java基础之十四->常用类

一、基本数据类型的包装类

包装类是将基本数据类型封装成一个类，包含属性和方法
使用：
在使用过程中，会涉及到自动装箱和自动拆箱
装箱：将基本数据类型转换成包装类型
拆箱：将包住类型转换成基本数据类型

示例

package com.msbline.basic.common;
public class IntegerDemo {
    public static void main(String[] args) {
        int a = 10;
        Integer i = new Integer(10);
        //通过方法进行类型的转换
 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;
        //这两个结果的不一致，是因为在自动拆箱的时候,
 //如果value不在规定范围内（-128到127）的时候，会 new Interger, //导致比较的时候两个地址不一致
 //Interger 中自动拆箱的方法：
 //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、字符串相关类

1、不可变字符序列：String

注意：
1、常量池在jdk1.7之后放在了对空间中
2、字符串的本质
--字符串的本质是字符数组或者叫字符序列
--String类使用final修饰，不可以被继承
--使用equals方法比较的是字符数组的每个位置的值

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 线程安全 效率低

StringBuilder 线程不安全 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使用大致一样
 StringBuilder stringBuilder = new StringBuilder();
    }
}

小结

三、时间处理相关类

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);
        //过时的方法
        System.out.println(date.getYear());
        //返回当前日期的毫秒值
        System.out.println(date.getTime());
        //yyyy-MM-dd HH:mm:ss
        DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        //将date类按照规范转换为字符串格式
        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);
        //设置指定时间的日历类
        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();
    }
}