Java Gotcha's

1
Java Gotcha's

• Most of these examples are from this book
• Java Puzzlers Traps, Pitfalls, and Corner Cases
  Author(s) Joshua Bloch and Neal Gafter
  Publisher Addison Wesley Professional Date 24
  June 2005

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 bitmask with and operation '' (faster)
• return (n 1) ! 0
• // n 1 ands the rightmost bit with 1
• // if n 25, n 1 is
• // 00011001 00000001 00000001
• // Expression is 0 unless n is odd
• // if n 24, n 1 is
• // 00011000 00000001 00000000

5
Simple Arithmetic

• _at_Test
• public void simpleArithmetic()
• // Does this assertion pass?
• assertEquals(444, 123 32l)

6
Ö

• Eyes Deceive
• It is 123 32L, which is 155
• Use L instead of l for Long

7
The laughs are on me

• _at_Test
• public void simpleChars()
• // Which, if any, of these 2 assertion pass?
• assertEquals("Ha", "H" "a") // a.
• assertEquals("Ha", 'H' 'a') // b.

8
Ö

• Answer
• Only a.
• java.lang.AssertionError
• expectedltHagt but waslt169gt

9
String constants interned

• _at_Test
• public void stringIntern()
• String pig "length 10"
• String dog "length " pig.length()
• // Which, if any, of these 2 assertion pass?
• assertEquals("Animals equal false",
• "Animals equal " pig dog)
• assertEquals("Animals equal true",
• "Animals equal " pig dog)

10
Ö

• Neither
• goes before the actual value is
• ("Animals equal " pig) dog

11
To intern or not to intern

• _at_Test
• public void testIntegerInterns()
• Integer a -128
• Integer b -128
• Integer c 127
• Integer d 127
• Integer e 345
• Integer f 345
• // 1. Which, if any, of these assertions fail?
• assertTrue(a b) // a.
• assertTrue(c d) // b.
• assertTrue(e f) // c.

12
Ö

• 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.

13
Are Doubles interned?

• _at_Test
• public void testEqualEquals()
• Double a 4.2
• Double b 4.2
• // 2. Which, if any, of these assertions
  fail?
• assertTrue(a gt b) // a.
• assertTrue(a lt b) // b.
• assertTrue(a.equals(b)) // c.
• assertTrue(a b) // d.
• assertTrue(a.compareTo(b) 0) // e.
• assertTrue(a.compareTo(4.2) 0) // f.

14
Ö

• assertTrue(a b) // d. Does not pass
• The compares reference values, not the numeric
  values

15
Transitivity, what Transitivity?

• _at_Test
• public void testTransitivity()
• double a 4.2
• Double b a
• Double c a
• // 2. Which, if any, of these assertions fail?
• assertTrue(b a) // a.
• assertTrue(c a) // b.
• assertTrue(b c) // c.

16
Ö

• Consistent?
• // Which, if any, of these lines does not compile
• int n2 0 // a.
• double x 0 // b.
• Integer n 0 // c.
• Double x 0 // d.

17
Output from this program?

• \u0070\u0075\u0062\u006c\u0069\u0063\u0020\u0020\u
  0020\u0020
• \u0063\u006c\u0061\u0073\u0073\u0020\u0055\u0067\u
  006c\u0079
• \u007b\u0070\u0075\u0062\u006c\u0069\u0063\u0020\u
  0020\u0020
• \u0020\u0020\u0020\u0020\u0073\u0074\u0061\u0074\u
  0069\u0063
• \u0076\u006f\u0069\u0064\u0020\u006d\u0061\u0069\u
  006e\u0028
• \u0053\u0074\u0072\u0069\u006e\u0067\u005b\u005d\u
  0020\u0020
• \u0020\u0020\u0020\u0020\u0061\u0072\u0067\u0073\u
  0029\u007b
• \u0053\u0079\u0073\u0074\u0065\u006d\u002e\u006f\u
  0075\u0074
• \u002e\u0070\u0072\u0069\u006e\u0074\u006c\u006e\u
  0028\u0020
• \u0022\u0048\u0065\u006c\u006c\u006f\u0020\u0077\u
  0022\u002b
• \u0022\u006f\u0072\u006c\u0064\u0022\u0029\u003b\u
  007d\u007d

18
Ö

• Answer
• Hello World
• \u0070 in hexadecimal is 112 in decimal or the
  character 'c'
• Unicode not readable
• Suggest avoid Unicode unless you need it
• System.out.println("\u20ac" 123.45)
• 123.45

19
assertEquals('_at_', '\u0040') assertEquals('A',
'\u0041') assertEquals('B', '\u0042')
assertEquals('', '\u0060') assertEquals('a',
'\u0061') assertEquals('b', '\u0062')
assertEquals('', '\u20ac')
20
What's the Output

• package edu.arizona.cs335
• public class WhatsUp
• public static void main(String args)
• System.out.println(WhatsUp.class.getName())
• System.out.println(
• WhatsUp.class.getName().replaceAll(".",
• "/") ".class")
• edu.arizona.cs335.WhatsUp.class
• What is output from 2nd println?

21
Ö

• Answer
• /////////////////////////.class
• The first argument to replace all is a regular
  expression
• Try this instead
• replaceAll("\\.", "/")
• edu/arizona/cs335/WhatsUp.class

22
A little regex

• Pattern p Pattern.compile(".....") // 5 of any
• Matcher m p.matcher("A string to look at.")
• m.find()
• assertEquals("A str", m.group())
• p Pattern.compile("1-9") // the first digit
• m p.matcher("first 2nd 3rd")
• m.find()
• assertEquals("2", m.group())

23
Is true true?

• _at_Test
• public void trueOr()
• // Does this assertion pass?
• assertEquals("Compare 5 to 4", "true", 5 gt 4)

24
Ö

• Answer
• No
• "true" is not true
• JUnit show this
• java.lang.AssertionError
• Compare 5 to 4 expectedlttruegt but waslttruegt

25
Output?

• _at_Test
• public void simpleChars()
• int choice 2
• switch (choice)
• case 1
• System.out.println("one")
• case 2
• System.out.println("two")
• case 3
• System.out.println("three")

26
Ö

• two
• three
• Add breaks
• int choice 2
• switch (choice)
• case 1
• System.out.println("one")
• break
• case 2
• System.out.println("two")
• break
• case 3
• System.out.println("three")
• break

27
Java plus plus

• _at_Test
• public void simpleChars()
• int j 0
• for (int i 0 i lt 10 i)
• j j
• // Does this assertion pass
• assertEquals(10, j)

28
Ö

• Answer No
• j j is postfix increment operator
• When you use a postfix operator as part of a
  larger expression, the expression's value is
  returned before the postfix operator is processed
• Use j

29
Output?

• int j 0
• int k 0
• System.out.println(j) //? _____
• System.out.println(k) //? _____
• System.out.println(j) //? _____
• int x 1, 2, 3
• int i 0
• System.out.println(i " " xi) //? _____
• System.out.println(i " " xi) //? _____

30
Output?

• public class PuzzlersTrapsPitfallsCornerCases
• public static final int END
  Integer.MAX_VALUE
• public static final int START END - 100
• _at_Test
• public void simpleChars()
• int count 0
• for (int i START i lt END i)
• count
• System.out.println(count)

31
Ö

• Answer No output
• Even with wraparound
• It never ends
• Frustrated?
• "I laughed, I cried, I threw up (my hands in
  admiration)."
• Tim Peierls, president, Prior Artisans LLC, and
  member of the JSR 166 Expert Group How well do
  you really know Java?

32
Is there any Output?

• public class Huh
• public static void main(String args)
• new B()
• public class B
• int j
• String s
• System.out.println("Hello world " j " "
  s)

33
Ö

• Answer Yes
• Hello world 0 null
• This is an initializer, a method with no heading
  

34
Add to 0 three times

• _at_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)

35
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)

36
No Warning

• _at_Test
• public void testHashMap()
• HashMapltString, BigIntegergt hm
• new HashMapltString,
  BigIntegergt()
• hm.put("a", new BigInteger("123456"))
• hm.put("b", new BigInteger("1234567"))
• hm.put("c", new BigInteger("1234567"))
• hm.put("a", new BigInteger("654321"))
• BigInteger aBigInt hm.get("a")
• // Does this assertion pass?
• assertEquals(123456, aBigInt.intValue())

37
Ö

• No, the first mapping was destroyed
• // Return old value null if there was a mapping
• hm.put("a", new BigInteger("123456"))
• BigInteger bi hm.put("a", new
  BigInteger("9999"))
• int actual bi.intValue()
• assertFalse(9999 actual)
• // Return null if there was no mapping with the
  key
• BigInteger bi2 hm.put("key", new
  BigInteger("123"))
• assertNull(bi2)

38
Which is/are valid classes

• // in E.java
• public class E
• private class F

• // in A.java
• public class A
• // in B.java
• public static class B
• // in C.java
• public class C
• private static class D

39
Ö

• A
• C and D
• E and F
• For B, you get this error because the top level
  class in a file can not be static
• Illegal modifier for the class B only public,
  abstract final are permitted

40
Which is/are valid classes?

• // in C.java
• public class C
• int n 0
• private class D
• public D()
• n

• // in C.java
• public class C
• int n 0
• private static class D
• public D()
• n

41
Ö

• The right column
• The left column has an error at n
• Cannot make a static reference to the non-static
  field n

42
Output

• int n 0
• try
• n n / 0
• catch (Exception e)
• System.out.println("A")
• finally
• System.out.println("B")
• System.out.println("C")

43
Ö

• A
• B
• C

44
Output

• int n 0
• try
• n n / 999
• catch (Exception e)
• System.out.println("A")
• finally
• System.out.println("B")
• System.out.println("C")

45
Ö

• A
• B

46
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")

47
Ö

• A
• B

48
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")

49
Ö

• A