String Formatting in Mercury

Our first program will demonstrate string formatting in Java. Here’s the full source code with examples of common string formatting tasks.

import java.util.Formatter;

class Point {
    int x, y;
    
    Point(int x, int y) {
        this.x = x;
        this.y = y;
    }
}

public class StringFormatting {
    public static void main(String[] args) {
        // Java offers several printing methods for formatting general Java values.
        // For example, this prints an instance of our Point class.
        Point p = new Point(1, 2);
        System.out.printf("struct1: %s%n", p);

        // If the value is an object, you can use reflection to include the field names.
        System.out.printf("struct2: x=%d, y=%d%n", p.x, p.y);

        // To print the type of a value, use getClass().getSimpleName().
        System.out.printf("type: %s%n", p.getClass().getSimpleName());

        // Formatting booleans is straightforward.
        System.out.printf("bool: %b%n", true);

        // There are many options for formatting integers.
        // Use %d for standard, base-10 formatting.
        System.out.printf("int: %d%n", 123);

        // This prints a binary representation.
        System.out.printf("bin: %s%n", Integer.toBinaryString(14));

        // This prints the character corresponding to the given integer.
        System.out.printf("char: %c%n", 33);

        // %x provides hex encoding.
        System.out.printf("hex: %x%n", 456);

        // There are also several formatting options for floats.
        // For basic decimal formatting use %f.
        System.out.printf("float1: %f%n", 78.9);

        // %e and %E format the float in (slightly different versions of) scientific notation.
        System.out.printf("float2: %e%n", 123400000.0);
        System.out.printf("float3: %E%n", 123400000.0);

        // For basic string printing use %s.
        System.out.printf("str1: %s%n", "\"string\"");

        // To double-quote strings, you can manually add the quotes.
        System.out.printf("str2: \"%s\"%n", "string");

        // To print a representation of a pointer (object reference in Java), use %h.
        System.out.printf("pointer: %h%n", p);

        // When formatting numbers you will often want to control the width and precision of the resulting figure.
        // To specify the width of an integer, use a number after the % in the format specifier.
        System.out.printf("width1: |%6d|%6d|%n", 12, 345);

        // You can also specify the width of printed floats,
        // though usually you'll also want to restrict the decimal precision at the same time.
        System.out.printf("width2: |%6.2f|%6.2f|%n", 1.2, 3.45);

        // To left-justify, use the - flag.
        System.out.printf("width3: |%-6.2f|%-6.2f|%n", 1.2, 3.45);

        // You may also want to control width when formatting strings,
        // especially to ensure that they align in table-like output.
        System.out.printf("width4: |%6s|%6s|%n", "foo", "b");

        // To left-justify use the - flag as with numbers.
        System.out.printf("width5: |%-6s|%-6s|%n", "foo", "b");

        // String.format() formats and returns a string without printing it anywhere.
        String s = String.format("sprintf: a %s", "string");
        System.out.println(s);

        // You can format+print to other output streams using Formatter.
        Formatter stderr = new Formatter(System.err);
        stderr.format("io: an %s%n", "error");
        stderr.close();
    }
}

To run the program, compile it and use java:

$ javac StringFormatting.java
$ java StringFormatting
struct1: Point@4617c264
struct2: x=1, y=2
type: Point
bool: true
int: 123
bin: 1110
char: !
hex: 1c8
float1: 78.900000
float2: 1.234000e+08
float3: 1.234000E+08
str1: "string"
str2: "string"
pointer: 4617c264
width1: |    12|   345|
width2: |  1.20|  3.45|
width3: |1.20  |3.45  |
width4: |   foo|     b|
width5: |foo   |b     |
sprintf: a string
io: an error

This example demonstrates various string formatting techniques in Java, which are similar to those in other languages but with some Java-specific syntax and methods. The System.out.printf() method is used for most formatting operations, which is analogous to fmt.Printf(). For creating formatted strings without printing, Java uses String.format().