Converting Between Numbers and Strings 숫자와 문자 사이의 변환

숫자와 문자 사이의 변환   Converting Between Numbers and Strings

문자를 숫자로 변환하기 Converting Strings to Numbers

빈번하게, 프로그램은 문자열 객체 안의 숫자 데이터로 끝나는데, 예를 들명 사용자가 입력 한 값으로 끝납니다.

원시 숫자 타입(Byte, Integer, Double, Float, Long 및 Short)을 둘러싼 서브클래스 Number들은 문자열을 해당 타입의 객체로 변화하는 valueOf 라는 이름의 클래스 메소드를 제공합니다.  객체에 문자열을 변환 클래스의 방법을 제공한다. 명령 줄에서 2개의 문자열을 얻는, 예제  ValueOfDemo는 그들을 숫자로 변환시키고, 그 값들 위에서 연산을 수행합니다:

Frequently, a program ends up with numeric data in a string object—a value entered by the user, for example.

The Number subclasses that wrap primitive numeric types ( Byte, Integer, Double, Float, Long, and Short) each provide a class method named valueOf that converts a string to an object of that type. Here is an example, ValueOfDemo , that gets two strings from the command line, converts them to numbers, and performs arithmetic operations on the values:

public class ValueOfDemo {
    public static void main(String[] args) {

        // this program requires two 
        // arguments on the command line 
        if (args.length == 2) {
            // convert strings to numbers
            float a = (Float.valueOf(args[0])).floatValue(); 
            float b = (Float.valueOf(args[1])).floatValue();

            // do some arithmetic
            System.out.println("a + b = " +
                               (a + b));
            System.out.println("a - b = " +
                               (a - b));
            System.out.println("a * b = " +
                               (a * b));
            System.out.println("a / b = " +
                               (a / b));
            System.out.println("a % b = " +
                               (a % b));
        } else {
            System.out.println("This program " +
                "requires two command-line arguments.");
        }
    }
}

The following is the output from the program when you use 4.5 and 87.2 for the command-line arguments:

a + b = 91.7
a - b = -82.7
a * b = 392.4
a / b = 0.0516055
a % b = 4.5

---

Note: Each of the Number subclasses that wrap primitive numeric types also provides a parseXXXX() method (for example, parseFloat()) that can be used to convert strings to primitive numbers. Since a primitive type is returned instead of an object, the parseFloat() method is more direct than the valueOf() method. For example, in the ValueOfDemo program, we could use:

float a = Float.parseFloat(args[0]);
float b = Float.parseFloat(args[1]);

---

Converting Numbers to Strings

Sometimes you need to convert a number to a string because you need to operate on the value in its string form. There are several easy ways to convert a number to a string:

int i;
// Concatenate "i" with an empty string; conversion is handled for you.
String s1 = "" + i;

or

// The valueOf class method.
String s2 = String.valueOf(i);

Each of the Number subclasses includes a class method, toString(), that will convert its primitive type to a string. For example:

int i;
double d;
String s3 = Integer.toString(i); 
String s4 = Double.toString(d); 

The ToStringDemo example uses the toString method to convert a number to a string. The program then uses some string methods to compute the number of digits before and after the decimal point:

public class ToStringDemo {
    
    public static void main(String[] args) {
        double d = 858.48;
        String s = Double.toString(d);
        
        int dot = s.indexOf('.');
        
        System.out.println(dot + " digits " +
            "before decimal point.");
        System.out.println( (s.length() - dot - 1) +
            " digits after decimal point.");
    }
}

The output of this program is:

3 digits before decimal point.
2 digits after decimal point.

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