Answers to Questions and Exercises: Characters and Strings - Java Tutorial 5.0 英文版

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Answers to Questions and Exercises: Characters and Strings

Questions

Question 1: What is the initial capacity of the following string buffer?

StringBuffer sb = new StringBuffer("Able was I ere I saw Elba.");

Answer 1: It's the length of the initial string + 16: 26 + 16 = 42.

Question 2: Consider the following string:

String hannah = "Did Hannah see bees? Hannah did.";

Question 2a: What is the value displayed by the expression hannah.length()?
Answer 2a: 32.

Question 2b: What is the value returned by the method call hannah.charAt(12)?
Answer 2b: e.

Question 2c: Write an expression that refers to the letter b in the string referred to by hannah.
Answer 2c: hannah.charAt(15).

Question 3: How long is the string returned by the following expression? What is the string?

"Was it a car or a cat I saw?".substring(9, 12)

Answer 3: It's 3 characters in length: car. It does not include the space after car.

Question 4: In the following program, called ComputeResult, what is the value of result after each numbered line executes?

public class ComputeResult {
    public static void main(String[] args) {
        String original = "software";
        StringBuffer result = new StringBuffer("hi");
        int index = original.indexOf('a');

/*1*/   result.setCharAt(0, original.charAt(0));
/*2*/   result.setCharAt(1, original.charAt(original.length()-1));
/*3*/   result.insert(1, original.charAt(4));
/*4*/   result.append(original.substring(1,4));
/*5*/   result.insert(3, (original.substring(index, index+2) + " "));

        System.out.println(result);
    }
}

Answer 4:

1. si
2. se
3. swe
4. sweoft
5. swear oft

Exercises

Exercise 1: Show two ways to concatenate the following two strings together to get the string "Hi, mom.":

String hi = "Hi, ";
String mom = "mom.";

Answer 1: hi.concat(mom) and hi + mom.

Exercise 2: Write a program that computes your initials from your full name and displays them.
Answer 2: ComputeInitials

public class ComputeInitials {
    public static void main(String[] args) {
        String myName = "Fred F. Flintstone";
        StringBuffer myInitials = new StringBuffer();
        int length = myName.length();

        for (int i = 0; i < length; i++) {
            if (Character.isUpperCase(myName.charAt(i))) {
                myInitials.append(myName.charAt(i));
            }
        }
        System.out.println("My initials are: " + myInitials);
    }
}

Exercise 3: An anagram is a word or a phrase made by transposing the letters of another word or phrase; for example, "parliament" is an anagram of "partial men," and "software" is an anagram of "swear oft." Write a program that figures out whether one string is an anagram of another string. The program should ignore white space and punctuation.
Answer 3: Anagram

 
/** 
 * This class compiles with v 1.2 and 1.3, but not previous
 * versions, because its sort method uses the Arrays class,
 * which was added in 1.2.  To make Anagram work with early
 * versions of the Java platform, you need to reimplement
 * the sort method. 
 */

public class Anagram {

    /**
     * Tests whether the passed-in strings are anagrams --
     * containing the exact same number of each letter.
     * Punctuation, case, and (obviously) order don't matter.
     * 
     * @return true if the strings are anagrams; otherwise, false
     */
    public static boolean areAnagrams(String string1,
                                      String string2) {

        String workingCopy1 = removeJunk(string1);
        String workingCopy2 = removeJunk(string2);

	workingCopy1 = workingCopy1.toLowerCase();
	workingCopy2 = workingCopy2.toLowerCase();

	workingCopy1 = sort(workingCopy1);
	workingCopy2 = sort(workingCopy2);

        return workingCopy1.equals(workingCopy2);
    }

    /**
     * Removes punctuation, spaces -- everything except letters
     * and digits from the passed-in string.
     * 
     * @return a stripped copy of the passed-in string
     */
    protected static String removeJunk(String string) {
        int i, len = string.length();
  	StringBuffer dest = new StringBuffer(len);
	char c;

	for (i = (len - 1); i >= 0; i--) {
	    c = string.charAt(i);
	    if (Character.isLetterOrDigit(c)) {
		dest.append(c);
	    }
	}

        return dest.toString();
    }

    /** 
     * Sorts the passed-in string.  Reimplement this method
     * if you want to use this class in pre-Java-2 versions
     * of the platform.
     * 
     * @return a sorted copy of the passed-in string
     */
    protected static String sort(String string) {
	int length = string.length();
        char[] charArray = new char[length];

	string.getChars(0, length, charArray, 0);

	//NOTE: The following line of code causes pre-1.2
	//compilers to choke.
	java.util.Arrays.sort(charArray);

        return new String(charArray);
    }

    public static void main(String[] args) {
        String string1 = "Cosmo and Laine:";
        String string2 = "Maid, clean soon!";

        System.out.println();
        System.out.println("Testing whether the following "
                         + "strings are anagrams:");
        System.out.println("    String 1: " + string1);
        System.out.println("    String 2: " + string2);
        System.out.println();

        if (areAnagrams(string1, string2)) {
            System.out.println("They ARE anagrams!");
        } else {
            System.out.println("They are NOT anagrams!");
        }
        System.out.println();
    }
}

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