9.8 (GUI) File Dialogs 329

334 Chapter 9 Strings and Text I/O

9.18Show the output of the following program:

public class Test {

public static void main(String[] args) { String s = "Java";

StringBuilder builder = new StringBuilder(s); change(s, builder);

System.out.println(s);

System.out.println(builder);

}

private static void change(String s, StringBuilder builder) { s = s + " and HTML";

builder.append(" and HTML");

}

}

Section 9.5

9.19This book declares the main method as

public static void main(String[] args)

Can it be replaced by one of the following lines?

public static void main(String args[])

public static void main(String[] x)

public static void main(String x[])

static void main(String x[])

9.20Show the output of the following program when invoked using

1.java Test I have a dream

2.java Test "1 2 3"

3.java Test

4.java Test "*"

5.java Test *

public class Test {

public static void main(String[] args) { System.out.println("Number of strings is " + args.length); for (int i = 0; i < args.length; i++)

System.out.println(args[i]);

}

}

Section 9.6

9.21What is wrong about creating a File object using the following statement?

new File("c:\book\test.dat");

Programming Exercises 335

9.22How do you check whether a file already exists? How do you delete a file? How do you rename a file? Can you find the file size (the number of bytes) using the File class?

9.23Can you use the File class for I/O? Does creating a File object create a file on the disk?

Section 9.7

9.24How do you create a PrintWriter to write data to a file? What is the reason to declare throws Exception in the main method in Listing 9.7, WriteData.java? What would happen if the close() method were not invoked in Listing 9.7?

9.25Show the contents of the file temp.txt after the following program is executed.

public class Test {

public static void main(String[] args) throws Exception { java.io.PrintWriter output = new

java.io.PrintWriter("temp.txt"); output.printf("amount is %f %e\r\n", 32.32, 32.32);

output.printf("amount is %5.4f %5.4e\r\n", 32.32, 32.32); output.printf("%6b\r\n", (1 > 2)); output.printf("%6s\r\n", "Java");

output.close();

}

}

9.26How do you create a Scanner to read data from a file? What is the reason to define throws Exception in the main method in Listing 9.8, ReadData.java? What would happen if the close() method were not invoked in Listing 9.8?

9.27What will happen if you attempt to create a Scanner for a nonexistent file? What will happen if you attempt to create a PrintWriter for an existing file?

9.28Is the line separator the same on all platforms? What is the line separator on Windows?

9.29Suppose you enter 45 57.8 789, then press the Enter key. Show the contents of the variables after the following code is executed.

Scanner input = new Scanner(System.in); int intValue = input.nextInt();

double doubleValue = input.nextDouble(); String line = input.nextLine();

9.30Suppose you enter 45, the Enter key, 57.8, the Enter key, 789, the Enter key. Show the contents of the variables after the following code is executed.

Scanner input = new Scanner(System.in); int intValue = input.nextInt();

double doubleValue = input.nextDouble(); String line = input.nextLine();

PROGRAMMING EXERCISES

Sections 9.2–9.3

9.1* (Checking SSN) Write a program that prompts the user to enter a social security number in the format DDD-DD-DDDD, where D is a digit. The program displays "Valid SSN" for a correct social security number and "Invalid SSN" otherwise.

336 Chapter 9 Strings and Text I/O

9.2** (Checking substrings) You can check whether a string is a substring of another string by using the indexOf method in the String class. Write your own method for this function. Write a program that prompts the user to enter two strings, and check whether the first string is a substring of the second.

9.3** (Checking password) Some Websites impose certain rules for passwords. Write a method that checks whether a string is a valid password. Suppose the password rule is as follows:

■A password must have at least eight characters.

■A password consists of only letters and digits.

■A password must contain at least two digits.

Write a program that prompts the user to enter a password and displays "Valid

Password" if the rule is followed or "Invalid Password" otherwise.

9.4(Occurrences of a specified character) Write a method that finds the number of occurrences of a specified character in the string using the following header:

public static int count(String str, char a)

For example, count("Welcome", 'e') returns 2. Write a test program that prompts the user to enter a string followed by a character and displays the number of occurrences of the character in the string.

9.5** (Occurrences of each digit in a string) Write a method that counts the occurrences of each digit in a string using the following header:

public static int[] count(String s)

The method counts how many times a digit appears in the string. The return value is an array of ten elements, each of which holds the count for a digit. For example, after executing int[] counts = count("12203AB3"), counts[0] is 1, counts[1] is 1, counts[2] is 2, counts[3] is 2.

Write a test program that prompts the user to enter a string and displays the number of occurrences of each digit in the string.

9.6* (Counting the letters in a string) Write a method that counts the number of letters in a string using the following header:

public static int countLetters(String s)

Write a test program that prompts the user to enter a string and displays the number of letters in the string.

9.7* (Phone keypads) The international standard letter/number mapping found on the telephone is shown below:

Write a method that returns a number, given an uppercase letter, as follows:

public static int getNumber(char uppercaseLetter)

Programming Exercises 337

Write a test program that prompts the user to enter a phone number as a string. The input number may contain letters. The program translates a letter (upperor lowercase) to a digit and leaves all other characters intact. Here is a sample run of the program:

Enter a string: 1-800-Flowers 1-800-3569377

Enter a string: 1800flowers 18003569377

public static boolean isAnagram(String s1, String s2)

338 Chapter 9 Strings and Text I/O

Write a test program that prompts the user to enter two strings and, if they are anagrams, displays "anagram", otherwise displays "not anagram".

Section 9.5

9.13* (Passing a string to check palindromes) Rewrite Listing 9.1 by passing the string as a command-line argument.

9.14* (Summing integers) Write two programs. The first program passes an unspecified number of integers as separate strings to the main method and displays their total. The second program passes an unspecified number of integers delimited by one space in a string to the main method and displays their total. Name the two programs Exercise9_14a and Exercise9_14b, as shown in Figure 9.20.

FIGURE 9.20 The program adds all the numbers passed from the command line.

9.15* (Finding the number of uppercase letters in a string) Write a program that passes a string to the main method and displays the number of uppercase letters in a string.

Sections 9.7–9.8

9.16** (Reformatting Java source code) Write a program that converts the Java source code from the next-line brace style to the end-of-line brace style. For example, the Java source in (a) below uses the next-line brace style. Your program converts it to the end-of-line brace style in (b).

Your program can be invoked from the command line with the Java source-code file as the argument. It converts the Java source code to a new format. For example, the following command converts the Java source-code file Test.java to the end-of-line brace style.

java Exercise9_16 Test.java

9.17* (Counting characters, words, and lines in a file) Write a program that will count the number of characters (excluding control characters '\r' and '\n'), words, and lines, in a file. Words are separated by spaces, tabs, carriage return, or line-feed

Programming Exercises 339

characters. The file name should be passed as a command-line argument, as shown in Figure 9.21.

FIGURE 9.21 The program displays the number of characters, words, and lines in the given file.

9.18* (Processing scores in a text file) Suppose that a text file Exercise9_18.txt contains an unspecified number of scores. Write a program that reads the scores from the file and displays their total and average. Scores are separated by blanks.

9.19* (Writing/Reading data) Write a program to create a file named Exercise9_19.txt if it does not exist. Write 100 integers created randomly into the file using text I/O. Integers are separated by spaces in the file. Read the data back from the file and display the sorted data.

9.20** (Replacing text) Listing 9.9, ReplaceText.java, gives a program that replaces text in a source file and saves the change into a new file. Revise the program to save the change into the original file. For example, invoking

java Exercise9_20 file oldString newString

replaces oldString in the source file with newString.

9.21** (Removing text) Write a program that removes all the occurrences of a specified string from a text file. For example, invoking

java Exercise9_21 John filename

removes string John from the specified file.

Comprehensive

9.22** (Guessing the capitals) Write a program that repeatedly prompts the user to enter a capital for a state, as shown in Figure 9.22(a). Upon receiving the user input, the program reports whether the answer is correct, as shown in Figure 9.22(b). Assume that 50 states and their capitals are stored in a two-dimensional array, as shown in Figure 9.23. The program prompts the user to answer all ten states’ capitals and displays the total correct count.

FIGURE 9.22 The program prompts the user to enter the capital in (a) and reports the correctness of the answer.

Programming Exercises 341

public MyStringBuilder2 reverse();

public MyStringBuilder2 substring(int begin); public MyStringBuilder2 toUpperCase();

9.28* (Common prefix) Write a method that returns the common prefix of two strings. For example, the common prefix of "distance" and "disinfection" is "dis". The header of the method is as follows:

public static String prefix(String s1, String s2)

If the two strings have no common prefix, the method returns an empty string.

Write a main method that prompts the user to enter two strings and display their common prefix.

9.29** (New string split method) The split method in the String class returns an array of strings consisting of the substrings split by the delimiters. However, the delimiters are not returned. Implement the following new method that returns an array of strings consisting of the substrings split by the matches, including the matches.

public static String[] split(String s, String regex)

For example, split("ab#12#453", "#") returns ab, #, 12, #, 453 in an array of String, and split("a?b?gf#e", "[?#]") returns a, b, ?, b, gf, #, and e in an array of String.

9.30** (Financial: credit card number validation) Rewrite Exercise 5.31 using a string input for credit card number. Redesign the program using the following method:

/** Return true if the card number is valid */ public static boolean isValid(String cardNumber)

/** Get the result from Step 2 */

public static int sumOfDoubleEvenPlace(String cardNumber)

/** Return this number if it is a single digit; otherwise, * return the sum of the two digits */

public static int getDigit(int number)

/** Return sum of odd place digits in number */ public static int sumOfOddPlace(String cardNumber)

9.31*** (Game: hangman) Write a hangman game that randomly generates a word and prompts the user to guess one letter at a time, as shown in the sample run. Each letter in the word is displayed as an asterisk. When the user makes a correct guess, the actual letter is then displayed. When the user finishes a word, display the number of misses and ask the user whether to continue for another word. Declare an array to store words, as follows:

// Use any words you wish

String[] words = {"write", "that", ...};