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17.7: String Formatting

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  • In Section 3.6, we learned how to use printf to display formatted output. Sometimes programs need to create strings that are formatted a certain way, but not display them immediately, or ever. For example, the following method returns a time string in 12-hour format:

    public static String timeString(int hour, int minute) {
        String ampm;
        if (hour < 12) {
            ampm = "AM";
            if (hour == 0) {
                hour = 12;  // midnight
        } else {
            ampm = "PM";
            hour = hour - 12;
        return String.format("%02d:%02d %s", hour, minute, ampm);

    String.format takes the same arguments as System.out.printf: a format specifier followed by a sequence of values. The main difference is that System.out.printf displays the result on the screen; String.format creates a new string, but does not display anything.

    In this example, the format specifier \%02d means “two digit integer padded with zeros”, so timeString(19, 5) returns the string "07:05 PM".

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