Aug 17, 2012
OCJP Exercise -6 (Strings)
1. Given:
import java.util.regex.*;
class Regex2 {
public static void main(String[] args) {
Pattern p = Pattern.compile(args[0]);
Matcher m = p.matcher(args[1]);
boolean b = false;
while(b = m.find()) {
System.out.print(m.start() + m.group());
}
}
}
And the command line:
java Regex2 "\d*" ab34ef
What is the result?
A. 234
B. 334
C. 2334
D. 0123456
E. 01234456
F. 12334567
G. Compilation fails
Answer:
E is correct. The \d is looking for digits. The * is a quantifier that looks for 0 to many
occurrences of the pattern that precedes it. Because we specified *, the group() method
returns empty Strings until consecutive digits are found, so the only time group() returns
a value is when it returns 34 when the matcher finds digits starting in position 2. The
start() method returns the starting position of the previous match because, again,
we said find 0 to many occurrences.
A, B, C, D, F, and G are incorrect based on the above. (Objective 3.5)
A, B, C, D, F, and G are incorrect based on the above. (Objective 3.5)
2. Given:
import java.io.*;
class Player {
Player() { System.out.print("p"); }
}
class CardPlayer extends Player implements Serializable {
CardPlayer() { System.out.print("c"); }
public static void main(String[] args) {
CardPlayer c1 = new CardPlayer();
try {
FileOutputStream fos = new FileOutputStream("play.txt");
ObjectOutputStream os = new ObjectOutputStream(fos);
os.writeObject(c1);
os.close();
FileInputStream fis = new FileInputStream("play.txt");
ObjectInputStream is = new ObjectInputStream(fis);
CardPlayer c2 = (CardPlayer) is.readObject();
is.close();
} catch (Exception x ) { }
}
}
What is the result?
A. pc
B. pcc
C. pcp
D. pcpc
E. Compilation fails
F. An exception is thrown at runtime
Answer:
C is correct. It's okay for a class to implement Serializable even if its superclass doesn't.
However, when you deserialize such an object, the non-serializable superclass must run its
constructor. Remember, constructors don't run on deserialized classes that implement
Serializable.
A, B, D, E, and F are incorrect based on the above. (Objective 3.3)
A, B, D, E, and F are incorrect based on the above. (Objective 3.3)
3. Given:
class TKO {
public static void main(String[] args) {
String s = "-";
Integer x = 343;
long L343 = 343L;
if(x.equals(L343)) s += ".e1 ";
if(x.equals(343)) s += ".e2 ";
Short s1 = (short)((new Short((short)343)) / (new Short((short)49)));
if(s1 == 7) s += "=s ";
if(s1 < new Integer(7+1)) s += "fly ";
System.out.println(s);
} }
Which of the following will be included in the output String s? (Choose all that apply.)
A. .e1
B. .e2
C. =s
D. fly
E. None of the above
F. Compilation fails
G. An exception is thrown at runtime
Answer:
B, C, and D are correct. Remember, that the equals() method for the integer wrappers
will only return true if the two primitive types and the two values are equal. With C, it's
okay to unbox and use ==. For D, it's okay to create a wrapper object with an expression,
and unbox it for comparison with a primitive.
A, E, F, and G are incorrect based on the above. (Remember that A is using the equals() method to try to compare two different types.) (Objective 3.1)
A, E, F, and G are incorrect based on the above. (Remember that A is using the equals() method to try to compare two different types.) (Objective 3.1)
4. Given:
import java.io.*;
class Keyboard { }
public class Computer implements Serializable {
private Keyboard k = new Keyboard();
public static void main(String[] args) {
Computer c = new Computer();
c.storeIt(c);
}
void storeIt(Computer c) {
try {
ObjectOutputStream os = new ObjectOutputStream(
new FileOutputStream("myFile"));
os.writeObject(c);
os.close();
System.out.println("done");
} catch (Exception x) {System.out.println("exc"); }
}
}
What is the result? (Choose all that apply.)
A. exc
B. done
C. Compilation fails
D. Exactly one object is serialized
E. Exactly two objects are serialized
Answer:
A is correct. An instance of type Computer Has-a Keyboard. Because Keyboard doesn't
implement Serializable, any attempt to serialize an instance of Computer will cause an
exception to be thrown.
B, C, D, and E are incorrect based on the above. If Keyboard did implement Serializable then two objects would have been serialized. (Objective 3.3)
B, C, D, and E are incorrect based on the above. If Keyboard did implement Serializable then two objects would have been serialized. (Objective 3.3)
5. Using the fewest fragments possible (and filling the fewest slots possible), complete the code below so that the class builds a directory named "dir3" and creates a file named "file3" inside "dir3". Note you can use each fragment either zero or one times. Code:
import java.io.______________
class Maker {
public static void main(String[] args) {
___________ ___________ ___________
___________ ___________ ___________
___________ ___________ ___________
___________ ___________ ___________
___________ ___________ ___________
___________ ___________ ___________
___________ ___________ ___________
} }
Fragments:
| File; | FileDescriptor; | FileWriter; | Directory; |
| try { | .createNewDir(); | File dir | File |
| { } | (Exception x) | ("dir3"); | file |
| file | .createNewFile(); | = new File | = new File |
| dir | (dir, "file3"); | (dir, file); | .createFile(); |
| } catch | ("dir3", "file3"); | .mkdir(); | File file |
Answer:
import java.io.File;
class Maker {
public static void main(String[] args) {
try {
File dir = new File("dir3");
dir.mkdir();
File file = new File(dir, "file3");
file.createNewFile();
} catch (Exception x) { }
} }
Notes: The new File statements don't make actual files or directories, just objects. You need the mkdir() and createNewFile() methods to actually create the directory and the file. (Objective 3.2)
6. Given that 1119280000000L is roughly the number of milliseconds from Jan. 1, 1970, to June 20, 2005, and that you want to print that date in German, using the LONG style such that "June" will be displayed as "Juni", complete the code using the fragments below. Note: you can use each fragment either zero or one times, and you might not need to fill all of the slots. Code:
import java.___________
import java.___________
class DateTwo {
public static void main(String[] args) {
Date d = new Date(1119280000000L);
DateFormat df = ___________________________
________________ , _________________ );
System.out.println(________________
}
}
Fragments:| io.*; | new DateFormat( | Locale.LONG |
| nio.*; | DateFormat.getInstance( | Locale.GERMANY |
| util.*; | DateFormat.getDateInstance( | DateFormat.LONG |
| text.*; | util.regex; | DateFormat.GERMANY |
| date.*; | df.format(d)); | d.format(df)); |
Answer:
import java.util.*;
import java.text.*;
class DateTwo {
public static void main(String[] args) {
Date d = new Date(1119280000000L);
DateFormat df = DateFormat.getDateInstance(
DateFormat.LONG, Locale.GERMANY);
System.out.println(df.format(d));
}
}
Notes: Remember that you must build DateFormat objects using static methods. Also
remember that you must specify a Locale for a DateFormat object at the time of instantiation.
The getInstance() method does not take a Locale. (Objective 3.4)
7. Given:
import java.io.*;
class Directories {
static String [] dirs = {"dir1", "dir2"};
public static void main(String [] args) {
for (String d : dirs) {
// insert code 1 here
File file = new File(path, args[0]);
// insert code 2 here
}
}
}
and that the invocationjava Directories file2.txt
is issued from a directory that has two subdirectories, "dir1" and "dir2", and that "dir1" has a file "file1.txt" and "dir2" has a file "file2.txt", and the output is "false true", which set(s) of code fragments must be inserted? (Choose all that apply.)
A. String path = d; System.out.print(file.exists() + " ");
B. String path = d; System.out.print(file.isFile() + " ");
C. String path = File.separator + d; System.out.print(file.exists() + " ");
D. String path = File.separator + d; System.out.print(file.isFile() + " ");
Answer:
A and B are correct. Because you are invoking the program from the directory whose
direct subdirectories are to be searched, you don't start your path with a File.separator
character. The exists() method tests for either files or directories; the isFile()
method tests only for files. Since we're looking for a file, both methods work.
C and D are incorrect based on the above. (Objective 3.2)
C and D are incorrect based on the above. (Objective 3.2)
8. Given:
import java.io.*;
public class TestSer {
public static void main(String[] args) {
SpecialSerial s = new SpecialSerial();
try {
ObjectOutputStream os = new ObjectOutputStream(
new FileOutputStream("myFile"));
os.writeObject(s); os.close();
System.out.print(++s.z + " ");
ObjectInputStream is = new ObjectInputStream(
new FileInputStream("myFile"));
SpecialSerial s2 = (SpecialSerial)is.readObject();
is.close();
System.out.println(s2.y + " " + s2.z);
} catch (Exception x) {System.out.println("exc"); }
}
}
class SpecialSerial implements Serializable {
transient int y = 7;
static int z = 9;
}
Which are true? (Choose all that apply.)
A. Compilation fails
B. The output is 10 0 9
C. The output is 10 0 10
D. The output is 10 7 9
E. The output is 10 7 10
F. In order to alter the standard deserialization process you would implement the readObject() method in SpecialSerial
G. In order to alter the standard deserialization process you would implement the defaultReadObject() method in SpecialSerial
Answer:
C and F are correct. C is correct because static and transient variables are not
serialized when an object is serialized. F is a valid statement.
A, B, D, and E are incorrect based on the above. G is incorrect because you don't implement the defaultReadObject() method, you call it from within the readObject()method, along with any custom read operations your class needs. (Objective 3.3)
A, B, D, and E are incorrect based on the above. G is incorrect because you don't implement the defaultReadObject() method, you call it from within the readObject()method, along with any custom read operations your class needs. (Objective 3.3)
9. Given:
3. public class Theory {
4. public static void main(String[] args) {
5. String s1 = "abc";
6. String s2 = s1;
7. s1 += "d";
8. System.out.println(s1 + " " + s2 + " " + (s1==s2));
9.
10. StringBuffer sb1 = new StringBuffer("abc");
11. StringBuffer sb2 = sb1;
12. sb1.append("d");
13. System.out.println(sb1 + " " + sb2 + " " + (sb1==sb2));
14. }
15. }
Which are true? (Choose all that apply.)
A. Compilation fails
B. The first line of output is abc abc true
C. The first line of output is abc abc false
D. The first line of output is abcd abc false
E. The second line of output is abcd abc false
F. The second line of output is abcd abcd true
G. The second line of output is abcd abcd false
Answer:
D and F are correct. While String objects are immutable, references to Strings are mutable.
The code s1 += "d"; creates a new String object. StringBuffer objects are mutable, so the
append() is changing the single StringBuffer object to which both StringBuffer references
refer.
A, B, C, E, and G are incorrect based on the above. (Objective 3.1)
A, B, C, E, and G are incorrect based on the above. (Objective 3.1)
10. Given:
3. import java.io.*;
4. public class ReadingFor {
5. public static void main(String[] args) {
6. String s;
7. try {
8. FileReader fr = new FileReader("myfile.txt");
9. BufferedReader br = new BufferedReader(fr);
10. while((s = br.readLine()) != null)
11. System.out.println(s);
12. br.flush();
13. } catch (IOException e) { System.out.println("io error"); }
16. }
17. }
And given that myfile.txt contains the following two lines of data:
ab cd
What is the result?
A. ab
B. abcd
C. ab cd
D. a b c d
E. Compilation fails
Answer:
E is correct. You need to call flush() only when you're writing data. Readers don't have
flush() methods. If not for the call to flush(), answer C would be correct.
A, B, C, and D are incorrect based on the above. (Objective 3.2)
A, B, C, and D are incorrect based on the above. (Objective 3.2)
11. Given:
3. import java.io.*;
4. public class Talker {
5. public static void main(String[] args) {
6. Console c = System.console();
7. String u = c.readLine("%s", "username: ");
8. System.out.println("hello " + u);
9. String pw;
10. if(c != null && (pw = c.readPassword("%s", "password: ")) != null)
11. // check for valid password
12. }
13. }
If line 6 creates a valid Console object, and if the user enters fred as a username and 1234 as a
password, what is the result? (Choose all that apply.)
A.
username:
password:
B.
username: fred
password:
C.
username: fred
password: 1234
D. Compilation fails
E. An exception is thrown at runtime
Answer:
D is correct. The readPassword() method returns a char[]. If a char[] were used,
answer B would be correct.
A, B, C, and E are incorrect based on the above. (Objective 3.2)
A, B, C, and E are incorrect based on the above. (Objective 3.2)
12. Given:
3. import java.io.*;
4. class Vehicle { }
5. class Wheels { }
6. class Car extends Vehicle implements Serializable { }
7. class Ford extends Car { }
8. class Dodge extends Car {
9. Wheels w = new Wheels();
10. }
Instances of which class(es) can be serialized? (Choose all that apply.)
A. Car
B. Ford
C. Dodge
D. Wheels
E. Vehicle
Answer:
A and B are correct. Dodge instances cannot be serialized because they "have" an instance
of Wheels, which is not serializable. Vehicle instances cannot be serialized even though the
subclass Car can be.
C, D, and E are incorrect based on the above. (Objective 3.3)
C, D, and E are incorrect based on the above. (Objective 3.3)
13. Given:
3. import java.text.*;
4. public class Slice {
5. public static void main(String[] args) {
6. String s = "987.123456";
7. double d = 987.123456d;
8. NumberFormat nf = NumberFormat.getInstance();
9. nf.setMaximumFractionDigits(5);
10. System.out.println(nf.format(d) + " ");
11. try {
12. System.out.println(nf.parse(s));
13. } catch (Exception e) { System.out.println("got exc"); }
14. }
15. }
Which are true? (Choose all that apply.)
A. The output is 987.12345 987.12345
B. The output is 987.12346 987.12345
C. The output is 987.12345 987.123456
D. The output is 987.12346 987.123456
E. The try/catch block is unnecessary
F. The code compiles and runs without exception
G. The invocation of parse() must be placed within a try/catch block
Answer:
D, F, and G are correct. The setMaximumFractionDigits() applies to the formatting
but not the parsing. The try/catch block is placed appropriately. This one might scare you
into thinking that you'll need to memorize more than you really do. If you can remember
that you're formatting the number and parsing the string you should be fine for the exam.
A, B, C, and E are incorrect based on the above. (Objective 3.4)
A, B, C, and E are incorrect based on the above. (Objective 3.4)
14. Given:
3. import java.util.regex.*;
4. public class Archie {
5. public static void main(String[] args) {
6. Pattern p = Pattern.compile(args[0]);
7. Matcher m = p.matcher(args[1]);
8. int count = 0;
9. while(m.find())
10. count++;
11. System.out.print(count);
12. }
13. }
And given the command line invocation:
java Archie "\d+" ab2c4d67
What is the result?
A. 0
B. 3
C. 4
D. 8
E. 9
F. Compilation fails
Answer:
B is correct. The "\d" metacharacter looks for digits, and the + quantifier says look for
"one or more" occurrences. The find() method will find three sets of one or more consecutive
digits: 2, 4, and 67.
A, C, D, E, and F are incorrect based on the above. (Objective 3.5)
A, C, D, E, and F are incorrect based on the above. (Objective 3.5)
15. Given:
import java.util.*;
public class Looking {
public static void main(String[] args) {
String input = "1 2 a 3 45 6";
Scanner sc = new Scanner(input);
int x = 0;
do {
x = sc.nextInt();
System.out.print(x + " ");
} while (x!=0);
}
}
What is the result?
A. 1 2
B. 1 2 3 45 6
C. 1 2 3 4 5 6
D. 1 2 a 3 45 6
E. Compilation fails
F. 1 2 followed by an exception
Answer:
F is correct. The nextXxx() methods are typically invoked after a call to a hasNextXxx(),
which determines whether the next token is of the correct type.
A, B, C, D, and E are incorrect based on the above. (Objective 3.5)
A, B, C, D, and E are incorrect based on the above. (Objective 3.5)
.
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