[Personal memo] Frequently used Java grammar updated from time to time

Self-introduction

This is my first year as a member of society who has just studied Java. It summarizes Java grammar like a memo. As for the grammar, it feels like "I'm having trouble finding it" and "I'm having trouble remembering it", so it's a completely personal memo.

• We plan to use it frequently in business and update it as soon as we find a useful grammar.

We are looking for advice on Java.

String split

Example

String str = new String("Minami Hoshino,Asuka Saito,Miona Hori");
String[] oshi = str.split(",");

//output
for (int i=0; i < oshi.length; i++) {
  System.out.println(oshi[i]);
}

output

Minami Hoshino
Asuka Saito
Miona Hori

String comparison

Example

String str1 = "Minami Hoshino";
String str2 = "Minami Hoshino";
String str3 = "Ranze Terada";

//Comparison
//str1 and str2
if (str1.equals(str2)) {
  System.out.println("equal");
} else {
  System.out.println("Not equal");
}

//Comparison
//str1 and str3
if (str1.equals(str3)) {
  System.out.println("equal");
} else {
  System.out.println("Not equal");
}

output

equal
Not equal

Regular expressions

date Determine if the date stored in the string ymd is in the form YYYY / MM / DD.

Example

Pattern p = Pattern.compile("^[0-9]{4}/[0-9]{2}/[0-9]{2}$");
Matcher m = p.matcher(ymd);
m.find(); //True if matched,False if not matched

time Judge whether the time stored in the character string time is in the form of HH: mm.

Example

Pattern p = Pattern.compile("^[0-9]{2}:[0-9]{2}$");
Matcher m = p.matcher(time);
m.find(); //True if matched,False if not matched

** Other regular expressions **

Example

// URL
Pattern p = Pattern.compile("^https?://([\w-]+\.)+[\w-]+(/[\w-./?%&=]*)?$");

//Domain name
Pattern p = Pattern.compile("^[a-zA-Z0-9][a-zA-Z0-9-]{1,61}[a-zA-Z0-9]\.[a-zA-Z-]{2,}$");

//Landline number
Pattern p = Pattern.compile("^0\d(-\d{4}|\d-\d{3}|\d\d-\d\d|\d{3}-\d)-\d{4}$");

//cell phone number
Pattern p = Pattern.compile("^0[789]0-\d{4}-\d{4}$");

//Free dial
Pattern p = Pattern.compile("^(0120|0800)-\d{3}-\d{3}$");

//Postal code
Pattern p = Pattern.compile("^\d{3}-\d{4}$");

Date and time format specification

Example

SimpleDateFormat df = new SimpleDateFormat("yyyy/MM/dd (E)", Locale.JAPANESE);
SimpleDateFormat de = new SimpleDateFormat("yyyy/MM/dd");
SimpleDateFormat dd = new SimpleDateFormat("HH:mm");
SimpleDateFormat dc = new SimpleDateFormat("HH:mm:ss");

Get the current date and time

Get the current date and time and store it in ** Today **

Example

SimpleDateFormat de = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");

//Get the current date
Date today = new Date();
today.toString();
String Today = de.format(today);

Date comparison (equal (), after (), before ())

Example

public void dateTodate() throws ParseException {
  SimpleDateFormat sdf = new SimpleDateFormat("yyyy/MM/dd");
  Date date1 = sdf.parse("2020/04/01");
  Date date2 = sdf.parse("2020/04/30");
  Date date3 = sdf.parse("2020/04/01");

  if (date1.equals(date3)) {
    System.out.println("date1 and date3 are the same day");
  }

  if (date1.before(date2)) {
    System.out.println("date1 is past compared to date2");
  }

  if (date2.after(date1)) {
    System.out.println("date2 is the future compared to date1");
  }
}

output

date1 and date3 are the same day
date1 is past compared to date2
date2 is the future compared to date1

Leap year judgment

Determine if the variable ymd_YYY (year) is a leap year.

Example

//Leap year judgment
if (Integer.parseInt(ymd_YYYY) % 4 == 0) {
	if ((Integer.parseInt(ymd_YYYY) % 100) == 0) {
		if ((Integer.parseInt(ymd_YYYY) % 400) == 0) {
			//leap year
		}
	} else {
		//leap year
	}
}

Copy of 2D array

Example

String[][] oshi = { {"Minami Hoshino", "Asuka Saito", "Miona Hori"}, {"South", "Asuka", "Miona"}, {"Chiba", "Tokyo", "Gifu Prefecture"}};
String[][] oshi_copy = new String[oshi.length][];

for (int i=0; i < oshi.length; i++) {
	oshi_copy[i] = new String[oshi[i].length];
	
	for (int j=0; j < oshi[i].length; j++) {
		oshi_copy[i][j] = oshi[i][j];
	}
}

//output
for (int i=0; i < oshi_copy.length; i++) {
	for (int j=0; j < oshi_copy[i].length; j++) {
		System.out.println(oshi_copy[i][j]);
    }
}

output

Minami Hoshino
Asuka Saito
Miona Hori
South
Asuka
Miona
Chiba
Tokyo
Gifu Prefecture

for statement (for, extension for, foreach)

Example

// for
//Standard
for (int i=0; i < 10; i++) {
  //processing
}

//Extension for
//Used when processing all elements of an array or list in order
for (String str : Array) {
  //processing
}

// forEach
//Can be written more concisely than the top two(I don't fully understand...)
str.forEach( i -> //processing);

in conclusion

If you have any Java grammar that you often use, we are looking for it in the comment section.