Java Reference
In-Depth Information
The starting point for outputting a data value is to create a character representation for it. Once we have
that, we need to extend it to the right to the required field width with spaces. We can implement a
helper method,
pad()
, in our
FormatWriter
class that will accept a
String
object as an argument,
and pad out the string appropriately before returning it:
// Helper method to form string
private String pad(String str) {
if (width == 0) {
return str;
}
int blanks = width - str.length(); // Number of blanks needed
StringBuffer result = new StringBuffer(); // Will hold the output
if(blanks<0) { // Data does not fit
for(int i = 0 ; i<width ; i++)
result.append('X'); // so append X's
return result.toString(); // and return the result
}
if(blanks>0) // If we need some blanks
for(int i = 0 ; i<blanks ; i++)
result.append(' '); // append them
// Insert the value string at the beginning or the end
result.insert(justification == LEFT
_
JUSTIFIED ? 0 : result.length(),
str);
return result.toString();
}
We will only use this method inside the class, so we make it
private
. If the
width
is zero then we just
return the original string. Otherwise, we assemble the string to be output in the
StringBuffer
object,
result
. If the string,
str
, has more characters than the field width, then we fill
result
with X's.
Alternatively, you could mess up the nice neat columns here and output the whole string as-is instead.
If the length of
str
is less than the field width, we append the appropriate number of spaces to
result
. We then insert
str
at the beginning of result (index position 0), or the end (index position
result.length())
. Finally, we return a
String
object that we create from
result
by calling its
toString()
method.
We can now implement the
print()
methods and
println()
methods in our class very easily using
the
pad()
method. Here's how the
print()
method for type
long
looks:
// Output type long formatted in a given width
public void print(long value) {
super.print(pad(String.valueOf(value))); // Pad to width and output
}
The
print(long
value)
method calls the static
valueOf()
method in the
String
class to convert
the value to a character string. The string is then passed to the
pad()
method to create a string of the
required length, and this is passed to the
print()
method belonging to the superclass,
PrintWriter
.
