Unit 2: Java Introduction to Programming

2.2 Information on Program Appearance and Printing

Comments

Comments are explanatory notes that a programmer can include in a program.

They are not code and they do not affect how the program runs.

On any line where “//” appears, whatever follows this pair of symbols is treated as a comment.

Likewise, anything that appears between “/*” and “*/” will be treated as a comment.

First Program with comments Here is a commented version of the first

program:

/* This is my first program. */ public class FirstProg{

public static void main(String[] args){

// A method is called here.System.out.println("Hello World");

}}

Quick note about indenting

Notice how the examples so far have indents. It is true that the code will run the same without them. However, it’s good practice (and extremely helpful) to indent so that it is clear where a block of code begins and ends.

Eclipse should do this for you automatically.

All examples given will be shown in this form.

Using println()

Here is a brief summary of some of the aspects of printing in Java when using println() and related methods.

1. The method println() prints a line of output followed by a new line or carriage return.

You can also make use of the method print(), which prints a line without a carriage return.

The complete call would be of the form:

System.out.print("Some Parameter");

Using println() with numbers

2. You can print arithmetic constants as well as strings of characters.

A call such as this would print out the numeric value 3:

 System.out.println(3);

Using println() with strings Note that what would appear on the

screen for the following call would be exactly the same, but that there is an important distinction in the meaning of the program code.

The example above prints a numeric value, while the example below prints a string containing the symbol “3”:

 System.out.println("3");

Using println() with + sign

3. You can print out more than one parameter at a time.

The symbol that accomplishes this is the “+” sign. When printing strings, if you want blank spaces

between things, you need to supply them explicitly. If you did this:

 System.out.println("Hello" + "World");  You would see this: HelloWorld

Using println() with + sign

However, you could also do this: System.out.println("Hello" + " World");  Or this: System.out.println("Hello" + " " + "World");  And you would see: Hello World

Using println() with + sign 4. Using the “+” sign when printing

numeric values has a different effect. It does the arithmetic.

If you did this: System.out.println(3 + 4);  You would see this: 7

Using println() with + sign 5. You can also combine a parameter in quotes

with a numerical value without quotes using a “+” sign.

In this case the system automatically converts the numerical value to a string and does concatenation.

If you did this: System.out.println("Hello" + 7);  You would see this: Hello7

Using println() with + sign

The full explanation of the different ways in which the “+” sign works will be given later.

For the time being you should simply be able to use it correctly with strings and numbers.

Using println() with a dot 6. Although the dot has a special syntactic

meaning in various places in Java code, in places where simple parameters in the form of numbers are allowed, such as when printing, it has its customary meaning—that of a decimal point.

Thus, if you did this: System.out.println(3.4);  You would see this value: 3.4

The power of the backslash 7. In Java printing, the backslash “\” is

used as the escape sequence. This means that any symbol immediately

following the backslash is simply treated as a printable character.

That character is not treated as having any syntactic meaning.

This allows you to print characters that would otherwise be ambiguous or syntactically incorrect in a printing parameter.

The power of the backslash

If you wanted to print a double quote, you could do this:

System.out.println("\"");  If you wanted to print the backslash

itself, you could do this: System.out.println("\\");

The power of the backslash The backslash can also be used to insert

special printing instructions into a printed string. \n represents a new line.

Thus, if you did this: System.out.println("Hello\nWorld");  You would see this: HelloWorld