Java基础.J.1

#### 两个整数相除得到小数并限制小数位数

    int a = 1, b = 3;    System.out.println("---->result: " + (a / b));    //获取小数    System.out.println("---->result: " + ((float) a / b));    System.out.println("---->result: " + (a / (float) b));    System.out.println("---->result: " + ((float) a / (float) b));    //限制小数位数    float result = (float) a / b;    DecimalFormat df = new DecimalFormat("0.00");//表示两位小数    System.out.println("---->result: " + df.format(result));// format() 返回的是String    DecimalFormat df1 = new DecimalFormat("#.000");    System.out.println("---->result: " + df1.format(result));

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#### Math 的 floor、round、ceil 函数

    

/**     *  Math.floor(num) : 返回小于 num 的最大整数     *  Math.ceil(num) : 返回大于 num 的最小整数     *  Math.round(num) ：返回 num “四舍五入” 后的整数     *  经典题：Math.round(11.5) 的结果为 12 ，Math.round(-11.5) 的结果为 -11     */    float[] numList = {1.40f, 1.5f, 1.6f, -1.40f, -1.5f, -1.6f};    for (float num : numList) {        System.out.println("---->result: " + new DecimalFormat("0").format(Math.floor(num)));        System.out.println("---->result: " + Math.ceil(num));        System.out.println("---->result: " + Math.round(num));    }

#### 获取星期名字数组和月份名字数组

  

  Locale locale = Locale.getDefault();//获取当前默认的区域    System.out.println(locale);    //DateFormatSymbols dateFormatSymbols = new DateFormatSymbols(Locale.US);    DateFormatSymbols dateFormatSymbols = new DateFormatSymbols();    //返回星期名字数组的长度为 8 ，其中 0 的时候为空    String[] weekShortName = dateFormatSymbols.getShortWeekdays();    String[] weekName = dateFormatSymbols.getWeekdays();    for(int i = 1;i<weekShortName.length;i++){        //System.out.println(weekShortName[i]);        System.out.println(weekShortName[i].substring(2,3));        System.out.println(weekName[i]);    }    //返回月份名字数组的长度为 13 ，其中 12 的时候为空    String[] monthShortName = dateFormatSymbols.getShortMonths();    String[] monthName = dateFormatSymbols.getMonths();    for(int i = 0;i<monthName.length;i++){        System.out.println(monthShortName[i]);        System.out.println(monthName[i]);    }

#### Java 引用

&emsp;&emsp; Java 引用分强引用、软引用、弱引用、虚引用。强引用 (SoftReference) 用来保存对象实例，除非 JVM 即将 OutOfMemory ，否则是不会被 GC 回收，这个特性使得它特别适合设计对象Cache。弱引用 (WeekReference) 用来保存的对象实例可以被GC回收掉。这个类通常用于在某处保存对象引用，而又不干扰该对象被GC回收，通常用于Debug、内存监视工具等程序中。

   

 public static void main(String[] args){        A a = new A();        a.str = "Hello,reference";        //弱引用        WeakReference<A> weakReference = new WeakReference<A>(a);        //强引用        SoftReference<A> softReference = new SoftReference<A>(a);        a = null;        int i = 0;    //        while(weakReference.get()!=null){        while(softReference.get()!=null){    //            System.out.println(String.format("Get str from object of weakReference:%s,count:%d",    //                    weakReference.get().str,++i));            System.out.println(String.format("--str-->: %s ,count: %d ",                    softReference.get().str,++i));            if(i%10==0){                System.gc();                System.out.println("System.gc() was invoked!");            }            try{                Thread.sleep(500);            }catch (InterruptedException e){            }        }        System.out.println("object A was cleared by JVM!");    }    static class A{        public String str;    }

#### 日历获取月份天数

    //获取上个月的第一天
    Calendar calendar = Calendar.getInstance();
    calendar.add(Calendar.MONTH, -1);
    calendar.set(Calendar.DAY_OF_MONTH, 1);
    System.out.println(calendar.get(Calendar.DAY_OF_MONTH));
    //获取上个月的最后一天
    Calendar calendar1 = Calendar.getInstance();
    calendar1.set(Calendar.DAY_OF_MONTH, 1);
    calendar1.add(Calendar.DATE, -1);
    System.out.println(calendar1.get(Calendar.DAY_OF_MONTH));
    //获取这个月的最后一天
    Calendar calendar2 = Calendar.getInstance();
    //        calendar2.add(Calendar.MONTH, 1);
    //        calendar2.set(Calendar.DAY_OF_MONTH,1);
    //        calendar2.add(Calendar.DATE,-1);
    calendar2.set(Calendar.DATE, calendar.getActualMaximum(Calendar.DATE));
    System.out.println(calendar2.get(Calendar.DAY_OF_MONTH));

#### 获取日历每月显示的数组

    Calendar calendar = Calendar.getInstance();
    calendar.add(Calendar.MONTH, 1);//设置月份

    calendar.set(Calendar.DAY_OF_MONTH, 1);
    int dayOfWeek = calendar.get(Calendar.DAY_OF_WEEK) < 1 ? 7 : calendar.get(Calendar.DAY_OF_WEEK);

    System.out.println(dayOfWeek);

    calendar.set(Calendar.DATE, calendar.getActualMaximum(Calendar.DATE));
    int lastDayOfMonth = calendar.get(Calendar.DAY_OF_MONTH);

    calendar.set(Calendar.DAY_OF_MONTH, 1);
    calendar.add(Calendar.DATE, -1);
    int lastDayOfLastMonth = calendar.get(Calendar.DAY_OF_MONTH);

    for (int raw = 0; raw < 6; raw++) {
        for (int column = 0; column < 7; column++) {
            int day = (raw * 7 + column + 1) - dayOfWeek + 1;
            if (day < 1) {
                day = lastDayOfLastMonth + day;
            } else if (day > lastDayOfMonth) {
                day = day - lastDayOfMonth;
            }
            System.out.printf("%2d" + " ", day);
        }
        System.out.println("");
    }