You have already seen multiple time a basic way to print text and messages in programs:
System.out.println("hello!");
If we deconstruct this, we see three parts:
Next to println, System.out provides other services to print out values towards the user.
System.out is an object of type PrintStream. The services offered by System.out are provided by any object of the type PrintStream.
The full documentation of the services offered by PrintStream is available in the JDK manual; for the time being, you need only remember the following:
Print a message, but do not begin a new line of text afterwards. The next use of print or println will append its message to the same line of output. For example:
System.out.print("hello");
System.out.print("world");
will print “helloworld” on a single line.
A common task to perform in programs is “alignment”: print data in columns, ensuring that values under each other are properly aligned. Another common task is to format numbers according to some application-defined standard, for example “all grades should be printed with one decimal of accuracy” to say the program should print “7.0” and not simply “7”.
Unfortunately, print and println do not know anything about alignment, decimal places and the like. To perform these additional tasks, the service printf is provided.
It is defined as follows:
printf(String format, ...)
This syntax indicates printf takes at least one argument, something that evaluates to a string, and then zero or more additional arguments, which can be of any type.
The semantics of printf are as follows:
- a “d”: the next argument in the remaining argument list is printed out in decimal;
- a “s”: the next argument in the remaining argument list is printed as a string;
- a “c”: the next argument in the remaining argument list is printed as a character;
- a “f”: the next argument is printed as a number with a decimal point;
- a “e”: the next argument is printed as a decimal number in scientific notation;
- a “%”: the character “%” is printed;
Some examples:
| Program text | What is printed |
|---|---|
| printf("") | (nothing) |
| printf("hello") | hello |
| printf("hi %d", 123) | hi 123 |
| printf("hi %d, again", 123) | hi 123, again |
| printf("%d - %d: %d", 4, 8, 12) | 4 - 8: 12 |
| printf("hi %s!", "bob") | hi bob! |
| printf("%s%s", "your", "world") | yourworld |
| printf("score = %d%%.", 13) | score = 13%. |
| printf("pi = %f", 3.14) | pi = 3.140000 |
| printf("10^3 = %e", 1000) | 10^3 = 1.000000e+03 |
As you can see, each combination of % followed by a character in the format string becomes a “hole” to be filled in by one of the remaining arguments. This group formed by a % followed by a special character is called a format specifier. The last character in a format specifier (s, c, d, f, etc) is called the format conversion. There exist other conversions, but in this course you will mostly use %d, %s, %c, %f and %%.
The definition above says about specifiers, “printf looks after the % sign until it encounters one of the following”. So there it is possible to express additional information between the “%” sign and the conversion, the special character that decides the type of value to convert. This extra information is used to control how the formatting is done.
In this course, you will use use the following format specifiers:
| Conversion | Meaning | Example | Prints |
|---|---|---|---|
| %f | Print the data as-is | printf(":%d:", 12) | :12: |
| %Nf | Print at least N columns, align right | printf(":%5d:", 12) | : 12: |
| %.Mf | Print M decimals (for floats only) | printf(":%.3f:", 3.14) | :3.140: |
| %N.Mf | Align right to N columns, print M decimals | printf(":%5.1f:", 3.14) | : 3.1: |
| %0Nf | Print at least N columns, align right, fill with 0 | printf(":%05d:", 12) | :00012: |
| %-Nf | Print at least N columns, align left | printf("%-5d:", 12) | :12 : |
printf behaves like print in that it never starts a new line in the output by itself. So if you have the following code:
System.out.printf("hello %d", 123);
System.out.printf("world");
the program will print “hello 123world” on a single line.
It is possible to force new lines using a special character combination: “\n” (a backslash, then small letter n). So you can reproduce the behavior of println for example as follows:
System.out.printf("hello\n");
It is also possible to use “\n” in the middle of a format string, for example:
System.out.println("hello,\nworld!\n");
will print “hello,” on a first line, then “world!” on a second line.
Tip
“\n” is one of multiple special combinations that can be found in strings. This will be handled in a later lecture on strings and character literals.
Copyright © 2014, Raphael ‘kena’ Poss. Permission is granted to distribute, reuse and modify this document according to the terms of the Creative Commons Attribution-ShareAlike 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by-sa/4.0/.