public class FormatSample {
    public static void main(String[] args) {
        int i = 100;
        double x = 52.3;
        String a = "ABC";
  
        System.out.printf("%d %5.2f %s\n", i, x, a);
 
        // format メソッドでも同じ結果になる
        System.out.format("%d %5.2f %s\n", i, x, a);
    }
}

C:\examples>java FormatSample
100 52.30 ABC
100 52.30 ABC
 
C:\examples>

    public PrintStream printf(String format, Object ... args) {
        return format(format, args);
    }
        
    public PrintStream format(String format, Object ... args) {
        try {
            synchronized (this) {
                ensureOpen();
                if ((formatter == null) 
                    || (formatter.locale() != Locale.getDefault()))
                    formatter = new Formatter((Appendable) this);
                formatter.format(Locale.getDefault(), format, args);
                if (autoFlush)
                    out.flush();
            }
        } catch (InterruptedIOException x) {
            Thread.currentThread().interrupt();
        } catch (IOException x) {
            trouble = true;
        }
        return this;
    }

import java.util.Formatter;
 
public class FormatterTest1 {
    public static void main(String[] args) {
        StringBuilder builder = new StringBuilder();
        Formatter formatter = new Formatter(builder);
 
        int i = 100;
        double x = 26.0;
        String text = "Text";
        formatter.format("Interger %d  Double %5.1f String %s", i, x, text);
 
        System.out.println(formatter.out());
    }
}

%[argument_index$][flags][width][.precision]conversion

import java.util.Formatter;

public class FormatterTest2 {
    public static void main(String[] args) {
        StringBuilder builder = new StringBuilder();
        Formatter formatter = new Formatter(builder);
 
        int i1 = 100;
        int i2 = 200;
        int i3 = 300;
        formatter.format("i3 = %3$d  i2 = %2$d i1 = %1d i2 = %2$d\n", i1, i2, i3);
 
        System.out.println(formatter.out());
    }
}

C:\examples>java FormatterTest2
i3 = 300  i2 = 200 i1 = 100 i2 = 200
 
C:\examples>

null は "null"
引数が Formatterble ならば arg.formatTo()
それ以外は arg.toString()

import java.util.Date;
import java.util.Formatter;
 
public class FormatterTest3 {
    public static void main(String[] args) {
        Formatter formatter = new Formatter((Appendable)System.out);
        Date date = new Date();
 
        formatter.format("tH: %tH%n", date);
        formatter.format("tI: %tI%n", date);
        formatter.format("tk: %tk%n", date);
        formatter.format("tl: %tl%n", date);
        formatter.format("tM: %tM%n", date);
        formatter.format("tS: %tS%n", date);
        formatter.format("tL: %tL%n", date);
 
         ... 以下、略 ...

0 で埋めるかどうか

import java.util.Formatter;

public class FormatterTest4 {
    public static void main(String[] args) {
        Formatter formatter = new Formatter((Appendable)System.out);
 
        int i = 1000;
        formatter.format("%%10d:  [%10d]%n", i);
        formatter.format("%%-10d: [%-10d]%n%n", i);
 
        formatter.format("%%o:  [%5o] %%x:  [%5x] %%X:  [%5X]%n", i, i, i);
        formatter.format("%%#o: [%#5o] %%#x: [%#5x] %%#X: [%#5X]%n%n", i, i, i);
 
        formatter.format("%%+d: [%+d] %% d: [% d]%n", i, i);
        formatter.format("%%010d: [%010d] %%,d: [%,d]%n", i, i);
        formatter.format("%%d: [%d] %%(d: [%(d]%n", -i, -i);
    }
}

C:\examples>java FormatterTest4
%10d:  [      1000]
%-10d: [1000      ]

%o:  [ 1750] %x:  [  3e8] %X:  [  3E8]
%#o: [01750] %#x: [0x3e8] %#X: [0X3E8]

%+d: [+1000] % d: [ 1000]
%010d: [0000001000] %,d: [1,000]
%d: [-1000] %(d: [(1000)]
 
C:\examples>

    public static String format(String format, Object ... args) {
        return new Formatter().format(format, args).toString(); 
    }

class Name implements Formattable {
    private String firstName;
    private String lastName;
 
    public Name(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public void formatTo(Formatter formatter, int flags, int width, int precision) {
        StringBuilder builder = new StringBuilder();
 
        boolean hasLimit = width != -1;
 
        if (hasLimit && width < lastName.length()) {
            createLimittedLastName(builder, width);
        } else if (hasLimit && width < firstName.length() + lastName.length()) {
            createLastName(builder);
        } else {
            createFullName(builder, flags, formatter.locale());
        }
 
        // 幅に足りない分のスペースを入れる
        if (hasLimit && builder.length() < width) {
            if ((flags & LEFT_JUSTIFY) != LEFT_JUSTIFY) {
                int num = width - builder.length();
                for (int i = 0; i < num; i++) {
                    builder.insert(0, ' ');
                }
            }
        }
 
        if ((flags & UPPERCASE) == UPPERCASE) {
            formatter.format(builder.toString().toUpperCase());
        } else {
            formatter.format(builder.toString());
        }
    }
 
    private void createLimittedLastName(StringBuilder builder, int width) {
        builder.append(lastName.substring(0, width - 1));
        builder.append('*');
    }
 
    private void createLastName(StringBuilder builder) {
        builder.append(lastName);
    }
 
    private void createFullName(StringBuilder builder, int flags, Locale locale) {
        if ((flags & ALTERNATE) == ALTERNATE 
          && Character.getType(firstName.charAt(0)) == Character.UPPERCASE_LETTER) {
            builder.append(firstName.substring(0, 1));
        } else {
            builder.append(firstName);
        }
        
        builder.append(' ');
        builder.append(lastName);
    }

    public static void main(String[] args) {
        Formatter formatter = new Formatter((Appendable)System.out);
        
        Name name = new Name("Yuichi", "Sakuraba");
        
        formatter.format("%%s [%1$s] %%#s [%1$#s]%n", name);
        formatter.format("%%5s [%1$5s] %%10s [%1$10s]%n", name);
        formatter.format("%%20s [%1$20s] %%-20s [%1$-20s]%n", name);
    }

C:\examples>java FormatterTest5
%s [Yuichi Sakuraba] %#s [Y Sakuraba]
%5s [Saku*] %10s [  Sakuraba]
%20s [     Yuichi Sakuraba] %-20s [Yuichi Sakuraba]
 
C:\examples>