >;</span>
<span >}</span>

}

6 拓展

6.1 ==和equals的区别

1.当使用比较时，如果相比较的两个变量是引用类型，那么比较的是两者的物理地值（内存地址），如果相比较的两个变量都是数值类型，那么比较的是具体数值是否相等。
2.当使用equals()方法进行比较时，比较的结果实际上取决于equals()方法的具体实现
众所周知，任何类都继承自Object类，因此所有的类均具有Object类的特性，比如String、integer等，他们在自己的类中重写了equals()方法，此时他们进行的是数值的比较，而在Object类的默认实现中，equals()方法的底层是通过来实现的。

6.2 练习: ==与equals测试

创建包: cn.tedu.api
创建类: Test.java

package cn.tedu.api;
/**本类用于测试==与equlas的区别*/
public class Test{
	public static void main(String[] args) {
		Person p1 = new Person("凹凸曼",16);
		Person p2 = new Person("凹凸曼",16);

	System<span >.</span>out<span >.</span><span >println</span><span >(</span>p1 <span >==</span> p2<span >)</span><span >;</span><span >//false,new了两个对象,p1和p2保存的地址值不同</span>
	System<span >.</span>out<span >.</span><span >println</span><span >(</span>p1<span >.</span>name <span >==</span> p2<span >.</span>name<span >)</span><span >;</span><span >//true,name是String类型,保存在常量池中,值是同一个</span>
	System<span >.</span>out<span >.</span><span >println</span><span >(</span>p1<span >.</span>age <span >==</span> p2<span >.</span>age<span >)</span><span >;</span><span >//true,age是int基本类型,保存的值都是18</span>
	
	<span >//第一次测试:结果false,使用的是Object默认的逻辑,也是用==来比较的</span>
	<span >//第二次测试:结果true,重写equals()后,就会使用子类重写后的功能,也就是说此时比较的不再是地址,而是类型&属性值</span>
	System<span >.</span>out<span >.</span><span >println</span><span >(</span>p1<span >.</span><span >equals</span><span >(</span>p2<span >)</span><span >)</span><span >;</span>
<span >}</span>

}

//1.创建Person类
class Person {
String name;
int age;

<span >public</span> <span >Person</span><span >(</span><span >)</span> <span >{<!-- --></span>
	System<span >.</span>out<span >.</span><span >println</span><span >(</span><span >"我是手动添加的无参构造"</span><span >)</span><span >;</span>
<span >}</span>
<span >public</span> <span >Person</span><span >(</span>String name<span >,</span> <span >int</span> age<span >)</span> <span >{<!-- --></span>
	<span >super</span><span >(</span><span >)</span><span >;</span>
	<span >this</span><span >.</span>name <span >=</span> name<span >;</span>
	<span >this</span><span >.</span>age <span >=</span> age<span >;</span>
<span >}</span>
<span >//需求:比较两个对象的属性值,如果属性值都一样,就是"同一个对象",要求equals返回true</span>
<span >@Override</span>
<span >public</span> <span >boolean</span> <span >equals</span><span >(</span>Object obj<span >)</span> <span >{<!-- --></span>
	<span >if</span> <span >(</span><span >this</span> <span >==</span> obj<span >)</span>
		<span >return</span> <span class="token boolean">true</span><span >;</span>
	<span >if</span> <span >(</span>obj <span >==</span> null<span >)</span>
		<span >return</span> <span class="token boolean">false</span><span >;</span>
	<span >if</span> <span >(</span><span >getClass</span><span >(</span><span >)</span> <span >!=</span> obj<span >.</span><span >getClass</span><span >(</span><span >)</span><span >)</span>
		<span >return</span> <span class="token boolean">false</span><span >;</span>
	Person other <span >=</span> <span >(</span>Person<span >)</span> obj<span >;</span>
	<span >if</span> <span >(</span>age <span >!=</span> other<span >.</span>age<span >)</span>
		<span >return</span> <span class="token boolean">false</span><span >;</span>
	<span >if</span> <span >(</span>name <span >==</span> null<span >)</span> <span >{<!-- --></span>
		<span >if</span> <span >(</span>other<span >.</span>name <span >!=</span> null<span >)</span>
			<span >return</span> <span class="token boolean">false</span><span >;</span>
	<span >}</span> <span >else</span> <span >if</span> <span >(</span><span >!</span>name<span >.</span><span >equals</span><span >(</span>other<span >.</span>name<span >)</span><span >)</span>
		<span >return</span> <span class="token boolean">false</span><span >;</span>
	<span >return</span> <span class="token boolean">true</span><span >;</span>
<span >}</span>

}

1.在线程安全上 :

–StringBuffer是旧版本就提供的，线程安全的。@since JDK1.0

–StringBuilder是jdk1.5后产生，线程不安全的。@since 1.5

2. 在执行效率上，StringBuilder > StringBuffer > String

3.源码体现：本质上都是在调用父类抽象类AbstractStringBuilder来干活，只不过Buffer把代码加了同步关键字，使得程序可以保证线程安全问题。

javaapp接口开发实例 java api接口开发_System_04

javaapp接口开发实例 java api接口开发_System_05