字符串
基础

java语法整理

包装类

为了方便操作基本数据类型，将其封装成了对象，在对象中定义了属性和行为丰富了该数据的操作。

Integer

public static void main(String[] args) {
    
    Integer i1 = new Integer(123);
    Integer i2 = new Integer("123");
    Integer i3 = new Integer(128);
    System.out.println(i1.equals(i2)); //true
    System.out.println(i1); //123   Integer 重写了 toString
    System.out.println(i1.compareTo(i3)); //-1
    System.out.println(i1.compareTo(i2)); //0
    System.out.println(i3.compareTo(i2)); //1
    Integer[] iiIntegers = {i1,i2,i3};

    //Integer==>int
    int a = i1.intValue();
    System.out.println(a); //123
    
    //int==>Integer
    Integer a1 = Integer.valueOf(a);
    System.out.println(a1); //123
    
    //String==>int
    int a2 = Integer.parseInt("123");
    System.out.println(a2); //123
    
    //int==>String
    String s1 = a2+"";
    System.out.println(s1); //123
    
    //String==>Integer
    Integer a3 = new Integer("123");
    System.out.println(a3); //123
    
    //Integer==>String
    String s2 = a3.toString();
    System.out.println(s2); //123
}

//进制转换
System.out.println(Integer.toBinaryString(60));
System.out.println(Integer.toOctalString(60));
System.out.println(Integer.toHexString(60));
System.out.println(Integer.toString(60, 2));

System.out.println(Integer.parseInt("111100", 2));

自动装箱拆箱

详解Java的自动装箱与拆箱(Autoboxing and unboxing)

public static void main(String[] args) {
		
    Integer integer = 23; //装箱 编译器自动调用valueOf方法
    int i = new Integer(123); //拆箱 调用了intValue方法
    
}

Integer integer1 = 127;
Integer integer2 = 127;
Integer integer3 = 128;
Integer integer4 = 128;
System.out.println(integer1==integer2); //true
System.out.println(integer3==integer4);  //false
  1. 自动装箱 auto-boxing 基本类型就自动地封装到与它相同的类型的包装类中 Integer i=100 编译器调用了valueOf()方法 Integer i=Integer.valueOf(100) Integer 中的缓存类 IntegerCache Cache 为[-128,127],IntegerCache 有一个静态的 Integer 数 组，在类加载时就将-128 到 127 的 Integer 对象创建了，并 保存在 cache 数组中，一旦程序调用 valueOf 方法，如果取 的值是在-128 到 127 之间就直接在 cache 缓存数组中去取 Integer 对象，超出范围就 new 一个对象。
  2. 自动拆箱 unboxing 包装类对象自动转换成基本类型数据 int a=new Integer(100) 编译器为我们添加了 int a=new Integer(100).intValue()

Date

Date date = new Date();
System.out.println(date); //Tue Mar 12 12:57:25 CST 2019

Date date1 = new Date(1001L);
System.out.println(date1); //Thu Jan 01 08:00:01 CST 1970

//比较时间
System.out.println(date1.getTime()>date.getTime()); //false
System.out.println(date1.before(date)); //true
System.out.println(date1.after(date)); //false
System.out.println(date1.compareTo(date)); //-1
System.out.println(date1.equals(date)); //false

//日期转字符串
SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
String strDateString = format.format(date);
System.out.println(strDateString); //2019-03-12 13:17:58
//字符串转日期
String string = "2019-03-12 13:17:58";
Date date3 = format.parse(string);
System.out.println(date3); //Tue Mar 12 13:17:58 CST 2019

//获取北京时间
Calendar calendar = Calendar.getInstance(Locale.CHINA);
Date date2 = calendar.getTime();
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
String dateString = dateFormat.format(date2);
System.out.println(dateString);

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;
import java.util.Scanner;

/**
 * 命令行打印日历
 * @author yuankun
 * @version 0.1
 */
public class PrintCalendar {

	public static void main(String[] args) throws ParseException {
		
		//获取日期输入
		Scanner scanner = new Scanner(System.in);
		System.out.println("请输入日期：(yyyy-mm-dd)");
		String inputString = scanner.next();
		
		//获取日历对象
		DateFormat format = new SimpleDateFormat("yyyy-MM-dd");
		Date date = format.parse(inputString);
		Calendar calendar = new GregorianCalendar();
		calendar.setTime(date);
		
		//获取输入日期
		int inputDate = calendar.get(Calendar.DAY_OF_MONTH);
		
		//获取当月1号的星期数
		calendar.set(Calendar.DAY_OF_MONTH, 1);
		int dayOfweek = calendar.get(Calendar.DAY_OF_WEEK);
		
		//获取当月天数
		int maxDay = calendar.getActualMaximum(Calendar.DAY_OF_MONTH);
		
		System.out.println("日\t一\t二\t三\t四\t五\t六");
		
		//打印1号前的空号
		for(int i=1;i<dayOfweek;i++) {
			System.out.print("\t");
		}
		
		//打印日期
		for(int i=1;i<=maxDay;i++) {
			System.out.print(i);
			if(i==inputDate) {
				System.out.print("*");
			}
			System.out.print("\t");
			if(calendar.get(Calendar.DAY_OF_WEEK)==Calendar.SATURDAY) {
				System.out.println();
			}
			calendar.add(Calendar.DAY_OF_MONTH, 1);
		}

	}

}

Math

//静态导入
import static java.lang.Math.*;
public class TestMath {
	public static void main(String[] args) {
		//返回1-10的随机数
		System.out.println((int)(random()*9)+1);
		//四舍五入取整
		System.out.println(round(3.1415));
		//取最值
		System.out.println(max(1, 2.1));
	}
}

File

//创建文件
File file = new File("D:\\a.txt");
File file1 = new File("a.txt");
File file2 = new File("D:/dir");

System.out.println(file.getAbsolutePath());
System.out.println(file1.getAbsolutePath());
System.out.println(file2.getAbsolutePath());

System.out.println(file.getName());
System.out.println(file1.getName());
System.out.println(file2.getName());

System.out.println(file.createNewFile());
System.out.println(file1.createNewFile());
System.out.println(file2.createNewFile());

//创建目录
File file3 = new File("D:\\test\\aa\\bb");
file3.mkdirs();
File parentFile = file3.getParentFile();
file3.delete();
parentFile.delete(); //目录下没有东西才能删除

//遍历目录
File file4 = new File("D:\\");
File[] files = file4.listFiles();
for (File f : files) {
	System.out.println(f);
}
System.out.println("-----------------------------");
String[] strFileStrings = file4.list();
for (String string : strFileStrings) {
	System.out.println(string); //文件名，没有盘符
}

/**
 * 树状结构展示目录
 * @author yuankun
 *
 */
public class FilesTree {
	public static void main(String[] args) {
		File file = new File("F:\\my-github\\my-java");
		
		PrintFiles(file, 0);
	}
	public static void PrintFiles(File file, int level) {
		
		for(int i=0;i<level;i++) {
			System.out.print("-");
		}
		System.out.println(file.getName());
		
		if(file.isDirectory()) {
			File[] files = file.listFiles();
			for (File f : files) {
				PrintFiles(f, level+1);
			}
		}
		
	}
}

枚举

public enum Gender {
	
	男,女;
	
	private String name;

	public void setName(String name){
		this.name = name;
	}

	public String getName(){
		return this.name;
	}
}
public class Person {
	
	private String name;
	private int age;
	private Gender gender;
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}
	public Gender getGender() {
		return gender;
	}
	public void setGender(Gender gender) {
		this.gender = gender;
	}
	public Person(String name, int age, Gender gender) {
		super();
		this.name = name;
		this.age = age;
		this.gender = gender;
	}
	public Person() {
		super();
	}
}
public class EnumMain {
	public static void main(String[] args) {
		
		Gender sex = Gender.女;
		sex.setName("张静");
		System.out.println(sex.getName()); //张静
		
		Person person = new Person("旺财", 12, Gender.女);
		System.out.println(person.getGender());
	}
}
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