List unsortList = new ArrayList();
unsortList.add("CCC");
unsortList.add("111");
unsortList.add("AAA");
Collections.sort(unsortList);
列表搜索:
int index = arrayList.indexOf(obj);
finding an object by value in a hashmap:
hashmap.containsValue(obj);
finding an object by key in a hashmap:
hashmap.containsKey(obj);
二分搜索:
int[] nums = new int[]{7,5,1,3,6,8,9,2};
Arrays.sort(nums);
int index = Arrays.binarySearch(nums,6);
System.out.println("6 is at index: "+ index);
arrayList 转化为 array:
Object[] objects = arrayList.toArray();
将 hashmap 转化为 array:
Object[] objects = hashmap.entrySet().toArray();
时间与日期类型打印时间与日期:
Date todaysDate = new Date(); //todays date
SimpleDateFormat formatter = new SimpleDateFormat("EEE, dd MMM yyyy HH:mm:ss"); //date format
String formattedDate = formatter.format(todaysDate);
System.out.println(formattedDate);
将日期转化为日历:
Date mDate = new Date();
Calendar mCal = Calendar.getInstance();
mCal.setTime(mDate);
将 calendar 转化为 date:
Calendar mCal = Calendar.getInstance();
Date mDate = mDate.getTime();
字符串解析为日期格式:
public void StringtoDate(String x) throws ParseException{
String date = "March 20, 1992 or 3:30:32pm";
DateFormat df = DateFormat.getDateInstance();
Date newDate = df.parse(date);
}
date arithmetic using date objects:
Date date = new Date();
long time = date.getTime();
time += 5*24*60*60*1000; //may give a numeric overflow error on IntelliJ IDEA
Date futureDate = new Date(time);
System.out.println(futureDate);
long startTime = System.currentTimeMillis();
//times flies by..
long finishTime = System.currentTimeMillis();
long timeElapsed = startTime-finishTime;
System.out.println(timeElapsed);
正则表达式使用 REGEX 寻找匹配字符串:
String pattern = "[TJ]im";
Pattern regPat = Pattern.compile(pattern,Pattern.CASE_INSENSITIVE);
String text = "This is Jim and that"s Tim";
Matcher matcher = regPat.matcher(text);
if (matcher.find()){
String matchedText = matcher.group();
System.out.println(matchedText);
}
替换匹配字符串:
String pattern = "[TJ]im";
Pattern regPat = Pattern.compile(pattern,Pattern.CASE_INSENSITIVE);
String text = "This is jim and that"s Tim";
Matcher matcher = regPat.matcher(text);
String text2 = matcher.replaceAll("Tom");
System.out.println(text2);
使用 StringBuffer 替换匹配字符串:
Pattern p = Pattern.compile("My");
Matcher m = p.matcher("My dad and My mom");
StringBuffer sb = new StringBuffer();
boolean found = m.find();
while(found){
m.appendReplacement(sb,"Our");
found = m.find();
}
m.appendTail(sb);
System.out.println(sb);
String pattern = "d$"; //any single digit
String text = "line one
line two
line three
";
Pattern regPat = Pattern.compile(pattern, Pattern.MULTILINE);
Matcher matcher = regPat.matcher(text);
while (matcher.find()){
System.out.println(matcher.group());
}
regex:
beginning of a string: ^
end of a string: $
0 or 1 times: ?
0 or more times: (*) //without brackets
1 or more times: +
alternative characters: [...]
alternative patterns: |
any character: .
a digit: d
a non-digit: D
whitespace: s
non-whitespace: S
word character: w
non word character: W
数字与数学操作处理built-in types:
byte: 8bits, Byte
short: 16bits, Short
long: 64bits, Long
float: 32bits, Float
判断字符串是否为有效数字:
String str = "dsfdfsd54353%%%";
try{
int result = Integer.parseInt(str);
}
catch (NumberFormatException e){
System.out.println("not valid");
}
比较 Double:
Double a = 4.5;
Double b= 4.5;
boolean result = a.equals(b);
if (result) System.out.println("equal");
rounding:
double doubleVal = 43.234234200000000234040324;
float floatVal = 2.98f;
long longResult = Math.round(doubleVal);
int intResult = Math.round(floatVal);
System.out.println(longResult + " and " + intResult); // 43 and 3
