原文：Java8 日期/時間（Date Time）API指南-趣讀
擷取日期：2018/07/27

st = st.replaceAll("\\s+","");

出處：Removing whitespace from strings in Java - Stack Overflow

Java 的 String 有個 API：「java.lang.String.contains(CharSequence s)」
可以用來比對，是否包含傳入的字串。

但是我想不分大小寫比對怎麼辦呢？

參考：

List<String> list = new ArrayList<String>(Arrays.asList("", "Hi", null, "How"));
System.out.println(list);
list.removeAll(Arrays.asList("", null));
System.out.println(list);

來源：arrays - In Java, remove empty elements from a list of Strings - Stack Overflow

例如：

class Car {
}

class Audi extends Car {
}

class Seat extends Car {
}

class Mercedes extends Car {
}

Car car = new Audi();
// car is instance of Audi, Seat or Mercedes ?

答：「使用 "instanceof" 關鍵字。」

public class Test {

    static {
        System.out.println("Static");
    }

    {
        System.out.println("Non-static block");
    }

    public static void main(String[] args) {
        Test t = new Test();
        Test t2 = new Test();
    }
}

輸出結果：

java.util.Formatter、System.out.printf

一直看不懂「%3$d」中的「$」前字號(dollar sign)是什麼意思...
終於看懂了

Java 8 u171

    String[] timeZoneIds = java.util.TimeZone.getAvailableIDs() ;
    for ( String aTimeZoneId : timeZoneIds ) {
        System.out.println( aTimeZoneId );
    }
    System.out.println( "Total: " + timeZoneIds.length );

java.util.Calendar、java.util.Date、java.sql.timestamp

取得現在時間，"toString()"結果：

• java.util.Calendar

      java.util.Calendar cal = java.util.Calendar.getInstance();
      System.out.println( cal );  // "java.util.GregorianCalendar[time=1525760873174,areFieldsSet=true,areAllFieldsSet=true,lenient=true,zone=sun.util.calendar.ZoneInfo[id="GMT+08:00",offset=28800000,dstSavings=0,useDaylight=false,transitions=0,lastRule=null],firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=1,YEAR=2018,MONTH=4,WEEK_OF_YEAR=19,WEEK_OF_MONTH=2,DAY_OF_MONTH=8,DAY_OF_YEAR=128,DAY_OF_WEEK=3,DAY_OF_WEEK_IN_MONTH=2,AM_PM=1,HOUR=2,HOUR_OF_DAY=14,MINUTE=27,SECOND=53,MILLISECOND=174,ZONE_OFFSET=28800000,DST_OFFSET=0]"
  

• java.util.Date

      java.util.Date utilDate = new java.util.Date();
      System.out.println( utilDate );  // "Tue May 08 14:24:13 GMT+08:00 2018"
  

• java.sql.timestamp

      Timestamp timestamp = new Timestamp( System.currentTimeMillis() );
      System.out.println( sqlTstp );  // "2018-05-08 14:24:13.338"
  

參考：

方法1：

String excepStr = org.apache.commons.lang.exception.ExceptionUtils.getStackTrace( throwable ) ;

方法2：

StringWriter errors = new StringWriter();
ex.printStackTrace(new PrintWriter(errors));
return errors.toString();

StringWriter PrintWriter Throwable

/**
 * @author Robert Chen
 */
public class CharUtil
{
    /**
     * Ref: http://www.micmiu.com/lang/java/java-check-chinese/
     * @param ch
     * @return
     */
    public static boolean IsChinese( char ch ) {
        Character.UnicodeBlock ub = Character.UnicodeBlock.of( ch );
        if ( ub == Character.UnicodeBlock.CJK_UNIFIED_IDEOGRAPHS
            || ub == Character.UnicodeBlock.CJK_COMPATIBILITY_IDEOGRAPHS
            || ub == Character.UnicodeBlock.CJK_UNIFIED_IDEOGRAPHS_EXTENSION_A
            || ub == Character.UnicodeBlock.CJK_UNIFIED_IDEOGRAPHS_EXTENSION_B
            || ub == Character.UnicodeBlock.CJK_SYMBOLS_AND_PUNCTUATION
            || ub == Character.UnicodeBlock.HALFWIDTH_AND_FULLWIDTH_FORMS
            || ub == Character.UnicodeBlock.GENERAL_PUNCTUATION) {
            return true;
        }
        return false;
    }
    
    /**
     * Printable characters
     * @param ch
     * @return
     */
    public static boolean IsPrintableAsciiChar( char ch ) {
        if ( 32 <= ch && ch <= 126 )
            return true ;
        return false ;
    }
    
    
    public static void main( String[] args ) {
        // Ref: http://tw.gitbook.net/java/lang/character.unicodeblock_of.html
        
        // 32~126: Printable characters
        for ( char ch = 0; ch < 128 ; ++ch )
            System.out.println( Character.UnicodeBlock.of( ch ) );  // All is "BASIC_LATIN".
        System.out.println();
        
        
        
        // Ref: https://www.yiibai.com/java/lang/character.unicodeblock_of_codepoint.html#article-start
        System.out.println( Character.UnicodeBlock.of(950) );  // "GREEK"
    }
}

使用：

簡單記錄：

private static final String FILENAME = "E:\\test\\filename.txt";

File file = new File(FILENAME);

// if file doesnt exists, then create it
if (!file.exists()) {
  file.createNewFile();
}

// true = append file
FileWriter fw = new FileWriter(file.getAbsoluteFile(), true);
BufferedWriter bw = new BufferedWriter(fw);
bw.write(data);

bw.close();
fw.close();

來源：Java – How to append content to file

// 在這裡做個筆記暫存

一個命令輕鬆切換Java版本

Window內開啟指定JDK環境懶人法