1. Java Gotcha's By Rick Mercer with help from the book:
Java™ Puzzlers: Traps, Pitfalls, and Corner Cases Joshua Bloch and Neal Gafter

2. Every programming language has its quirks.
Are you a code sleuth?
Have you ever spent days chasing a bug caused by a trap or pitfall in Java or its libraries?
Here are some diabolical puzzles

3. Oddity // Does this method return true if n is odd?
public boolean isOdd(int n) {
return n % 2 == 1; }

4. Ö Yes, but it's wrong when n is negative // Try this
return n % 2 != 0;
// or use bitwise AND operation (faster)
return (n & 1) == 1;
// n & 1 ands the rightmost bit with 1
// if n is 25, n & 1 is 1
// & is
// Expression is 0 unless n is odd
// if n is 24, n & 1 is 0
// & is

5. Simple Arithmetic @Test public void simpleArithmetic() {
// Does this assertion pass?
assertEquals(444, l); }

6. Ö Eyes Deceive It is 123 + 32L, which is 155
Use L instead of l for Long

7. The laughs are on me @Test public void simpleChars() {
// Which, if any, of these two assertions pass?
assertEquals("Ha", "H" + "a"); // a.
assertEquals("Ha", 'H' + 'a'); // b. }

8. Ö Answer Only a. java.lang.AssertionError:
expected:<Ha> but was:<169>

9. Operator Precedence @Test public void stringIntern() {
String pig = "length: 10";
String dog = "length: " + pig.length();
// Which, if any, of these two assertions pass?
assertEquals("Animals equal: false",
"Animals equal: " + pig == dog);
assertEquals("Animals equal: true", }

10. Ö
Neither. Precedence rules have + evaluating before ==. The actual value (2nd arg to assert) is
"Animals equal: " + pig == dog
which evaluates to
"Animals equal: pig" == dog
which as an argument evaluates to false
assertEquals uses the equals method
when the type do not match, equals returns false
"string".equals(true) is false
These asssertions pass
assertEquals("Animals equal: false", "Animals equal: " + (pig == dog));
assertEquals("Animals equal: true", "Animals equal: " + (pig.equals(dog)));

11. From a 227 Student Can this recursive solution ever work?
// Part of escape obstacle course in findExit
if(escape = false)
findExit(r-1, c); // row above
findExit(r, c+1); // col to the right
Can this recursive solution ever work?

12. Ö No The boolean expressions are ALWAYS false
An assignment statement evaluates to the value of the right value (expression to the right of =)
What is the value of booleanVar = true

13. From a 127B Student Can this recursive solution ever work? @Test
public void testRecursion() {
assertEquals(6, sumInts(3)); }
int sumInts(int n) {
if(n <= 1)
return n;
else
return n * sumInts(n--);
Can this recursive solution ever work?

14. Ö No n is not decremented until after the function call Use f(n-1)
f(n--) results in a StackOverflowError
Use f(n-1)

15. Output from this program?
\u0070\u0075\u0062\u006c\u0069\u0063\u0020\u0020\u0020\u0020
\u0063\u006c\u0061\u0073\u0073\u0020\u0055\u0067\u006c\u0079
\u007b\u0070\u0075\u0062\u006c\u0069\u0063\u0020\u0020\u0020
\u0020\u0020\u0020\u0020\u0073\u0074\u0061\u0074\u0069\u0063
\u0076\u006f\u0069\u0064\u0020\u006d\u0061\u0069\u006e\u0028
\u0053\u0074\u0072\u0069\u006e\u0067\u005b\u005d\u0020\u0020
\u0020\u0020\u0020\u0020\u0061\u0072\u0067\u0073\u0029\u007b
\u0053\u0079\u0073\u0074\u0065\u006d\u002e\u006f\u0075\u0074
\u002e\u0070\u0072\u0069\u006e\u0074\u006c\u006e\u0028\u0020
\u0022\u0048\u0065\u006c\u006c\u006f\u0020\u0077\u0022\u002b
\u0022\u006f\u0072\u006c\u0064\u0022\u0029\u003b\u007d\u007d

16. Ö Answer: Unicode not very readable
Hello World
\u0070 in hexadecimal is 112 in decimal or the character 'p'
Unicode not very readable
Suggestion" avoid Unicode till you need it
System.out.println( " \u20ac" ); €

17. '\u0040');
assertEquals('A', '\u0041');
assertEquals('B', '\u0042');
assertEquals('`', '\u0060');
assertEquals('a', '\u0061');
assertEquals('b', '\u0062');
assertEquals('€', '\u20ac');

18. Is “true” true? @Test public void trueOr() {
// Does this assertion pass?
assertEquals("Compare 5 to 4", "true", 5 > 4); }

19. Ö Answer "true" is not true However, JUnit show this: No
java.lang.AssertionError:
Compare 5 to 4 expected:<true> but was:<true>

20. Output? @Test public void whoopsForgotToBreak() { int choice = 2;
switch (choice) {
case 1:
System.out.println("one");
case 2:
System.out.println("two");
case 3:
System.out.println("three"); }

21. Ö Add breaks two three int choice = 2; switch (choice) { case 1:
System.out.println("one");
break;
case 2:
System.out.println("two");
case 3:
System.out.println("three"); }

22. BTW: Java 7 allows String in a switch statement String choice = "2";
switch (choice) {
case "1":
System.out.println("one");
break;
case "2":
System.out.println("two");
case "3":
System.out.println("three"); }

23. Java plus plus @Test public void thatDarnedPostIncrement() {
int j = 0;
for (int i = 0; i < 10; i++)
j = j++;
// Does this assertion pass?
assertEquals(10, j); }

24. Ö Answer: No Use ++j; instead j = j++; is postfix increment operator
j is 0 after the loop
When you use a postfix operator as part of a larger expression, the expression's value is returned before the postfix operator is processed
the assignment completes before the increment
Use ++j; instead

25. Output? int j = 0; int k = 0; System.out.println(j++); //? _____
System.out.println(++k); //? _____
System.out.println(j); //? _____
int[] x = { 5, 4, 3 };
int i = 0;
System.out.println(i + " " + x[i++]); //? _____
System.out.println(i + " " + x[i]); //? _____
System.out.println(i + " " + x[++i]); //? _____

26. O
0 1 With array

27. Is there any Output? public class Huh {
public static void main(String[] args) {
new B(); }
class B {
int j;
String s; {
System.out.println("Hello world " + j + " " + s);

28. Ö Answer: Yes This is an initializer, a method with no heading; { }
Hello world 0 null
This is an initializer, a method with no heading;
{ }

29. Add to 0 three times @Test public void testBigInt() {
BigInteger five = new BigInteger("5");
BigInteger fifty = new BigInteger("50");
BigInteger fiveHundred = new BigInteger("500");
BigInteger total = BigInteger.ZERO;
total.add(five);
total.add(fifty);
total.add(fiveHundred);
// Does this assertion pass?
assertEquals(555, total); }

30. Ö No BigInteger, like String is immutable This will pass
BigInteger total = BigInteger.ZERO;
total = total.add(five);
total = total.add(fifty);
total = total.add(fiveHundred);
// Does this assertion pass
assertEquals(555, total);

31. No Warning @Test public void testHashMap() {
HashMap<String, BigInteger> hm =
new HashMap<String, BigInteger>();
hm.put("a", new BigInteger("123456"));
hm.put("b", new BigInteger(" "));
hm.put("c", new BigInteger(" "));
hm.put("a", new BigInteger("654321"));
BigInteger aBigInt = hm.get("a");
// Does this assertion pass?
assertEquals(123456, aBigInt.intValue()); }

32. Ö No, the first mapping was destroyed
// Return old value if the key exists
// return null if there was no mapping to the key
hm.put("a", new BigInteger("123456"));
BigInteger bi = hm.put("a", new BigInteger("9999"));
// put returned the old value mapped to the key "a"
assertEquals(123456, bi.intValue());

33. Output? int n = 0; try { n = n / 0; } catch (Exception e) {
System.out.println("A");
} finally {
System.out.println("B"); }
System.out.println("C");

34. Ö A B C
Finally blocks always execute unless System.exit(0) is encountered first

35. Output? int n = 0; try { n = n / 999; } catch (Exception e) {
System.out.println("A");
} finally {
System.out.println("B"); }
System.out.println("C");

36. Ö B C

37. Output int n = 0; try { n = n / 999; } catch (Exception e) {
System.out.println("A");
System.exit(0);
} finally {
System.out.println("B"); }
System.out.println("C");

38. Ö B C

39. Output int n = 0; try { n = n / 0; } catch (Exception e) {
System.out.println("A");
System.exit(0);
} finally {
System.out.println("B"); }
System.out.println("C");

40. Ö A

41. To intern or not to intern
String data = new String("123"); String moreData = new String("123"); System.out.println(data==moreData);
To intern or not to intern
Which assertion(s) pass?
1 and 2
1 only
2 only
Neither
@Test
public void stringIntern() {
String s1 = "UofA";
String s2 = new String("UofA");
assertTrue(s1.equals(s2)); // 1
assertTrue(s1 == s2); // 2 }

42. Ö
b) 1 only
== compares reference values, with new, a new string is created. Without new, Java tries to find the characters in the string pool where all instances of String are stored. If found, Java returns a reference to the existing instance
In Java, String is a Flyweight
to save memory
Both of these assertions pass:
String s1 = "UofA";
String s2 = "UofA";
assertTrue(s1.equals(s2)); // 1
assertTrue(s1 == s2); // 2

43. To intern or not to intern
@Test
public void testIntegerInterns() {
Integer a = -128;
Integer b = -128;
Integer c = 127;
Integer d = 127;
Integer e = 345;
Integer f = 345;
// Which, if any, of these assertions fail?
assertTrue(a == b); // a.
assertTrue(c == d); // b.
assertTrue(e >= f); // c.
assertTrue(e == f); // d. }

44. Ö
d only
== compares reference values so you would think all 3 fail, but....
java.sun.com/docs/books/jls/download/langspec-3.0.pdf explicitly states that wrappers for values in the range -128 to 127 will be interned by any JVM. If you use the int literal, you get a reference to that existing instance
Flyweight: Do not create a new Integer(1), just return a reference to that instance from the existing pool of Integers
to save memory

45. Are Doubles interned? @Test public void testEqualEquals() {
Double a = 4.2;
Double b = 4.2;
// Which, if any, of these assertions fail?
assertTrue(a >= b); // a.
assertTrue(a <= b); // b.
assertTrue(a.equals(b)); // c.
assertTrue(a == b); // d.
assertTrue(a.compareTo(b) == 0); // e.
assertTrue(a.compareTo(4.2) == 0); // f. }

46. Ö The == compares reference values, not the numeric values
assertTrue(a == b); // d. fails
The == compares reference values, not the numeric values
Why does a == b evaluate to false?
There is no Double pool
a and b refer to two different objects with the same value
== compares references, not the 4.2s