java的一些問題

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1. 判斷是否是奇數：

public static boolean isOdd(int i) { return i %2 != 0 ; }

 

2. System.out.println(2.0 - 1.1); 輸出：0.89999999 99999999 （Double型的）

   System.out.println(new BigDecimal("2.00").subtract(new BigDecimal("1.10")));   --輸出 0.90 要加上引號，否則會有小數。

 System.out.println(new BigDecimal("2.01").subtract(new BigDecimal("1.65")));   -- 輸出 0.36

System.out.println(new BigDecimal(2.0).subtract(new BigDecimal(1.1)));  -- 輸出    0.899 99999999 99999111 82158029 98747676 61094665 52734375

System.out.printf("%.2f\n", 2.0-1.115);   --輸出：0.89

 

 

final long MICROS_PER_DAY = 24 * 60 * 60 * 1000 * 1000;
final long MILLIS_PER_DAY = 24 * 60 * 60 * 1000;
System.out.println(24 * 60 * 60 * 1000 * 1000);                              --- 輸出： 500654080  ， 當做了int型，導致越界。
System.out.println(24 * 60 * 60 * 1000 );                                        --- 輸出： 86400000　
System.out.println(MICROS_PER_DAY/MILLIS_PER_DAY);         --- 輸出 ：5

 

對於長整型的計算，要加上L：  System.out.println(24L * 60 * 60 * 1000 * 1000);  --- 輸出： 86400000 000

 

System.out.println("H" + "a"); 輸出： Ha

System.out.println("H" + ‘a‘);     Ha

System.out.println(‘H‘ + ‘a‘);    169

 

char[] numbers = {‘1‘,‘2‘,‘3‘};

System.out.println("a" + numbers);     輸出：a[[email protected]

void java.io.PrintStream.println(String x)

Prints a String and then terminate the line. This method behaves as though it invokes print(String) and then println().

 

System.out.println(numbers);  輸出：123

void java.io.PrintStream.println(char[] x)     重載的方法

Prints an array of characters and then terminate the line. This method behaves as though it invokes print(char[]) and then println().

 

 

\u0022 是雙引號的unicode編碼。

System.out.println("a\u0022 + \u0022b ".length());   相當於： System.out.println("a" + "b ".length());  ， 輸出 a2  。（b後面有一個空格）

 

 

System.out.println(Test.class.getName().replace(".","/"));   輸出： com/Test 

String java.lang.String.replace(CharSequence target, CharSequence replacement)

Replaces each substring of this string that matches the literal target sequence with the specified literal replacement sequence. The replacement proceeds from the beginning of the string to the end, for example, replacing "aa" with "b" in the string "aaa" will result in "ba" rather than "ab".

 

System.out.println(Test.class.getName().replaceAll(".","/"));   輸出：////////              

System.out.println(Test.class.getName().replaceAll("\\.","/"));   輸出：com/Test

String java.lang.String.replaceAll(String regex, String replacement)

Replaces each substring of this string that matches the given regular expression with the given replacement.

An invocation of this method of the form str.replaceAll(regex, repl) yields exactly the same result as the expression

java.util.regex.Pattern. compile( regex). matcher( str). replaceAll( repl)

 

StringBuffer word = null;
word = new StringBuffer(‘P‘);

‘P’ 當做了int數。

java.lang.StringBuffer.StringBuffer(int capacity)

Constructs a string buffer with no characters in it and the specified initial capacity.

Parameters:

capacity the initial capacity.

System.out.println(word);                           輸出換行
System.out.println(word.append("a"));      輸出 a

 

 

int j = 0;
for(int i = 0; i < 100; i++){
j = j++;
System.out.println(j);
}

輸出100 行 0 （j的值總是 0）：

0

0

……

 

 

final int END=Integer.MAX_VALUE;                    //2147483647
final int START = END - 100;
int count = 0;
for(int i = START; i <= END; i++){
　　count++;
　　System.out.println(i);     -- 死迴圈。  當 i 達到 2147483647 ，再增加會變成負數。
}

 

java的一些問題