TOP 30 Java Interview Coding Tasks with Winning Solutions.pdf

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Matthew Urban
1. Reverse a text.
From my experience, reverse a text is a very popular coding task used during
job interview. Text manipulation methods are the ones mostly used by pro-
grammers. Although you do not need to implement them, it is desired to un-
derstand how such operations are performed under the hood. The low-level
details summed together have a significant impact on overall system per-
formance.
Solution
The
String
class represents a text value in Java which is built from an array
of characters. To implement a custom method to reverse a
String
you also
have to operate on an array of characters. There are many solutions to re-
verse a
String
, but the most optimal should not allocate additional memory
if there is no need. It is recommended to take an input
char[]
array, iterate
through it and switch the first element with the last, the second element
with the penultimate, etc. until half of the array is reached as presented in
Listing 1.1.
Listing 1.1
– Reverse a text algorithm.
public class
StringUtils {
public static
String reverse(String input) {
if
(input ==
null)
{
return
"";
}
char[]
array = input.toCharArray();
final
int
halfLength = array.length/2;
int
idx2;
char
clipboard;
for
(int idx1=0; idx1 < halfLength; idx1++) {
idx2 = array.length -
1
- idx1;
clipboard = array[idx1];
array[idx1] = array[idx2];
array[idx2] = clipboard;
}
return
String.valueOf(array);
}
}
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Top 30 Java Interview Coding Tasks With Winning Solutions
Tests
The
StringUtils.reverse()
method is a good example of code which is
easy to test with unit tests. Listing 1.2 presents an example implementation
of such a unit test. In this case, a test is parametrized with two variables:
expected
and
input
values. The
data()
method, is a factory method which
returns all combinations of input and expected data that need to be tested.
Remember, preparing test cases and a method body simultaneously (TDD)
results in high-quality code.
Listing 1.2
– Unit test used to verify the
StringUtils.reverse()
method.
@RunWith(Parameterized.class)
public class
StringUtilsReverseTest {
@Parameters(name
=
"({index}): reverse({1})={0}")
public static
Collection data() {
return
Arrays.asList(new Object[][]{
{"",
null},
{"",
""},
{"a",
"a"},
{"1234567890",
"0987654321"},
{"aBbA",
"AbBa"}
});
}
@Parameter(value
=
0)
public
String expected;
@Parameter(value
=
1)
public
String input;
@Test
public void
reverse() {
assertEquals(expected,
StringUtils.reverse(input));
}
}
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