java进阶开发

java进阶开发

（一），不可变集合

import java.util.List;
import java.util.Map;//不可变集合
public class CollectionDemo {public static void main(String[] args) {//不可变ListList<Double> lists= List.of(543.2,134.1,123.4,135.1);//lists.add(23.4);//lists.set(1.23.4);System.out.(2));//可以查看//Map和Set集合都一样Map<String, Integer> maps = Map.of("ssss",1,"wwww",2);}
}

 （二）、Stream流（流水线）

1.Stream初始

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
//初步体验Stream流
//找出姓张两个字的人
public class StreamTest {public static void main(String[] args) {List<String> list1 = new ArrayList<>();Collections.addAll(list1,"张三","张威威","张是");/*List<String> zhang1 = new ArrayList<>();for (String s : list1) {if (s.startsWith("张")){zhang1.add(s);}}List<String> zhang2 = new ArrayList<>();for (String s : zhang1) {if(s.length() == 3){zhang2.add(s);}}*///Stream简化集合、数组操作的API，结合Lambda表达式（格式像传送带一样）list1.stream().filter(s -> s.startsWith("张")).filter(s ->             s.length()==3).forEach(s -> System.out.println(s));}
}

2.Stream流的获取

import java.util.*;
import java.util.stream.Stream;//Stream流的获取
public class SreamTest2 {public static void main(String[] args) {//集合CollectionCollection<String> list = new ArrayList<>();Stream<String> s = list.stream();//Map集合Map<String,Integer> maps = new HashMap<>();//键流Stream<String> KeyStream = maps.keySet().stream();//值流Stream<Integer> value = maps.values().stream();//键值对流Stream<Map.Entry<String, Integer>> entries = Set().stream();//数组String[] st = {"aaa","sss","ddd"};Stream<String> names = Arrays.stream(st);Stream<String> name1 = Stream.of(st);}
}

3.Stream流的API

//Stream流API，不会改变原来的元素
import java.util.ArrayList;
import java.util.List;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.stream.Stream;public class StreamTest1 {public static void main(String[] args) {List<String> list = new ArrayList<>();list.add("张大");list.add("张参数");list.add("张给");list.add("张完全");//filter()过滤器list.stream().filter(new Predicate<String>() {@Overridepublic boolean test(String s) {return s.startsWith("张");}});//之后Lambda表达式简化list.stream().filter(s -> s.startsWith("张")).filter(s -> s.length()==3).forEach(s -> System.out.println(s));//Lambda取值互不影响//limit()取前几个数，skip()跳过前几个数//在集合加元素(加工)list.stream().map(new Function<String, String>(){@Overridepublic String apply(String s) {return "无敌的" + s;}});list.stream().map(s -> "无双的" + s);//对象加工list.stream().map(s -> new Student0(s));//合并流(Stream.cincat())Stream<String> s1 = list.stream().filter(s -> s.startsWith("张"));Stream<String> s2 = Stream.of("ww","dd");Stream<String> s3 = at(s1,s2);}
}

4.Stream流的相关应用

import java.math.BigDecimal;
import java.math.RoundingMode;
import java.util.ArrayList;
import java.util.List;public class StreamDemo4 {public static double allMoney;public static double allMoney1;public static void main(String[] args) {List<Employee> one = new ArrayList<>();one.add(new Employee("aaa",'a',2323,224223,null));one.add(new Employee("ddd",'v',3223,238723,"asas"));one.add(new Employee("ccc",'a',4523,235323,null));one.add(new Employee("ddd",'d',1323,233223,null));List<Employee> two = new ArrayList<>();two.add(new Employee("gfd",'f',2376,2243,null));two.add(new Employee("adf",'h',3224,2723,null));two.add(new Employee("afs",'q',4524,5223,null));two.add(new Employee("ryg",'w',4574,5423,null));two.add(new Employee("zzx",'t',1321,7523,"adf"));//找最大值Tapperfor employee = one.stream().max((e1, e2) -> Salary() + e1.getBouns() , e2.getSalary() + e2.getBouns())).map(employee1 -> new Name(),Salary()+ Bouns())).get();System.out.println(employee);one.stream().sorted((e1, e2) -> Salary() + e1.getBouns() , e2.getSalary() + e2.getBouns())).skip(1).limit(one.size()-2).forEach(employee1 -> {allMoney1 += (Salary()+ Bouns());//skip跳过第一个数，limit取前面的数（在跳过的基础上取）});//解决精度问题BigDecimal a = BigDecimal.valueOf(allMoney1);BigDecimal b = BigDecimal.valueOf(one.size()-2);System.out.println(a.divide(b,2, RoundingMode.HALF_UP));}
}

5.Stream流收集

//收集Stream到数组或集合里
import java.util.ArrayList;
import java.util.List;
import java.util.Set;
import java.util.function.IntFunction;
import java.util.stream.Collectors;
import java.util.stream.Stream;public class StreamDemo5 {public static void main(String[] args) {List<String> list = new ArrayList<>();list.add("张额外");list.add("张稿文");list.add("张个人");list.add("张的");Stream<String> s = list.stream().filter(s1 -> s1.startsWith("张"));List<String> zhanglist = s.List());//可以toSet,但是是不可变集合Set<String> zhanglist1 = s.Set());Object[] arrs = s.toArray();//String(arrs)String[] arrs1 = s.toArray(new IntFunction<String[]>() {@Overridepublic String[] apply(int value) {return new String[value];}});System.out.println(zhanglist);System.out.println(zhanglist1);//流只能使用一次}
}

 （二）、异常机制
1.异常处理类型

//运行时异常，继承RuntimeException
public class ExceptionDemo {public static void main(String[] args) {System.out.println("开始编译...");//1.数组异常：ArrayIndexOutOfBoundsExceptionint[] arr = {1,2,3,4};//System.out.println(arr[4]);//2.空指针异常：NullPointerExceptionString name = null;System.out.println(name);//System.out.println(name.length());运行时错误//3.类型转换异常：ClassCastExecptionObject o = 11;//String s = (String) o;//4.数学操作异常：ArithmeticExecption//int c = 10/0;//5.数字转换异常：NumberFormatExecptionString num = "23kk";int it = Integer.valueOf(num);System.out.println(it);}
}

2.异常处理意义（提醒你可能发生的错误）

ParseException;
SimpleDateFormat;
import java.util.Date;public class ExecptionDemo2 {public static void main(String[] args) throws ParseException {String date = "2015-01-12 10:23:21";//创建一个简单的格式化类SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-DD HH:KK:LL");//解析字符串时间成为对象Date d = sdf.parse(date);System.out.println(d);}
}

 3.异常处理使用

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.InputStream;
ParseException;
SimpleDateFormat;
import java.util.Date;public class ExecptionDemo01 {public static void main(String[] args) throws ParseException, FileNotFoundException {System.out.println("程序开始");parseTime("2011-11-11 11:11:11");//连续抛出对应System.out.println("，程序结束...");}public static void parseTime(String date) throws ParseException, FileNotFoundException {SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-ddd FF:NN:KK");Date d = sdf.parse(date);//连贯抛出System.out.println(d);InputStream is = new FileInputStream("E:/mi");//多次抛出//可以直接Exception,但是没有提醒}
}import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.InputStream;
ParseException;
SimpleDateFormat;
import java.util.Date;public class ExecptionDemo01 {public static void main(String[] args){System.out.println("程序开始");parseTime("2011-11-11 11:11:11");//连续抛出对应System.out.println("，程序结束...");}public static void parseTime(String date) {SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-ddd FF:NN:KK");Date d = null;//连贯抛出try {d = sdf.parse(date);} catch (ParseException e) {//解析出现System.out.println("出现了解析时间异常！");}System.out.println(d);try {InputStream is = new FileInputStream("E:/mi");//多次抛出} catch (FileNotFoundException e) {System.out.println("你这文件找不到");}//有异常就执行catch后面的}
}

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.InputStream;
ParseException;
SimpleDateFormat;
import java.util.Date;public class ExecptionDemo01 {public static void main(String[] args){System.out.println("程序开始");parseTime("2011-11-11 11:11:11");//连续抛出对应System.out.println("，程序结束...");}public static void parseTime(String date) {try {SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-ddd FF:NN:KK");Date d = sdf.parse(date);//连贯抛出System.out.println(d);InputStream is = new FileInputStream("E:/mi");} catch (ParseException e) {System.out.println("解析错误");//e.printStackTrace();解析异常问题} catch (FileNotFoundException e) {System.out.println("找文件");}/*catch(异常|异常){//Exception e 统领}*///有错误之后就处理该异常后直接停止，后面的异常也不处理了//还有一种，内部往外抛（throws）,外部解决异常（try,catch）}
}

public class ExecptionDemo3 {public static void main(String[] args) {System.out.println("程序开始...");try {chu(10,0);} catch (Exception e) {System.out.println("运算非法");}System.out.println("程序结束...");}public static void chu(int a,int b){System.out.println(a);System.out.println(b);int c = a/b;System.out.println(c);}
}import java.util.Scanner;public class Test2 {public static void main(String[] args) {Scanner sc = new Scanner(System.in);while(true){try {System.out.println("请输入价格：");String price = sc.nextLine();//转化为double类型double p = Double.valueOf(price);} catch (Exception e) {System.out.println("用户输入的数据有问题！");}}}
}
//会出现的异常，提前处理，为了让程序更好的执行，让代码更稳健

4.定义异常

public class ExecptionDemo6 {
//定义编译时异常，编译器不会自带的异常定义public static void main(String[] args) {try {checkAge(-23);} catch (ItheimaAgelleagaIException e) {e.printStackTrace();}}public static void checkAge(int age) throws ItheimaAgelleagaIException {if(age < 0 || age > 200){//抛出一个异常//throw在方法内部创建异常对象，从中抛出//thows在方法申明，抛出方法内部异常throw new ItheimaAgelleagaIException(age + "is illeagal");}else {System.out.println("年龄合法！");}}
}
public class ItheimaAgelleagaIException extends Exception{
//extends RunTimeExecption,可定义运行时异常，也就是声明，但异常不用抛出，运行时才会出现，但建议使用，为了高效的编译代码public ItheimaAgelleagaIException() {}public ItheimaAgelleagaIException(String message) {super(message);}
}