The program shown in Example 5-5 prints a table of Fahrenheit temperatures (still used in daily life weather reporting in the United States) and the corresponding Celsius temperatures (used in science everywhere, and in daily life in most of the world).
Example 5-5. TempConverter.java
import java.text.*;
/* Print a table of fahrenheit and celsius temperatures
*/
public class TempConverter {
public static void main(String[] args) {
TempConverter t = new TempConverter( );
t.start( );
t.data( );
t.end( );
}
protected void start( ) {
}
protected void data( ) {
for (int i=-40; i<=120; i+=10) {
float c = (i-32)*(5f/9);
print(i, c);
}
}
protected void print(float f, float c) {
System.out.println(f + " " + c);
}
protected void end( ) {
}
}This works, but these numbers print with about 15 digits of (useless)
decimal fractions! The second version of this program subclasses the
first and uses a
DecimalFormat
to control the formatting of the
converted temperatures (Example 5-6).
Example 5-6. TempConverter2.java
import java.text.*;
/* Print a table of fahrenheit and celsius temperatures, a bit more neatly.
*/
public class TempConverter2 extends TempConverter {
protected DecimalFormat df;
public static void main(String[] args) {
TempConverter t = new TempConverter2( );
t.start( );
t.data( );
t.end( );
}
// Constructor
public TempConverter2( ) {
df = new DecimalFormat("##.###");
}
protected void print(float f, float c) {
System.out.println(f + " " + df.format(c));
}
protected void start( ) {
System.out.println("Fahr Centigrade.");
}
protected void end( ) {
System.out.println("-------------------");
}
}This works, and the results are better than the first version’s, but still not right:
C:javasrc umbers>java TempConverter2 Fahr Centigrade. -40.0 -40 -30.0 -34.444 -20.0 -28.889 -10.0 -23.333 0.0 -17.778 10.0 -12.222 20.0 -6.667 30.0 -1.111 40.0 4.444 50.0 10
It would look neater if
we lined up the decimal points, but
Java has nothing in its standard API for doing this. This is
deliberate! They wanted to utterly break the ties with the ancient
IBM 1403 line printers and similar monospaced devices such as
typewriters, “dumb” terminals,[20] and DOS
terminal windows. However, with a bit of simple arithmetic, the
FieldPosition
from Section 5.11 can be used to figure out how many spaces need
to be prepended to line up the columns; the arithmetic is done in
print( ), and the spaces are put on in
prependSpaces( ). The result is much prettier:
C:javasrc umbers>java TempConverter3 Fahr Centigrade. -40 -40 -30 -34.444 -20 -28.889 -10 -23.333 0 -17.778 10 -12.222 20 -6.667 30 -1.111 40 4.444 50 10 60 15.556 70 21.111 80 26.667 90 32.222 100 37.778 110 43.333 120 48.889 -------------------
And the code (Example 5-7) is only ten lines longer!
Example 5-7. TempConverter3.java
import java.text.*;
/* Print a table of fahrenheit and celsius temperatures, with decimal
* points lined up.
*/
public class TempConverter3 extends TempConverter2 {
protected FieldPosition fp;
protected DecimalFormat dff;
public static void main(String[] args) {
TempConverter t = new TempConverter3( );
t.start( );
t.data( );
t.end( );
}
// Constructor
public TempConverter3( ) {
super( );
dff = new DecimalFormat("##.#");
fp = new FieldPosition(NumberFormat.INTEGER_FIELD);
}
protected void print(float f, float c) {
String fs = dff.format(f, new StringBuffer(), fp).toString( );
fs = prependSpaces(4 - fp.getEndIndex( ), fs);
String cs = df.format(c, new StringBuffer(), fp).toString( );
cs = prependSpaces(4 - fp.getEndIndex( ), cs);
System.out.println(fs + " " + cs);
}
protected String prependSpaces(int n, String s) {
String[] res = {
"", " ", " ", " ", " ", " "
};
if (n<res.length)
return res[n] + s;
throw new IllegalStateException("Rebuild with bigger "res" array.");
}
}Remember, though, that the fields will line up only if you use a
fixed-width font, such as Courier or
LucidaSansTypewriter. If you want to line it up in
a graphical display, you’ll need to
use Java’s
font capability (see Section 12.6) or use a JTable (see the
Javadoc for javax.swing.JTable or the
O’Reilly book Java Swing).
[20] My children are quick to remind me that “dumb” means “incapable of speech.” Nobody who has used, say, a TTY33 or a DecWriter 100 dumb terminal will claim that they are incapable of speech. Intelligible speech yes, but they certainly did talk at you while they were printing . . .