The program shown in Example 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).
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 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 frac-
tions! The second version of this program subclasses the first and uses a
DecimalFormat to control the formatting of the converted temperatures
Example 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("#0.00"); } 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\numbers>java Fahr Centigrade. -40.00 -40.00 -30.00 -34.44 -20.00 -28.89 -10.00 -23.33 0.00 -17.78 10.00 -12.22 20.00 -6.67 30.00 -1.11 40.00 4.44 50.00 10.00 60.00 15.56 70.00 21.11 80.00 26.67 90.00 32.22 100.00 37.78 110.00 43.33 120.00 48.89 TempConverter2
It would look neater if we lined up the decimal points, but Java had 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, * and DOS terminal windows. However, with a bit of
simple arithmetic, the FieldPosition from Recipe 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\numbers>java Fahr Centigrade. -40 -40 -30 -34.444 -20 -28.889 -10 -23.333 0 -17.778 10 -12.222 20 -6.667 TempConverter30 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 is only ten lines longer!
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."); } }
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