Input, Output, and Exception Handling

Question 1

What class is at the top of the exception hierarchy?

Answer 1

Throwable is at the top of the exception hierarchy.

Question 2

Briefly explain how to use try and catch.

Answer 2

The try and catch statements work together. Program statements that you want to monitor for exceptions
are contained within a try block. An exception is caught using catch.

Question 3

What is wrong with this fragment?

// …
vals[18] = 10;
catch (ArrayIndexOutOfBoundsException exc) {
// handle error
}

Answer 3

There is no try block preceding the catch statement.

Question 4

What happens if an exception is not caught?

Answer 4

If an exception is not caught, abnormal program termination results.

Question 5

What is wrong with this fragment?

class A extends Exception { …

class B extends A { …

// …

try {
// …
}
catch (A exc) { … }
catch (B exc) { … }

Answer 5

In the fragment, a superclass catch precedes a subclass catch. Since the superclass catch will catch all subclasses too, unreachable code is created.

Question 6

Can an inner catch rethrow an exception to an outer catch?

Answer 6

Yes, an exception can be rethrown.

Question 7

The finally block is the last bit of code executed before your program ends. True or False? Explain your answer.

Answer 7

False. The finally block is the code executed when a try block ends.

Question 8

What type of exceptions must be explicitly declared in a throws clause of a method?

Answer 8

All exceptions except those of type RuntimeException and Error must be declared in a throws clause.

Question 9

What is wrong with this fragment?

class MyClass { // … }
// …
throw new MyClass();

Answer 9

MyClass does not extend Throwable. Only subclasses of Throwable can be thrown by throw.

Question 10

In question 3 of the Chapter 6 Self Test, you created a Stack class. Add custom exceptions to your class that report stack full and stack empty conditions.

Answer 10

// An exception for stack-full errors.
class StackFullException extends Exception {
int size;

StackFullException(int s) { size = s; }

public String toString() {
return “\nStack is full. Maximum size is “ +
size;
}
}
// An exception for stack-empty errors.
class StackEmptyException extends Exception {

public String toString() {
return “\nStack is empty.”;
}
}

// A stack class for characters.
class Stack {
private char[] stck; // this array holds the stack
private int tos; // top of stack

// Construct an empty Stack given its size.
Stack(int size) {
stck = new char[size]; // allocate memory for stack
tos = 0;
}

// Construct a Stack from a Stack.
Stack(Stack ob) {
tos = ob.tos;
stck = new char[ob.stck.length];

// copy elements
for(int i=0; i < tos; i++)
  stck[i] = ob.stck[i];   }

// Construct a stack with initial values.
Stack(char[] a) {
stck = new char[a.length];

for(int i = 0; i < a.length; i++) {
  try {
    push(a[i]);
  }
  catch(StackFullException exc) {
    System.out.println(exc);
  }
}   }

// Push characters onto the stack.
void push(char ch) throws StackFullException {
if(tos==stck.length)
throw new StackFullException(stck.length);
stck[tos] = ch;
tos++;
}

// Pop a character from the stack.
char pop() throws StackEmptyException {
if(tos==0)
throw new StackEmptyException();
tos–;
return stck[tos];
}
}

Question 11

What are the three ways that an exception can be generated?

Answer 11

An exception can be generated by an error in the JVM, by an error in your program, or explicitly via
a throw statement.

Question 12

What are the two direct subclasses of Throwable?

Answer 12

Error and Exception

Question 13

What is the multi-catch feature?

Answer 13

The multi-catch feature allows one catch clause to catch two or more exceptions.

Question 14

Should your code typically catch exceptions of type Error?

Answer 14

No.

Question 15

Why does Java define both byte and character streams?

Answer 15

The byte streams are the original streams defined by Java. They are especially useful for binary I/O,
and they support random-access files. The character streams are optimized for Unicode.

Question 16

Even though console input and output is text-based, why does Java still use byte streams for this purpose?

Answer 16

The predefined streams, System.in, System.out, and System.err, were defined before Java added the
character streams.

Question 17

Show how to open a file for reading bytes.

Answer 17

Here is one way to open a file for byte input:

FileInputStream fin = new FileInputStream(“test”);

Question 18

Show how to open a file for reading characters.

Answer 18

Here is one way to open a file for reading characters:

FileReader fr = new FileReader(“test”);

Question 19

Show how to open a file for random-access I/O.

Answer 19

Here is one way to open a file for random access:

randfile = new RandomAccessFile(“test”, “rw”);

Question 20

How do you convert a numeric string such as “123.23” into its binary equivalent?

Answer 20

To convert numeric strings into their binary equivalents, use the parsing methods defined by the type wrappers, such as Integer or Double.

Question 21

Write a program that copies a text file. In the process, have it convert all spaces into hyphens. Use the byte stream file classes. Use the traditional approach to closing a file by explicitly calling close( ).

Answer 21

/* Copy a text file, substituting hyphens for spaces.

This version uses byte streams.

To use this program, specify the name
of the source file and the destination file.
For example,

java Hyphen source target
*/

import java.io.*;

class Hyphen {
public static void main(String[] args)
{
int i;
FileInputStream fin = null;
FileOutputStream fout = null;

// First make sure that both files have been specified.
if(args.length !=2 ) {
  System.out.println("Usage: Hyphen From To");
  return;
}

// Copy file and substitute hyphens.
try {
  fin = new FileInputStream(args[0]);
  fout = new FileOutputStream(args[1]);

  do {
    i = fin.read();

    // convert space to a hyphen
    if((char)i == ' ') i = '-';

    if(i != -1) fout.write(i);
  } while(i != -1);
} catch(IOException exc) {
  System.out.println("I/O Error: " + exc);
} finally {
  try {
    if(fin != null) fin.close();
  } catch(IOException exc) {
    System.out.println("Error closing input file."); 
  } 

  try {
    if(fin != null) fout.close();
  } catch(IOException exc) {
    System.out.println("Error closing output file.");
  }
}   } }

Question 22

Rewrite the program in question 7 so that it uses the character stream classes. This time, use the try-with-resources statement to automatically close the file.

Answer 22

/* Copy a text file, substituting hyphens for spaces.

This version uses character streams.

To use this program, specify the name
of the source file and the destination file.
For example,

java Hyphen2 source target

*/

import java.io.*;

class Hyphen2 {
public static void main(String[] args)
throws IOException
{
int i;

// First make sure that both files have been specified.
if(args.length !=2 ) {
  System.out.println("Usage: CopyFile From To");
  return;
} // Copy file and substitute hyphens.
// Use the try-with-resources statement.
try (FileReader fin = new FileReader(args[0]);
     FileWriter fout = new FileWriter(args[1]))
{
  do {
    i = fin.read();

    // convert space to a hyphen 
    if((char)i == ' ') i = '-'; 

    if(i != -1) fout.write(i);
  } while(i != -1);
} catch(IOException exc) {
  System.out.println("I/O Error: " + exc);
}

Question 23

What type of stream is System.in?

Answer 23

InputStream

Question 24

What does the read( ) method of InputStream return when an attempt is made to read at the end of the stream?

Answer 24

–1

Question 25

What type of stream is used to read binary data?

Answer 25

DataInputStream

Question 26

Reader and Writer are at the top of the ____________ class hierarchies.

Answer 26

character-based I/O

Question 27

The try-with-resources statement is used for ___________ ____________ ____________.

Answer 27

automatic resource management

Question 28

If you are using the traditional method of closing a file, then closing a file within a finally block is generally a good approach. True or False?

Answer 28

True