Chapter 9 - Debugging

Question 1

59. You coded the following on line 14 of the Test.java class:

int a[2][ ] = { { 2, 7 }, { 9, 2 } }; // line 14
When you compile, you get the following message:

Test.java:14: error: ‘]’ expected
int a[2][ ] = { { 2, 7 }, { 9, 2 } }; // line 14
^
Test.java:14: error: not a statement
int a[2][ ] = { { 2, 7 }, { 9, 2 } }; // line 14
^
Test.java:14: error: ‘;’ expected
int a[2][ ] = { { 2, 7 }, { 9, 2 } }; // line 14
^
Test.java:14: error: illegal start of expression
int a[2][ ] = { { 2, 7 }, { 9, 2 } }; // line 14
^
Test.java:14: error: not a statement
int a[2][ ] = { { 2, 7 }, { 9, 2 } }; // line 14
^
Test.java:14: error: ‘;’ expected
int a[2][ ] = { { 2, 7 }, { 9, 2 } }; // line 14
^
6 errors
Explain what the problem is and how to fix it.

Answer 1

You cannot use dimensions when declaring an array

Example of correct code: int a[][] = { { 2, 7 }, { 9, 2 } };

Question 2

60. You coded the following in the Test.java class:

int [ ][ ] a = { { 1, 2, 3, 4 },
{ 10, 20, 30 } };
for ( int i = 0; i < a.length; i++ )
{
for ( int j = 0; j < a[0].length; j++ )
{
System.out.println( a[i][j] ); // line 14
}
}
The code compiles properly but when you run, you get the following output:

 1
 2
 3
 4
10
20
30
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 3
         at Test.main(Test.java: 14)
Explain what the problem is and how to fix it.

Answer 2

Correct code is a[i].length, not a[0].length.

Row 1 only has 3 elements, not 4

Question 3

61. You coded the following in the Test.java class in order to output the smallest element in the array a:

int [ ][ ] a = { { 9, 8, 7, 6 },
                 { 10, 20, 30, 40 } };
int min = a[0][0];
for ( int i = 1; i < a.length; i++ )
{
      for ( int j = 0; j < a[i].length; j++ )
      {
            if ( a[i][j] < min )
                  min = a[i][j];
      }
}
System.out.println( "The minimum is " + min );

The code compiles properly, but when you run, you get the following output:

The minimum is 9

You expected the value of min to be 6. Explain what the problem is and how to fix it.

Answer 3

Other than a[0][0], the first row is not taken into account because i is initialized to
1 in the outer loop. It should be int i = 0; not int i = 1;

Question 4

62. You coded the following in file Test.java:

int [ ][ ] a = { { 9, 8, 7, 6 },
{ 10, 20, 30, 40 } };
for ( int j = 0; j <= a[1].length; j++ )
{
if ( a[1][j] == 20 ) // line 14
{
System.out.println( “Found 20 at column index “ + j
+ “ of second row” );
}
}
The code compiles properly, but when you run, you get the following output:

Found 20 at column index 1 of second row
Exception in thread “main” java.lang.ArrayIndexOutOfBoundsException: 4
at Test.main(Test.java:14)
Explain what the problem is and how to fix it.

Answer 4

It should be j < a[1].length, not j <= a[1].length. When j is equal to
a[1].length, the index [1][j] is out of bounds.

Question 5

63. You coded the following in the Test.java class:

// cars is an ArrayList of Auto objects
// cars has already been declared and instantiated
for ( Auto a ; cars ) // line 12
{
System.out.println( a.toString( ) );
} // line 15
When you compile, you get the following message :

Test.java:12: error: ';' expected
  for ( Auto a ; cars ) // line 12
                     ^
1 error
Explain what the problems are and how to fix them.

Answer 5

Syntax error (semicolon used instead of a colon). It should be Auto a : cars,
not Auto ; cars

Question 6

64. You coded the following in the Test.java class:

ArrayList a = new ArrayList( );
a.add( “Cloudy” );
a.add( “Snowy” );
a.add( “Cloudy” );
System.out.println( “Weather is “ + a.get( 3 ) ); // line 14
The code compiles properly, but when you run, you get the following output:

Exception in thread “main” java.lang.IndexOutOfBoundsException: Index 3 out-of-bounds for length 3

Explain what the problem is and how to fix it.

Answer 6

Index 3 is out of bounds. There are only 3 elements in a; the last index is 2.

Question 7

65. You coded the following in the file Test.java:

ArrayList a = new ArrayList( );
When you compile (using Xlint), you get the following warning message:

Test.java:10: warning: [rawtypes] found raw type: ArrayList
    ArrayList a = new ArrayList( );
                               ^
    missing type arguments for generic class ArrayList
    where E is a type-variable:
    E extends Object declared in class ArrayList

Test.java:10: warning: [unchecked] unchecked conversion
ArrayList a = new ArrayList( );
^
required: ArrayList
found: ArrayList
2 warnings
Explain what the problem is and how to fix it.

Answer 7

The type of elements must be specified when calling the constructor. The
statement should be: ArrayList a = new ArrayList( );

Question 8

66. You coded the following in the file Test.java:

ArrayList a = new ArrayList( );
a.add( 2.3 );
a.add( 8.4 );
a.add( 5 ); // line 11
When you compile, you get the following message:

Test.java:11: error: no suitable method found for add(int)
a.add( 5 ); // line 11
^
Explain what the problem is and how to fix it.

Answer 8

An int cannot be converted to a Double. Change line 11 to a.add( 5.0 );

Question 9

67. You coded the following in the file Test.java:

ArrayList a = new ArrayList( );
a.add( 'X' );
a.add( 'A' );
a.add( 'V' );
a.add( 'A' );
a.set( 1, 'J' );
for ( Character c : a )
  System.out.print( c + " " );
The code compiles properly, but when you run, you get the following output:

X J V A
when you expected:

J A V A
Explain what the problem is and how to fix it.

Answer 9

Because ArrayList elements begin at index 0, the statement
a.set( 1, ‘J’ );
sets the value of the second element of the ArrayList. To set the value
of the first element, use this statement:
a.set( 0, ‘J’ );