How to format a number 0..9 to display with 2 digits (it's NOT a date)

Quick dive into String.format()

The %02d format specifier in the String.format() method ensures a consistent 2-digit display. For example, when formatting a single digit number like "5", it becomes "05". The 0 in %02d is a flag indicating zero-padding and the 2 specifies a width of 2 characters.

A bit more: Advanced formatting options

DecimalFormat & NumberFormat

The DecimalFormat and NumberFormat classes offer greater control for complex or variable requirements. With DecimalFormat, 2-digit formatting is as simple as:

DecimalFormat decimalFormat = new DecimalFormat("00");
String result = decimalFormat.format(8); // We got "08" in the house!
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In NumberFormat, here's how you achieve the same:

NumberFormat numFormat = NumberFormat.getInstance();
numFormat.setMinimumIntegerDigits(2);
String numStrings = numFormat.format(7); // Watch out! "07" passing through.
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The crafty string operation approach

For those who love the good ol' string operations, a bit of zero concatenation and substring() magic will give you a padded number:

String fmtNum = ("000000" + num).substring(String.valueOf(num).length()); // I raise you "007"!
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Though a bit crude, it can come handy in no-library situations or for quick scripts.

Formatting variations for assorted applications

Using Android resources

For Android developers, String.format()'s sister getString() can format integers similarly, using placeholders (%1$02d):

<string name="formatted_number">%1$02d</string>
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And inside our Java code:

String formatted = getString(R.string.formatted_number, myNumber); // Returns "09", it's got 99% battery!
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Beware the octal pitfall

Remember, in Java, an integer number with a leading zero denotes an octal number. Be cautious!

int num = 010; // Oops! It's an 8 in disguise, sneaky!
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So, always be sure of your context when formatting numbers this way.

Tips & Pecks

• Type safe: Ensure the parameter passed to the formatter is an integer data type. You're better safe than sorry!
• Performance: If you're processing huge datasets or long arrays, efficiency matters: always opt for the most efficient method.