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) == 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

5

Simple Arithmetic

@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

@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

String constants interned

@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",

"Animals equal: " + pig == dog);

}

10

Ö

Neither

Precedence rules have + evaluating before ==. The actual value is"Animals equal: " + pig == dog

"Animals equal: pig" == dog

11

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.

}

12

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.

13

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.

}

14

Ö

assertTrue(a == b); // d. Does not pass

The == compares reference values, not the numeric values

Changing Double to double does what?

From a 227 Student

// Part of escape obstacle course in findExit

if(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?

15

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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17

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

18

Ö

Answer:

Hello World

\u0070 in hexadecimal is 112 in decimal or the character 'c'

Unicode not very readable

Suggest: avoid Unicode unless 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);

}

21

Ö

Answer

No

"true" is not true

JUnit show this:

java.lang.AssertionError:

Compare 5 to 4 expected:<true> but was:<true>

22

Output?

@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");

}

}

23

Ö

two

three

Add breaksint 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 simpleChars() {

int j = 0;

for (int i = 0; i < 10; i++)

j = j++;

// Does this assertion pass?

assertEquals(10, j);

}

25

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

Use j++;

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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]); //? _____

O

0

1

1

With array

0 5

1 4

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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);

}

}

29

Ö

Answer: YesHello world 0 null

This is an initializer, a method with no heading; { }

30

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);

}

31

Ö

No

BigInteger, like String is immutable

This will passBigInteger 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());

}

33

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"));

int actual = bi.intValue();

assertFalse(9999 == actual);

assertTrue(123456 == actual);

// Return null if there was no mapping with the key

BigInteger bi2 = hm.put("key", new BigInteger("123"));

assertNull(bi2);

34

Output

int n = 0;

try {

n = n / 0;

} catch (Exception e) {

System.out.println("A");

} finally {

System.out.println("B");

}

System.out.println("C");

35

Ö

A

B

C

Finally blocks always execute unless System.exit(0) is encountered first

36

Output

int n = 0;

try {

n = n / 999;

} catch (Exception e) {

System.out.println("A");

} finally {

System.out.println("B");

}

System.out.println("C");

37

Ö

B

C

38

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");

39

Ö

B

C

40

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");

41

Ö

A