1 java gotcha's by rick mercer with help from the book: java™ puzzlers: traps, pitfalls, and...
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Java Gotcha's
By Rick Mercer with help from the book:Java™ Puzzlers: Traps, Pitfalls, and Corner Cases
Joshua Bloch and Neal Gafter
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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
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Oddity// Does this method return true if n is odd?
public boolean isOdd(int n) {
return n % 2 == 1;
}
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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
// 00011001 & 00000001 is 00000001
// Expression is 0 unless n is odd
// if n is 24, n & 1 is 0
// 00011000 & 00000001 is 00000000
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Simple Arithmetic @Test
public void simpleArithmetic() {
// Does this assertion pass?
assertEquals(444, 123 + 32l);
}
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Ö
Eyes Deceive It is 123 + 32L, which is 155 Use L instead of l for Long
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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.
}
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Ö
Answer Only a.
java.lang.AssertionError:
expected:<Ha> but was:<169>
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Operator Precedence
@Testpublic 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", "Animals equal: " + pig == dog);}
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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)));
From a 227 Student
// Part of escape obstacle course in findExitif(escape = false)
findExit(r-1, c); // row above
if(escape = false)
findExit(r, c+1); // col to the right
Can this recursive solution ever work?
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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
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From a 127B Student
@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?
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Ö
No n is not decremented until after the function call f(n--) results in a StackOverflowError Use f(n-1)
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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
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Ö
Answer: 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(123.45 + " \u20ac" );
123.45 €
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assertEquals('@', '\u0040');
assertEquals('A', '\u0041');
assertEquals('B', '\u0042');
assertEquals('`', '\u0060');
assertEquals('a', '\u0061');
assertEquals('b', '\u0062');
assertEquals('€', '\u20ac');
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Is “true” true?
@Test
public void trueOr() {
// Does this assertion pass?
assertEquals("Compare 5 to 4", "true", 5 > 4);
}
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Ö
Answer No
"true" is not true However, JUnit show this:
java.lang.AssertionError:
Compare 5 to 4 expected:<true> but was:<true>
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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"); } }
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Ö
twothree
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; }
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"); break; case "3": System.out.println("three"); break; }
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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);
}
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Ö
Answer: No 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
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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]); //? _____
O
0
1
1
With array
0 5
1 4
1 3
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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); }}
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Ö
Answer: YesHello world 0 null
This is an initializer, a method with no heading;
{ }
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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);
}
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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);
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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("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()); }
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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());
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Output?
int n = 0;
try {
n = n / 0;
} catch (Exception e) {
System.out.println("A");
} finally {
System.out.println("B");
}
System.out.println("C");
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Ö
A
B
C
Finally blocks always execute unless System.exit(0) is encountered first
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Output?
int n = 0;
try {
n = n / 999;
} catch (Exception e) {
System.out.println("A");
} finally {
System.out.println("B");
}
System.out.println("C");
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Ö
B
C
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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");
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Ö
B
C
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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");
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Ö
A
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To intern or not to intern
Which assertion(s) pass? a)1 and 2 b)1 onlyc)2 onlyd)Neither
@Test public void stringIntern() { String s1 = "UofA"; String s2 = new String("UofA"); assertTrue(s1.equals(s2)); // 1 assertTrue(s1 == s2); // 2 }
String data = new String("123"); String moreData = new String("123"); System.out.println(data==moreData);
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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
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To intern or not to intern@Testpublic 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.}
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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 -128..127
to save memory
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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. }
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ÖassertTrue(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