CS 139 Lab: Input and
output in Java
Objectives:
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Students will practice:
- declaring and using variables
- using concatenation operators
- reading values from the keyboard
- manipulating output
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Background:
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Java
is a strongly typed language. In some cases, widening
conversion may be done to make both operands the same type before the
operation is carried out. In other cases, automatic conversion cannot
be done. This lab explores some of these features while also exploring
the Scanner class. You will find information about Scanner in chapter
2.13 of the text.
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Key Terms:
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- Scanner
- A Java provided class that enables us to do input functions.
- concatenation
- Process of combining two strings
- data type
- The kind of data that a container can hold.
- Strongly typed
- A language in which containers may only be declared to be of one type and they can only hold that type. Operations must use the same data types.
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Materials:
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Concat.java
Types.java
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Prerequisites: |
You should have viewed the Scanner tutorial or read the pertinent section in your textbook. |
Part 1: Startup
Set up your programming environment for this lab. Create a folder on
your
n-drive and download the files found in the materials section.
Part 2: Two meanings of +
In Java, the symbol + can be used to add
numbers
or to concatenate strings. This lab illustrates both uses.
When using a String literal (a sequence of characters enclosed in
double
quotation marks) in Java the complete String must fit on one line. The
following is NOT legal (it would result in a compile-time error).
System.out.println ("It is NOT okay to go to the next line
in a LONG string!!!");
The solution is to break the long String up into two shorter strings
that
are joined using the concatenation operator (which is the + symbol). So the
following would be legal.
System.out.println ("It is OKAY to break a long string into "
+ "parts and join them with a + symbol.");
(Note how the continued line is indented one space to indicate it is
a
continuation line. Also note that the operator is placed at the
beginning of
the continuation line instead of at the end of the first line, to
improve
readability.)
So, when working with strings the +
symbol means
to concatenate the Strings (join them). BUT, when working with numbers
the + means
what it has always meant, add!
- Download the program Concat.java
. Do not compile or run the program (yet).
- Unlike primitive data types (do you remember the 8 primitive data
types?), Strings are Java classes just like the Hello program was a
class. As such, it has methods or actions that can be done on Strings.
- This program uses the length method of
the String class. length
is a method that returns a value which, in this example, is the number
of
characters in sample. Or another way to read
this is "sample's length".
- "Hand trace" the program. That is, act as if you were a computer
and execute each statement in the program like a computer would. Draw
storage boxes on a piece of paper to simulate variable/constant storage
containers.
- Write the output your hand tracing produces on your paper.
- Compile and run the program. Compare your "expected result" with
the "actual result".
- For each line where the "expected result" differed from the
"actual result", do you understand why your prediction was wrong? If
you cannot figure it out so you know for sure, ASK!
Correct the second output statement so blanks are printed around the
55.
Recompile and run.
Before turning in this assignment, be sure to fill in the standard
header
information.
Part 3: Input in Java
- Download the program Types.java.
- Open it up in the editor of your choice.
- Read through the program, noticing the following new features:
- Scanner - The Scanner
class provides a mechanism for reading in values from the keyboard
(among other things).
- The very first line of the program is an import statement. This
import makes the Scanner class available to this program.
- Before we can use the Scanner, we
must make and instance of it. We call an instance of a class an object.
So we need to create a Scanner object.
- So, we first declare a Scanner
variable named keyboard. Then in the
initializations, we instantiate or make a new Scanner
object. The value in the parentheses is System.in
which refers to standard system input or the keyboard.
- Whenever we expect the user of a program to enter something at
the command line, we have to prompt them. The line System.out.print("Enter a number: "); is performing
the prompt. When you run this program, the line will display, then the
program will stop executing until the user enters a value on the
command line. The line num1 = keyboard.nextInt();
is the place where the keyboard Scanner is reading what the user has
typed in and assigning it to the variable num1.
- We then echo the value of num1 in the following line.
- Compile the program and execute it using any integer value for
num1. If you enter something that is not an integer the program
will crash (have a run-time error);
- Now, duplicate the three lines at the end of the program and
alter them to read a value into val1.
You will need to use the nextDouble()
method instead of nextInt().
- Compile and execute the program with your changes.
- Finally, duplicate the last three lines and alter them to read a
value into text. You will need to
use the nextLine() method instead of nextDouble(). NOTE: When
reading in both numbers and text, you need to be aware that Scanner
leaves in the new line character that you enter as return.
When you read a number followed by text you need to have a single line
which will consume the new line character. So you would need to add the
line keyboard.nextLine(); after the prompt and before the actual read of the text value. See Gaddis, page 88-91.
Part 4: Data types and compatibility
The last part of the lab will have you practice with different data
types. The three shown in this lab are the ones you will most commonly
use.
- Add a statement to the end of the program to assign the value found in text to the variable val1. What happens when you try to compile it?
- Comment out that line (inline comments) and add a new line that assigns the value of val1 to num1. What happens when you try to compile it?
- Comment out that line and add a new line that assigns the value of val1 to val2. What happens when you try to compile it? Will it execute?
- Add a print statement to display the contents of val1 and val2 after the assignment.
- Add a statement to the end of the program to assign the value found in num1 to val2. What happens when you try to compile it? Will it execute?
- Add a print statement to display the contents of num1 and val2 after the assignment.
- What
can you say about compatibility among Java data types? In other words,
when can you assign values of different data types to one another?
Part 5: Optional - The cast operation
Let's say that you wanted only the integer portion of the value in val1. To do this, you must "cast" the value of val1
as an integer. Casting tells the compiler that it should treat the
value as if it were the cast type. See an example in your book, section
2.7, Conversion between Primitive Data Types.
- Uncomment the line you created in Step 4.2 (assigning val1 to num1).
- Add a cast operation to convert val1's value to an integer.
- Compile and execute the program.
Part 6: Optional - Going further
- Look at Programming Challenges, number 1 in the back of Chapter 2.
- Write a new program called NameAgeIncome that will read in the three values described and print them back out again as described in the assignment.
- The income will print with as many decimal places as there are in the number with a minimun of 2. How can you format the number to print it nicely (in currency format). Hint: Loook for the DecimalFormatting class or printf.
