兩個整數相除得到小數并限制小數位數
int a = 1, b = 3;
System.out.println("---->result: " + (a / b));
//獲取小數
System.out.println("---->result: " + ((float) a / b));
System.out.println("---->result: " + (a / (float) b));
System.out.println("---->result: " + ((float) a / (float) b));
//限制小數位數
float result = (float) a / b;
DecimalFormat df = new DecimalFormat("0.00");//表示兩位小數
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));
Math 的 floor、round、ceil 函數
/**
* Math.floor(num) : 返回小于 num 的最大整數
* Math.ceil(num) : 返回大于 num 的最小整數
* Math.round(num) ：返回 num “四舍五入” 后的整數
* 經典題：Math.round(11.5) 的結果為 12 ，Math.round(-11.5) 的結果為 -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));
}
獲取星期名字數組和月份名字數組
Locale locale = Locale.getDefault();//獲取當前默認的區域
System.out.println(locale);
//DateFormatSymbols dateFormatSymbols = new DateFormatSymbols(Locale.US);
DateFormatSymbols dateFormatSymbols = new DateFormatSymbols();
//返回星期名字數組的長度為 8 ，其中 0 的時候為空
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]);
}
//返回月份名字數組的長度為 13 ，其中 12 的時候為空
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
?? Java 引用分強引用、軟引用、弱引用、虛引用。強引用 (SoftReference) 用來保存對象實例，除非 JVM 即將 OutOfMemory ，否則是不會被 GC 回收，這個特性使得它特別適合設計對象Cache。弱引用 (WeekReference) 用來保存的對象實例可以被GC回收掉。這個類通常用于在某處保存對象引用，而又不干擾該對象被GC回收，通常用于Debug、內存監視工具等程序中。
public static void main(String[] args){
A a = new A();
a.str = "Hello,reference";
//弱引用
WeakReference<A> weakReference = new WeakReference<A>(a);
//強引用
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;
}
日歷獲取月份天數
//獲取上個月的第一天
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));
//獲取上個月的最后一天
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));
//獲取這個月的最后一天
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));
獲取日歷每月顯示的數組
Calendar calendar = Calendar.getInstance();
calendar.add(Calendar.MONTH, 1);//設置月份
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("");
}
截取函數 substring() 和 split()
/**
* substring(int start);//截取索引start開始后面的字符串
* substring(int start, int end);//截取索引start開始end結束中間的字符串
*/
String num = "123456789";
System.out.println(num.substring(3));//截取3（包括3）開始后面的字符串
System.out.println(num.substring(2,5));//截取2（包括2）開始5（不包括5）結束中間的字符串
System.out.println(num.substring(1,1));//為空
/**
* split(String temp);//截掉temp字符串，返回截取剩下的字符串數組
* split(String temp,int limit);//限制limit個字符串
*/
String str = "123,4,56,789";
for (String temp : str.split(",")) {
System.out.println(temp);
}
for (String temp : str.split(",",2)) {
System.out.println(temp);
}