共计 4091 个字符，预计需要花费 11 分钟才能阅读完成。
一、根本数据类型的包装类
包装类是将根本数据类型封装成一个类，蕴含属性和办法
应用：
在应用过程中，会波及到主动装箱和主动拆箱
装箱：将根本数据类型转换成包装类型
拆箱：将包住类型转换成根本数据类型
示例
package com.msbline.basic.common;
public class IntegerDemo {public static void main(String[] args) {
int a = 10;
Integer i = new Integer(10);
// 通过办法进行类型的转换
Integer i2 = Integer.valueOf(a);
int i3 = i2.intValue();
System.out.println(a == i);
Integer b1 = 100;
Integer b2 = 100;
Integer b3 = 200;
Integer b4 = 200;
// 这两个后果的不统一，是因为在主动拆箱的时候,
// 如果 value 不在规定范畴内（-128 到 127）的时候，会 new Interger, // 导致比拟的时候两个地址不统一
//Interger 中主动拆箱的办法：//public static Integer valueOf(int i) {// if (i >= IntegerCache.low && i <= IntegerCache.high) // return IntegerCache.cache[i + (-IntegerCache.low)]; // return new Integer(i); // } System.out.println(b1 == b2);//true
System.out.println(b3 == b4);//false
}
}二、字符串相干类
1、不可变字符序列：String
留神：
1、常量池在 jdk1.7 之后放在了对空间中
2、字符串的实质
– 字符串的实质是字符数组或者叫字符序列
–String 类应用 final 润饰，不能够被继承
– 应用 equals 办法比拟的是字符数组的每个地位的值
示例
package com.msbline.basic.common;
public class StringDemo {public static void main(String[] args) {
String str = "abc";
String str1 = new String("abc");
System.out.println(str == str1);
System.out.println(str.equals(str1));
str1 = str1.intern();
System.out.println(str == str1);
System.out.println(str1.charAt(0));
System.out.println(str1.concat("def"));
System.out.println("-------------");
String a = "abc";
String b = "def";
String c = "abcdef";
String d = a+b;
// String d = (a+b).intern();
String e = "abc"+"def";
System.out.println(c == d);//false
System.out.println(c == e);//true
System.out.println("-------------");
String f = "a"+"b"+"c";// 一个对象
// 如下则是 4 个对象
String a1 = "a";
String a2 = "b";
String a3 = "c";
String f1 = a1+a2+a3;
}
}2、可变字符序列：StringBuffer、StringBuilder
StringBuffer 线程平安 效率低
StringBuilder 线程不平安 效率高
示例
package com.msbline.basic.common;
public class StringBufferDemo {public static void main(String[] args) {StringBuffer stringBuffer = new StringBuffer();
stringBuffer.append(1)
.append(1.234)
.append("abcaaaaaaaaaaaaa")
.append(true);
System.out.println(stringBuffer);
System.out.println(stringBuffer.charAt(1));
System.out.println(stringBuffer.length());
System.out.println(stringBuffer.capacity());
System.out.println("-----------------------------");
//StringBuilder 与 StringBuffer 应用大抵一样
StringBuilder stringBuilder = new StringBuilder();}
}小结
三、工夫解决相干类
1、Date、DateFormat、SimpleDateFormat、Calender
示例
package com.msbline.basic.common;
import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
public class DateDemo {public static void main(String[] args) throws ParseException {Date date = new Date();
System.out.println(date);
// 过期的办法
System.out.println(date.getYear());
// 返回以后日期的毫秒值
System.out.println(date.getTime());
//yyyy-MM-dd HH:mm:ss
DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
// 将 date 类依照标准转换为字符串格局
String str = dateFormat.format(date);
System.out.println(str);
// 将字符串转换成对应的日期类
Date date1 = dateFormat.parse("2010-10-10 20:20:20");
System.out.println(date1);
System.out.println("-----------------------------------------");
Calendar calendar = Calendar.getInstance();
System.out.println(calendar);
// 设置指定工夫的日历类
calendar.setTime(date1);
System.out.println(calendar);
System.out.println(calendar.get(Calendar.YEAR));
System.out.println(calendar.get(Calendar.MONTH));
System.out.println(calendar.get(Calendar.DAY_OF_MONTH));
System.out.println(calendar.get(Calendar.HOUR_OF_DAY));
System.out.println(calendar.get(Calendar.MINUTE));
System.out.println(calendar.get(Calendar.SECOND));
}
}四、Math 类
示例
package com.msbline.basic.common;
public class MathDemo {public static void main(String[] args) {System.out.println(Math.abs(-1));
System.out.println(Math.sqrt(2));
// 向上取整
System.out.println(Math.ceil(3.14));
// 向下取整
System.out.println(Math.floor(3.14));
// 2 的 3 次方
System.out.println(Math.pow(2,3));
}
}五、枚举类 jdk1.5
示例
package com.msbline.basic.common;
public enum Event {LAUNCH("launch"),PAGEVIEW("pageview"),EVENT("event");
private String name;
Event(String name){this.name = name;}
public void show(){System.out.println(this.name);
Event[] ee = values();
for(int i = 0; i < ee.length; i++){System.out.println(ee[i].name);
}
}
}package com.msbline.basic.common;
public enum Gender {男, 女}package com.msbline.basic.common;
public class Test {public static void main(String[] args) {
Gender gender = Gender. 女;
Gender gender1 = Gender. 男;
Event event = Event.LAUNCH;
System.out.println(event.name());
event.show();}
}
正文完
