Chapter 9 Strings and Text I/O
1.
s1 == s2 => true
s2 == s3 => false
s1.equals(s2) => true
s2.equals(s3) => true
s1.compareTo(s2) => 0
s2.compareTo(s3) => 0
s1 == s4 => true
s1.charAt(0) => W
s1.indexOf('j') => -1
s1.indexOf("to") => 8
s1.lastIndexOf('a') => 14
s1.lastIndexOf("o", 15) => 9
s1.length() => 16
s1.substring(5) => me to Java
s1.substring(5, 11) => me to
s1.startsWith("Wel") => true
s1.endsWith("Java") => true
s1.toLowerCase() => welcome to java!
s1.toUpperCase()=> WELCOME TO JAVA!
" Welcome ".trim() => Welcome
s1.replace('o', 'T') => WelcTme tT Java!
s1.replaceAll("o", "T") => WelcTme tT Java!
s1.replaceFirst("o", "T") => WelcTme tT Java!
s1.toCharArray() returns an array of characters consisting of W, e, l, c, o, m, e, , t, o, , J, a, v, a
(Note that none of the operation causes the contents of a string to change)
2. String s = new String("new string");
Answer: Correct
String s3 = s1 + s2;
Answer: Correct
String s3 = s1 - s2;
Answer: Incorrect
s1 == s2
Answer: Correct
s1 >= s2
Answer: Incorrect
s1.compareTo(s2);
Answer: Correct
int i = s1.length();
Answer: Correct
char c = s1(0);
Answer: Incorrect
char c = s1.charAt(s1.length());
Answer: Incorrect : it's out of bounds, even if the preceding problem is fixed.
- The output is
Welcome to Java
Welcabcme tabc Java
Hint: No method in the String class can change the content of the string. String is an immutable class.
- Check whether s1 is equal to s2 and assign the result to a Boolean variable isEqual.
boolean isEqual = s1.equals(s2);
- Check whether s1 is equal to s2 ignoring case and assign the result to a Boolean variable isEqual.
boolean isEqual = s1.equalsIgnoreCase(s2);
- Compare s1 with s2 and assign the result to an int variable x.
int x = s1.compareTo(s2);
- Compare s1 with s2 ignoring case and assign the result to an int variable x.
int x = s1.compareToIgnoreCase(s2);
- Check whether s1 has prefix "AAA" and assign the result to a Boolean variable b.
boolean b = s1.startsWith("AAA");
- Check whether s1 has suffix "AAA" and assign the result to a Boolean variable b.
boolean b = s1.endsWith("AAA");
- Assign the length of s1 to an int variable x.
int x = s1.length();
- Assign the first character of s1 to a char variable x.
char x = s1.charAt(0);
- Create a new string s3 that combines s1 with s2.
String s3 = s1 + s2;
- Create a substring of s1 starting from index 1.
String s3 = s1.substring(1);
- Create a substring of s1 from index 1 to index 4.
String s3 = s1.substring(1, 5);
- Create a new string s3 that converts s1 to lowercase.
String s3 = s1.lowercase();
- Create a new string s3 that converts s1 to uppercase.
String s3 = s1.uppercase();
- Create a new string s3 that trims blank spaces on both ends of s1.
String s3 = s1.trim();
- Replace all occurrence of character e with E in s1 and assign the new string to s3.
String s3 = s1.replaceAll(‘e’, ‘E’);
- Split "Welcome to Java and HTML" into an array tokens using delimited by a space.
String[] tokens = "Welcome to Java and HTML".split(‘ ‘);
- Assign the index of the first occurrence of character e in s1 to an int variable x.
int x = s1.indexOf(‘e‘);
- Assign the index of the last occurrence of string abc in s1 to an int variable x.
int x = s1.lastIndexOf(“abc”);
- No.
- 0.
- 用 String 类的静态重载方法。.
- The text is declared in Line 2 as a data field, but redeclared in Line 5 as a local variable. The local variable is assigned with the string passed to the constructor, but the data field is still null. In Line 10, test.text is null, which causes NullPointerException when invoking the toLowerCase() method.
- 不能有void.
- A lowercase letter is between ‘a’ and ‘z’. You can use the static isLowerCase(char) method in the Character class to test if a character is in lowercase. An uppercase letter is between ‘A’ and ‘Z’. You can use the static isUpperCase(char) method in the Character class to test if a character is in uppercase.
- An alphanumeric character is between ‘0’ and ‘9’, or ‘A’ and ‘Z’, or ‘a’ and ‘z’. You can use the static isLetterOrDigit(char ch) method in the Character class to test if a character is a digit or a letter.
- The StringBuilder class, introduced in JDK 1.5, is similar to StringBuffer except that the update methods in StringBuffer are synchronized.
- Use the StringBuilder’s constructor to create a string buffer for a string, and use the toString method in StringBuilder class to return a string from a StringBuilder.
- StringBuilder sb = new StringBuilder(s);
sb.reverse();
s = sb.toString();
- StringBuilder sb = new StringBuilder(s);
sb.delete(4, 10);
s = sb.toString();
- Both string and string buffer use arrays to hold characters. The array in a string is fixed once a string is created. The array in a string buffer may change if the buffer capacity is changed. To accommodate the change, a new array is created.
17.
(1) Java is fun
(2) JavaHTML
(3) Jais funva
(4) JHTMLava
(5) v
(6) 4
(7) Jav
(8) Ja
(9) avaJ
(10) JComputera
(11) av
(12) va
18.
The output is
Java
Java and HTML
NOTE:
Inside the method, the statement s = s + " and HTML" creates a new String object s, which is different from the original String object passed to the change(s, buffer) method. The original String object has not been changed. Therefore, the printout from the original string is Java.
Inside the method, the content of the StringBuilder object is changed to Java and HTML. Therefore, the printout from buffer is Java and HTML.
19.
public static void main(String[] args)
can be replaced by
public static void main(String args[])
public static void main(String[] x)
public static void main(String x[])
but not
static void main(String x[])
because it is not public.
20.
(1)
Number of strings is 4
I
have
a
dream
(2)
Number of strings is 1
1 2 3
(3)
Number of strings is 0
(4)
Number of strings is 1
*
(5)
Number of strings is (the number of files and directory from where the command is executed)
Displays all files and directory names in the directory where the command is executed.
21. \ 是特殊符号。应该用双斜杠\\表示。
- 用 exists() 。用 delete() 。 用 renameTo(File) 。使用File类不能获得文件的大小。
- File类不能进行输入输出。
- To create a PrintWriter for a file, use new PrintWriter(filename). This statement may throw an exception. Java forces you to write the code to deal with exceptions. One way to deal with it is to declare throws Exception in the method declaration. If the close() method is not invoked, the data may not be saved properly.
- The contents of the file temp.txt is:
amount is 32.320000 3.232000e+01
amount is 32.3200 3.2320e+01
false
Java
- To create a Scanner for a file, use new Scanner(new File(filename)). This statement may throw an exception. Java forces you to write the code to deal with exceptions. One way to deal with it is to declare throws Exception in the method declaration. If the close() method is not invoked, the problem will run fine. But it is a good practice to close the file to release the resource on the file.
- 会抛出异常。 If you attempt to create a Formatter for an existing file, the contents of the existing file will be gone.
28.不是，windows平台上是\r\n。
29.intValue contains 45. doubleValue contains 57.8, and line contains ' ', '7 ', '8 ', '9'.
30.intValue contains 45. doubleValue contains 57.8, and line is empty.
