java的一些问题

Posted 2020-09-27 zhoudingzhao

1. 判断是否是奇数：

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

 

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

   System.out.println(new BigDecimal("2.00").subtract(new BigDecimal("1.10")));   --输出 0.90 要加上引号，否则会有小数。

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

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

System.out.printf("%.2f\n", 2.0-1.115);   --输出：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);                              --- 输出： 500654080  ， 当做了int型，导致越界。
System.out.println(24 * 60 * 60 * 1000 );                                        --- 输出： 86400000　
System.out.println(MICROS_PER_DAY/MILLIS_PER_DAY);         --- 输出 ：5

 

对于长整型的计算，要加上L：  System.out.println(24L * 60 * 60 * 1000 * 1000);  --- 输出： 86400000 000

 

System.out.println("H" + "a"); 输出： Ha

System.out.println("H" + ‘a‘);     Ha

System.out.println(‘H‘ + ‘a‘);    169

 

char[] numbers = {‘1‘,‘2‘,‘3‘};

System.out.println("a" + numbers);     输出：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);  输出：123

void java.io.PrintStream.println(char[] x)     重载的方法

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

 

 

\u0022 是双引号的unicode编码。

System.out.println("a\u0022 + \u0022b ".length());   相当于： System.out.println("a" + "b ".length());  ， 输出 a2  。（b后面有一个空格）

 

 

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

System.out.println(Test.class.getName().replaceAll("\\.","/"));   输出：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’ 当做了int数。

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);                           输出换行
System.out.println(word.append("a"));      输出 a

 

 

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

输出100 行 0 （j的值总是 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);     -- 死循环。  当 i 达到 2147483647 ，再增加会变成负数。
}