Description
Terminology:
new operator
class
object
reference variable
instantiation
null
method
static method
constructor
dot (.) operator
parameter
return
alias
garbage collection
String class
immutable
Java API
package
import declaration
Random class
Math class
NumberFormat class
DecimalFormat class
wrapper class
autoboxing
unboxing
Course Coding Standard Additions
• When using the import declaration, import classes from the same package individually (i.e. do not
use the * notation). For example, you should have import java.util.Scanner and import
java.util.Random rather than having import java.util.* which imports everything in
the java.util package.
• When making calculations with π, use Math.PI rather than the literal value such as 3.141. You
should always use constants when they are available.
• From now on, when using a Scanner object to scan input (e.g., from the keyboard), use nextLine
to get numerical input from the user and convert it to a number using methods from the
corresponding wrapper class.
For example to read in an int value from Scanner userInput:
int myInt = Integer.parseInt(userInput.nextLine());
For example to read in a double value from Scanner scan:
double myDouble = Double.parseDouble(userInput.nextLine());
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Goals:
By the end of this activity you should be able to do the following:
Ø Understand how to instantiate an object using the new operator
Ø Use the Java standard library (the Java API) to look up a class
Ø Use the String class and the Scanner class
Ø Construct a viewer canvas and run your program in the canvas
Program Description:
You will create a program that exchanges letters in a String to encode a message.
In-lab Directions:
As you build your program incrementally by entering each of the code segments below, you should
compile and run your program after each segment is completed. This will allow you to verify that your
program is correct after each step as you progress toward the completed program.
MessageConverter.java
• This program will read in a message from the user and then require the user to enter a
number. The following will happen based on the number entered by user:
o If the user enters a 1, the message will be printed trimmed.
o If the user enters a 2, the message will be printed in lower case.
o If the user enters a 3, the message will be printed in upper case.
o If the user enters a 4, the message will be printed with all lower case vowels replaced
with underscores.
o If the user enters a 5, the message will be printed without the first and last character.
o Any other number should generate an appropriate message.
• Create a class called MessageConverter that includes a main method. Also add an import
statement for the java.util.Scanner class. The import statement should be the first line in your
Java file (i.e., it comes before the class declaration as well as comments).
• In the main method, declare the following variables:
– userInput: Scanner object for reading the user input from the keyboard.
Scanner userInput = new Scanner(System.in);
– message: String object for the user input; initialized to an empty String.
String message = “”;
– outputType: An int representing the type of output the user wants.
– result: String object for the converted message; initialized to an empty String.
String result = “”;
• Request input from the user then get user input using the nextLine() method of the Scanner
class.
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• Now get the user’s input for the output type. Make sure that you understand how the code
below works. Notice that this is a call to the print method rather than println.
• Set up an if statement that will print out the appropriate message based on the user’s selected
output type. Note that you can write code for alternate conditions in the if statement using
else if after the if block.
• After the entire if-else statement, add the following line of code to print out the result.
Compile now and after each step below as you fill in the if-else-if-else in the following steps.
• The first condition for outputType equal to 0 sets the result to the original message which
may include leading and/or trailing spaces.
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• Because message references an instance of the String class, we can use the methods in the
String class to return a new String that is a modified version of the original String referenced
by message (the original String object is not changed). To see the available methods in the
String class, go to the Java API at http://docs.oracle.com/javase/8/docs/api/ and find the
String class in the lower left pane on the page and left-click on it. This will open the String
page in large right window of the API as shown below. You should become familiar with the
API since it describes all of the classes that come with the JDK.
• In the main window on the right side, click method at the top or scroll down to Method
Summary to see the methods that our String object message can use.
• String methods do not modify the String object upon which they are called. Instead, if a
method has a String return type, then it returns a new String object which may be a modified
version of the original String.
String Class
The list of String
methods includes
the return type
and a description
of each method.
For example, the
charAt method
returns a char
value at the
specified index.
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• The second condition for outputType equal to 1 sets the result to the trimmed value of the
message; i.e., we can use the trim method removes any leading or trailing whitespace. The
API entry for a method shows the return type for the method in the left and the method name
with formal parameters, if any, on the right along with a brief description of what the method
does. So the entry shown below for the trim method indicates that it returns a String and
takes no parameters. Remember, the trim method does not modify the String object upon
which it is called. Instead it returns a new String object which is the trimmed version of
original String.
Returns a String Method name – there are no formal parameters for this method
Be sure to compile each time you complete a part of the if-else-if-else below.
• The third condition for outputType equal to 2 sets the result to the lower case value of the
message; i.e., we can use the toLowerCase method. The Java API entry shown below for the
toLowerCase method indicates that it returns a String and takes no parameters.
Returns a String Method name – there are no formal parameters for this method
Based on this knowledge, you can now invoke the toLowerCase method on our message
object to return its value in lower case. Place the following code in second block of the if
statement:
• Look at the API description for the toUpperCase method and place the following line in the
second block of the if statement:
• Now write code that will replace specified characters in the message with other characters. In
order to change a character, we need to use the replace method of String to change certain
characters of our string. Take a look at the replace method in the Java API. We see that it
returns a String and that it requires two parameters: the character you want to replace and the
new character. Note there are two versions of replace: the first takes parameters of type char
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(e.g., ‘a’ or ‘e’).and the second takes parameters of type CharSequence which includes String
literals (e.g., “a” or “abc”). In the code below, we will use the char version.
Returns a String Method name and two formal parameters
As with the other String methods, the replace method does not modify the String object it.
Instead it returns a new String object in which the new character has replaced the old
character.
• First, use the replace method to change all of the a’s to underscores. The following line of
code replaces the ‘a’ characters of the message value and then assigns the new version of
message to result. Again, note that message itself remains unchanged.
Now complete the translation by converting the remaining lower case vowels to underscores
for outputType equal to 4 .
• Now use the substring method and length method to extract a part (or substring) of the
message.
Returns a String Method name and formal parameters
Recall the characters in a String are indexed beginning at 0. Since the requirement here is for
result to include everything except the first and last character, use the second version of
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substring with 1 as the parameter beginIndex and message.length() – 1 as endIndex.
• Finally, if the user enters a number other than 1, 2, 3, 4, or 5, result should be set to an error
message.
• Test your program under each of the conditions.
Original (“as is”): For the message, enter: “ This is a test.”
Be sure to include the five leading spaces, but omit the quotes. For choice, enter 0.
Type in a message and press enter:
> This is a test.
Output types:
0: As is
1: Trimmed
2: lower case
3: UPPER CASE
4: v_w_ls r_pl_c_d
5: Without first and last character
Enter your choice: 0
This is a test.
Trimmed: Use the Up arrow key to retrieve the message you entered above, then for choice,
enter 1. Notice the five leading spaces have been removed in the output.
Type in a message and press enter:
> This is a test.
Output types:
0: As is
1: Trimmed
2: lower case
3: UPPER CASE
4: v_w_ls r_pl_c_d
5: Without first and last character
Enter your choice: 1
This is a test.
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Lower case: Use the Up arrow key to retrieve the message you entered above, then for
choice, enter 2. Notice the output is in all lower case but the five leading spaces have not
been removed.
Type in a message and press enter:
> This is a test.
Output types:
0: As is
1: Trimmed
2: lower case
3: UPPER CASE
4: v_w_ls r_pl_c_d
5: Without first and last character
Enter your choice: 2
this is a test.
Upper case: Use the Up arrow key to retrieve the message you entered above, then for
choice, enter 3. Notice the output is in all upper case but the five leading spaces have not
been removed.
Type in a message and press enter:
> This is a test.
Output types:
0: As is
1: Trimmed
2: lower case
3: UPPER CASE
4: v_w_ls r_pl_c_d
5: Without first and last character
Enter your choice: 3
THIS IS A TEST.
Vowels replaced: Use the Up arrow key to retrieve the message you entered above, then for
choice, enter 4. Notice the output has the lower case vowels removed but the five leading
spaces have not been removed.
Type in a message and press enter:
> This is a test.
Output types:
0: As is
1: Trimmed
2: lower case
3: UPPER CASE
4: v_w_ls r_pl_c_d
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5: Without first and last character
Enter your choice: 4
Th_s _s _ t_st.
Without first and last character: For the message, enter “This is a test.” This time
do not enter any leading or trailing spaces. Then for choice, enter 5. Notice the output has
the first and last character removed.
Type in a message and press enter:
> This is a test.
Output types:
0: As is
1: Trimmed
2: lower case
3: UPPER CASE
4: v_w_ls r_pl_c_d
5: Without first and last character
Enter your choice: 5
his is a test
Invalid input:
Type in a message and press enter:
> This is a test.
Output types:
0: As is
1: Trimmed
2: lower case
3: UPPER CASE
4: v_w_ls r_pl_c_d
5: Without first and last character
Enter your choice: 7
Error: Invalid choice input.
• Try the input “A message” for the message and choose to replace all vowels with
underscores. What do you notice about the output? Why do you think that one of the
vowels was not replaced? How would you go about correcting the issue? You don’t have
to make this change – – just think about it.
Web-CAT – After you have completed the program in this activity, you should submit your
MessageConverter.java file to Web-CAT. This is the only file you will submit for this activity.
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Using the Viewer Canvas with your MessageConverter program
Now we want to create a viewer canvas so we can see what is happening in the MessageConverter
program step-by-step as it runs. Note that we could have done this step as soon as the variables had
been declared above. For your project assignments, you should consider creating a canvas at any
point that you need insight into exactly what is happening in your program.
• With MessageConverter.java in the CSD edit window, click the Run in Canvas button on
the toolbar. In the CSD window, you should see that the program is stopped at the first
executable statement, and you should see an empty viewer canvas window as shown below.
• As indicated by the message in the canvas, click the debug step button to execute the
statement to create a Scanner object on System.in which is the keyboard. You should now
see the variable name userInput in the Debug tab. Now click the step button three
more times so that you see message, result, and outputType in the Debug tab. Now
drag each of these variables into the canvas window and arrange them as shown in the figure
below. Click the Save button on the canvas toolbar.
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• Now click the play button , and your program will begin auto-stepping until is pauses
waiting for input. After you enter the requested input each time, the viewers on canvas will
be updated. The run in canvas will continue until your program terminates at which time the
viewers on the canvas will display their last values as shown in the example below for
message “This is a test” and outputType = 4.
Although we created the canvas for the MessageConverter after the program was completed,
it would have been even more appropriate to create the canvas as the program was being
coded, especially if you were encountering logical errors in your program. For example, if
you sense that an error has occurred at particular place in the program, you should set a
breakpoint on the statement where you want to examine the behavior. [To set a breakpoint,
move the mouse over the margin to the left of the statement until you see the red octagon
(stop sign symbol) and then left-click the mouse.] Then instead of clicking the Run in
Canvas button on the toolbar, click the Debug button . The program will run to the
breakpoint and stop. You can single-step (or perhaps step-in ) and observe the variables
in the canvas and debug tab to help determine the nature of the error so that you can correct
it.