Assignment 7
Due date : September 17, 12 PM EST
Total Marks : 30
Today’s Exercise
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Topics |
Lab Exercises |
Points |
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String Class |
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Important Note:
· Work with your partner collaboratively. This means that you must not divide the work between the two of you. You will work together.
· Work on one computer only. If two computers are open, it will be assumed that you have divided the work and only half of the total grade will be given.
· Only one partner should upload all assignment files. The other partner will upload only the declaration form.
· Make sure that these two programs compile and run without any error. A program with compiler or runtime error will receive no credit.
· When you work on today’s problem set, get help from the following websites.
§ http://java.sun.com/j2se/1.4.2/docs/api/java/lang/String.html
§ http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Math.html
§ http://java.sun.com/j2se/1.4.2/docs/api/java/util/Random.html
Prelab Exercises
These exercises focus on the String, Random, and Math classes defined in the Java Standard Class Library. The main concepts are in the text in sections 3.2-3.5. The goals of the lab are for you to gain experience with the following concepts:
• Declaring a variable to be a reference to an object—for example, the following declares the variable quotation to be a reference to a String object:
String quotation;
• Declaring a variable to be a reference to an object and creating the object (instantiating it) using the new operator—for example,
String quotation = new String("I think, therefore I am.");
Random generator = new Random();
• Invoking a method to operate on an object using the dot operator—for example,
quotation.length()
invokes the length method which returns the length of the quotation String or
quotation.toLowerCase()
quotation except all letters are lower case. These invocations would be used in a program in a place appropriate for an integer (in the first case) or a String (in the second case) such as an assignment statement or a println statement.
• Invoking static or class methods—these are methods that are invoked using the class name rather than an object name. The methods in the Math class are static methods (basically because we don't need different instances of Math whereas there are lots of different String objects). Examples are
Math.sqrt(2) (which returns the square root of 2)
and
Math.pow(3, 2) (which returns 32)
• Importing the appropriate packages—usually when you use classes from a library you need to put the import declaration at the top of your program. The exception is for classes defined in the java.lang package (this includes String and Math) which is automatically imported into every Java program.
1. Fill in the blanks in the program below as follows: (Section 3.2, especially the example in Listing 3.1, should be helpful):
(a) declare the variable town as a reference to a String object and initialize it to "Anytown, USA".
(b) write an assignment statement that invokes the length method of the string class to find the length of the college String object and assigns the result to the stringLength variable
(c) complete the assignment statement so that change1 contains the same characters as college but all in upper case
(d) complete the assignment statement so that change2 is the same as change1 except all capital O's are replaced with the asterisk (*) character.
(e) complete the assignment statement so that change3 is the concatenation of college and town (use the concat method of the String class rather than the + operator)
// **************************************************
// StringPlay.java
//
// Play with String objects
// **************************************************
public class StringPlay
{
public static void main (String[] args)
{
String college = new String ("PoDunk College");
________________________________________________________; // part (a)
int stringLength;
String change1, change2, change3;
________________________________________________________; // part (b)
System.out.println (college + " contains " + stringLength + " characters.");
change1 = ______________________________________________; // part (c)
change2 = ______________________________________________; // part (d)
change3 = ______________________________________________; // part (e)
System.out.println ("The final string is " + change3);
}
}
2. The following program should read in the lengths of two sides of a right triangle and compute the length of the hypotenuse (recall that the length of the hypotenuse is the square root of side 1 squared plus side 2 squared). Complete it by adding statements to read the input from the keyboard and to compute the length of the hypotenuse (you need to use a Math class method for that).
// *******************************************************************
// RightTriangle.java
//
// Compute the length of the hypotenuse of a right triangle
// given the lengths of the sides
// *******************************************************************
import java.util.Scanner;
public class RightTriangle
{
public static void main (String[] args)
{
double side1, side2; // lengths of the sides of a right triangle
double hypotenuse; // length of the hypotenuse
Scanner scan = new Scanner(System.in);
// Get the lengths of the sides as input
System.out.print ("Please enter the lengths of the two sides of " +
"a right triangle (separate by a blank space): ");
_____________________________________________________________;
_____________________________________________________________;
// Compute the length of the hypotenuse
_____________________________________________________________;
// Print the result
System.out.println ("Length of the hypotenuse: " + hypotenuse);
}
}
3. In many situations a program needs to generate a random number in a certain range. The Java Random class lets the programmer create objects of type Random and use them to generate a stream of random numbers (one at a time). The following declares the variable generator to be an object of type Random and instantiates it with the new operator.
Random generator = new Random();
The generator object can be used to generate either integer or floating point random numbers using either the nextInt method (either with no parameter or with a single integer parameter) or nextFloat (or nextDouble) methods, respectively. The integer returned by nextIn(t could be any valid integer (positive or negative) whereas the number returned by nextInt(n) is a random integer in the range 0 to n-1. The numbers returned by nextFloat() or nextDouble() are floating point numbers between 0 and 1 (up to but not including the 1). Most often the goal of a program is to generate a random integer in some particular range, say 30 to 99 (inclusive). There are several ways to do this:
Math.abs(generator.nextInt()) % 70
will return numbers between 0 and 69 (because those are the only possible remainders when an integer is divided by 70 - note that the absolute value of the integer is first taken using the abs method from the Math class). In general, using % N will give numbers in the range 0 to N - 1. Next the numbers must be shifted to the desired range by adding the appropriate number. So, the expression
Math.abs(generator.nextInt()) % 70 + 30
will generate numbers between 30 and 99.
generator.nextInt(70)
will return numbers between 0 and 69 (inclusive). Next the numbers must be shifted to the desired range by adding the appropriate number. So, the expression
generator.nextInt(70) + 30
will generate numbers between 30 and 99.
generator.nextFloat() * 70
returns a floating point number between 0 and 70 (up to but not including 70). To get the integer part of the number we use the cast operator:
(int) (generator.nextFloat() * 70)
The result of this is an integer between 0 and 69, so
(int) (generator.nextFloat() * 70) + 30
shifts the numbers by 30 resulting in numbers between 30 and 99.
The method nextFloat can be replaced by nextDouble to get double precision floating point numbers rather than single precision.
Fill in the blanks in the following program to generate the random numbers as described in the documentation. NOTE that that java.util.Random must be imported to use the Random class.
// **************************************************
// LuckyNumbers.java
//
// To generate three random "lucky" numbers
// **************************************************
import java.util.Random;
public class LuckyNumbers
{
public static void main (String[] args)
{
Random generator = new Random();
int lucky1, lucky2, lucky3;
// Generate lucky1 (a random integer between 50 and 79) using the
// nextInt method (with no parameter)
lucky1 = ______________________________________________________;
// Generate lucky2 (a random integer between 90 and 100) using the
// nextInt method with an integer parameter
lucky2 = ______________________________________________________;
// Generate lucky3 (a random integer between 11 and 30) using nextFloat
lucky3 = ______________________________________________________;
System.out.println ("Your lucky numbers are " + lucky1 + ", " + lucky2
+ ", and " + lucky3);
}
}
Working with Strings
The following program illustrates the use of some of the methods in the String class. Study the program to see what it is doing.
// ***************************************************************
// StringManips.java
//
// Test several methods for manipulating String objects
// ***************************************************************
import java.util.Scanner;
public class StringManips
{
public static void main (String[] args)
{
String phrase = new String ("This is a String test.");
int phraseLength; // number of characters in the phrase String
int middleIndex; // index of the middle character in the String
String firstHalf; // first half of the phrase String
String secondHalf; // second half of the phrase String
String switchedPhrase; // a new phrase with original halves switched
// compute the length and middle index of the phrase
phraseLength = phrase.length();
middleIndex = phraseLength / 2;
// get the substring for each half of the phrase
firstHalf = phrase.substring(0,middleIndex);
secondHalf = phrase.substring(middleIndex, phraseLength);
// concatenate the firstHalf at the end of the secondHalf
switchedPhrase = secondHalf.concat(firstHalf);
// print information about the phrase
System.out.println();
System.out.println ("Original phrase: " + phrase);
System.out.println ("Length of the phrase: " + phraseLength +
" characters");
System.out.println ("Index of the middle: " + middleIndex);
System.out.println ("Character at the middle index: " +
phrase.charAt(middleIndex));
System.out.println ("Switched phrase: " + switchedPhrase);
System.out.println();
}
}
The file StringManips.java contains this program. Save the file to your directory and compile and run it. Study the output and make sure you understand the relationship between the code and what is printed. Now modify the file as follows:
1. Declare a variable of type String named middle3 (put your declaration with the other declarations near the top of the program) and use an assignment statement and the substring method to assign middle3 the substring consisting of the middle three characters of phrase (the character at the middle index together with the character to the left of that and the one to the right – use variables, not the literal indices for this particular string). Add a println statement to print out the result. Save, compile, and run to test what you have done so far.
2. Add an assignment statement to replace all blank characters in switchedPhrase with an asterisk (*). The result should be stored back in switchedPhrase (so switchedPhrase is actually changed). (Do not add another print—place your statement in the program so that this new value of switchedPhrase will be the one printed in the current println statement.) Save, compile, and run your program.
3. Declare two new variables city and state of type String. Add statements to the program to prompt the user to enter their hometown—the city and the state. Read in the results using the appropriate Scanner class method – you will need to have the user enter city and state on separate lines. Then using String class methods create and print a new string that consists of the state name (all in uppercase letters) followed by the city name (all in lowercase letters) followed again by the state name (uppercase). So, if the user enters Lilesville for the city and North Carolina for the state, the program should create and print the string
NORTH CAROLINAlilesvilleNORTH CAROLINA
Submit the following files:
All these programs must compile and run without any error.