Java Program: TempConverter

Java Program: TempConverter


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).

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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