Java Basic Learning final keyword, traversal collection, use of Date class objects, use of the Math class object, runtime class object usage, Time object date (two date subtraction)

Source: Internet
Author: User
Tags date1 dateformat

1.final keywords
As with the const keyword in. NET, it is the modifier of a constant, but final can also modify the class, method.
Specification: Constant All letters are capitalized, the middle of multiple words with "_" connection.

2. Iterating through the collection
arraylist<integer> list = new arraylist<integer> ();
List.add (1);
List.add (3);
List.add (5);
List.add (7);
Traverse List Method 1, using the normal for loop:
for (int i = 0; i < list.size (); i++) {
System.out.println (List.get (i));
List.remove (i);//using this traversal to delete elements can be problematic. List.size () will change after deletion
}
Traverse List Method 2, using the enhanced for loop (the generic definition type should be used!). ):
for (int temp:list) {
SYSTEM.OUT.PRINTLN (temp);
}
To Traverse list Method 3, use the iterator iterator:
for (Iterator iter = List.iterator (); Iter.hasnext ();) {
System.out.println (Iter.next ());
}
How to traverse the Delete collection using the following methods
Iterator Iter=list.iterator ();
while (Iter.hasnext ()) {
Object Obj=iter.next ();
Iter.remove ();//If you want to traverse the deletion of elements in a collection, this is the recommended way!
System.out.println (obj);
}

3. Use of Date class objects

Get current time
Calendar C = calendar.getinstance ();
C.set (2014, 2, 1);//change of time, date specified
int year = C.get (calendar.year);
int month = C.get (calendar.month) +1;
int date = C.get (calendar.date);
int hour = C.get (Calendar.hour_of_day);
int minute = C.get (Calendar.minute);
int second = C.get (Calendar.second);
Int week = C.get (Calendar.day_of_week);
String[] weeks = {"", "Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};
SYSTEM.OUT.PRINTLN (Year + "/" + month + "/" + Date + "" +hour + ":" +minute + ":" + second+ "" +weeks[week]);

Specify the format of the current time
SimpleDateFormat df = new SimpleDateFormat ("Yyyy-mm-dd HH:mm:ss");//Set Date format
System.out.println (Df.format (new Date ()));//new Date () to get the current system time

Use of 4.Math class objects
/*
* Math: Provides methods for manipulating mathematical operations, all static.
* Common methods:
* Ceil (): Returns the smallest integer greater than the parameter.
* Floor (): Returns the largest integer less than the parameter.
* ROUND (): Returns the rounded integer.
* POW (A, B): A is the second party.
*/
for (int i = 0; i < i++) {
//
int d = (int) (Math.random () * 10 + 1);//Generate 1-10 random number
//
int d= (int) Math.ceil (12.23);
// //
Double d = Math.ceil (12.31);
// //
Double D=math.floor (12.36);
// //
Long D=math.round (63.50);
Double D=math.pow (2, 3);

System.out.println (d);

}

Use of 5.Runtime class objects
/*
* Runtime: There is no constructor summary, stating that the class is not available to create objects.
* Also found that there are non-static methods. Indicates that the class should provide a static method to return the class object.
* And only one, indicating that the runtime class uses a singleton design pattern.
*
*/
Runtime r = Runtime.getruntime ();
//
Execute: Execute. Xxx.exe
Process p = r.exec ("notepad.exe");
Thread.Sleep (5000);
P.destroy ();

6. Time Object Date (two date subtraction)
String str_date1 = "2012-3-17";
String str_date2 = "2012-4-18";
DateFormat DateFormat = Dateformat.getdateinstance ();
DateFormat = new SimpleDateFormat ("Yyyy-mm-dd");
Date date1 = Dateformat.parse (str_date1);
Date date2 = Dateformat.parse (STR_DATE2);
Long time1 = Date1.gettime ();
Long time2 = Date2.gettime ();
Long time = Math.Abs (time1-time2);//The number of milliseconds to get two time difference
int day = (int) (TIME/1000/60/60/24);
System.out.println (day);//32 Days Difference

Java Basic Learning final keyword, traversal collection, use of Date class objects, use of the Math class object, runtime class object usage, Time object date (two date subtraction)

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