物件導向程式設計

 

 

      2.10 字串轉為數值

 

 

授課教師：陳慶瀚

WWW : http://www.miat.ee.isu.edu.tw/java

E-mail : pierre@isu.edu.tw  　

 
When processing user input it is often necessary to convert a String that the user enters into an int. The syntax is straightforward. It requires using the static Integer.valueOf(String s) and intValue() methods from the java.lang.Integer class. To convert the String "22" into the int 22 you would write

　 

int i = Integer.valueOf("22").intValue();
Doubles, floats and longs are converted similarly. To convert a String like "22" into the long value 22 you would write

long l = Long.valueOf("22").longValue();

To convert "22.5" into a float or a double you would write:

　 

double x = Double.valueOf("22.5").doubleValue();
float y = Float.valueOf("22.5").floatValue();
The various valueOf() methods are relatively intelligent and can handle plus and minus signs, exponents, and most other common number formats. However if you pass one something completely non-numeric like "pretty in pink," it will throw a NumberFormatException. You haven't learned how to handle exceptions yet, so try to avoid passing theses methods non-numeric data.

You can now rewrite the E = mc2 program to accept the mass in kilograms as user input from the command line. Many of the exercises will be similar. 

class mc2 
{
  public static void main (String args[]) 
  {
    double mass;
    double c = 2.998E8;  // meters/second
    double E;

    mass = Double.valueOf(args[0]).doubleValue(); 
    E = mass * c * c;
    System.out.println(E + " Joules");
  }
}
　

Here's the output:

C:> javac mc2.java
C:> java mc2 0.0456
4.09853e+15 Joules
C:>

　

課堂練習：

請將上述程式(mc2)的質量輸入改由ConsoleReader執行。(請參考第一章範例三、在console模式進行數值輸入輸出)

 

　

物件導向程式設計

義守大學電機系 陳慶瀚

2001.10.02