Clownfish Deployment Plan Patch In Trunk

The patch to support deployment plan for Clownfish is now in trunk (rev 36) :

http://clownfish.svn.sourceforge.net/viewvc/clownfish/clownfish-maven-plugin/trunk/

Integers and Equality Operator

On a lucky day, you could get away by comparing Integers using the equality operator:

1:  public class IntTest {  
2:      public static void main(String[] args) {  
3:          Integer a = 100;  
4:          Integer b = 100;  
5:          System.out.println(a == b);  
6:      }  
7:  }  

Running IntTest.class results to:

 true  

But you may not find yourself lucky if you have a code similar to the one found below:

1:  public class IntTest2 {  
2:      public static void main(String[] args) {  
3:          Integer a = 200;  
4:          Integer b = 200;  
5:          System.out.println(a == b);  
6:      }  
7:  }  

Running IntTest2.class results to:

 false  

Let's write another equality test to check our sanity:

1:  public class IntBox {  
2:      public static void main(String[] args) {  
3:          Integer a1 = 127;  
4:          Integer a2 = 127;  
5:          Integer b1 = -128;  
6:          Integer b2 = -128;  
7:          Integer c1 = 128;  
8:          Integer c2 = 128;  
9:          Integer d1 = -129;  
10:          Integer d2 = -129;  
11:          System.out.println("a1 == a1 " + (a1 == a2));  
12:          System.out.println("b1 == b2 " + (b1 == b2));  
13:          System.out.println("c1 == c2 " + (c1 == c2));  
14:          System.out.println("d1 == d2 " + (d1 == d2));  
15:      }  
16:  }  

Running the code gives us:

 a1 == a1 true  
 b1 == b2 true  
 c1 == c2 false  
 d1 == d2 false  

Yes, comparing 127 and -128 results to true but not 128 and -129.

Let's disassemble IntBox.class to investigate further by running "javap -c IntBox":

1:  Compiled from "IntBox.java"  
2:  public class IntBox extends java.lang.Object{  
3:  public IntBox();  
4:   Code:  
5:    0:    aload_0  
6:    1:    invokespecial    #1; //Method java/lang/Object."<init>":()V  
7:    4:    return  
8:  public static void main(java.lang.String[]);  
9:   Code:  
10:    0:    bipush    127  
11:    2:    invokestatic    #2; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;  
12:    5:    astore_1  
13:    6:    bipush    127  
14:    8:    invokestatic    #2; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;  
15:    11:    astore_2  
16:    12:    bipush    -128  
17:    14:    invokestatic    #2; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;  
18:    17:    astore_3  
19:    18:    bipush    -128  
20:    20:    invokestatic    #2; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;  
21:    23:    astore    4  
22:    25:    sipush    128  
23:    28:    invokestatic    #2; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;  
24:    31:    astore    5  
25:    33:    sipush    128  
26:    36:    invokestatic    #2; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;  
27:    39:    astore    6  
28:    41:    sipush    -129  
29:    44:    invokestatic    #2; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;  
30:    47:    astore    7  
31:    49:    sipush    -129  
32:    52:    invokestatic    #2; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;  
33:    55:    astore    8  
34:    57:    getstatic    #3; //Field java/lang/System.out:Ljava/io/PrintStream;  
35:    60:    new    #4; //class java/lang/StringBuilder  
36:    63:    dup  
37:    64:    invokespecial    #5; //Method java/lang/StringBuilder."<init>":()V  
38:    67:    ldc    #6; //String a1 == a1   
39:    69:    invokevirtual    #7; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;  
40:    72:    aload_1  
41:    73:    aload_2  
42:    74:    if_acmpne    81  
43:    77:    iconst_1  
44:    78:    goto    82  
45:    81:    iconst_0  
46:    82:    invokevirtual    #8; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder;  
47:    85:    invokevirtual    #9; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;  
48:    88:    invokevirtual    #10; //Method java/io/PrintStream.println:(Ljava/lang/String;)V  
49:    91:    getstatic    #3; //Field java/lang/System.out:Ljava/io/PrintStream;  
50:    94:    new    #4; //class java/lang/StringBuilder  
51:    97:    dup  
52:    98:    invokespecial    #5; //Method java/lang/StringBuilder."<init>":()V  
53:    101:    ldc    #11; //String b1 == b2   
54:    103:    invokevirtual    #7; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;  
55:    106:    aload_3  
56:    107:    aload    4  
57:    109:    if_acmpne    116  
58:    112:    iconst_1  
59:    113:    goto    117  
60:    116:    iconst_0  
61:    117:    invokevirtual    #8; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder;  
62:    120:    invokevirtual    #9; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;  
63:    123:    invokevirtual    #10; //Method java/io/PrintStream.println:(Ljava/lang/String;)V  
64:    126:    getstatic    #3; //Field java/lang/System.out:Ljava/io/PrintStream;  
65:    129:    new    #4; //class java/lang/StringBuilder  
66:    132:    dup  
67:    133:    invokespecial    #5; //Method java/lang/StringBuilder."<init>":()V  
68:    136:    ldc    #12; //String c1 == c2   
69:    138:    invokevirtual    #7; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;  
70:    141:    aload    5  
71:    143:    aload    6  
72:    145:    if_acmpne    152  
73:    148:    iconst_1  
74:    149:    goto    153  
75:    152:    iconst_0  
76:    153:    invokevirtual    #8; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder;  
77:    156:    invokevirtual    #9; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;  
78:    159:    invokevirtual    #10; //Method java/io/PrintStream.println:(Ljava/lang/String;)V  
79:    162:    getstatic    #3; //Field java/lang/System.out:Ljava/io/PrintStream;  
80:    165:    new    #4; //class java/lang/StringBuilder  
81:    168:    dup  
82:    169:    invokespecial    #5; //Method java/lang/StringBuilder."<init>":()V  
83:    172:    ldc    #13; //String d1 == d2   
84:    174:    invokevirtual    #7; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;  
85:    177:    aload    7  
86:    179:    aload    8  
87:    181:    if_acmpne    188  
88:    184:    iconst_1  
89:    185:    goto    189  
90:    188:    iconst_0  
91:    189:    invokevirtual    #8; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder;  
92:    192:    invokevirtual    #9; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;  
93:    195:    invokevirtual    #10; //Method java/io/PrintStream.println:(Ljava/lang/String;)V  
94:    198:    return  
95:  }  

From line 10 to 33, the compiler converts our integer assignments to calling Integer.valueOf(int).

Looking at Integer.java source, the Integer class performs special cache operation whenever valueOf(int i) method is called:

(From JDK6):

1:  public static Integer valueOf(int i) {  
2:      if(i >= -128 && i <= IntegerCache.high)  
3:        return IntegerCache.cache[i + 128];  
4:      else  
5:        return new Integer(i);  
6:  }  

and for private static class IntegerCache found in Integer.java (neglect line 8 to 15 to keep things simple for now):

1:  private static class IntegerCache {  
2:      static final int high;  
3:      static final Integer cache[];  
4:      static {  
5:        final int low = -128;  
6:        // high value may be configured by property  
7:        int h = 127;  
8:        if (integerCacheHighPropValue != null) {  
9:          // Use Long.decode here to avoid invoking methods that  
10:          // require Integer's autoboxing cache to be initialized  
11:          int i = Long.decode(integerCacheHighPropValue).intValue();  
12:          i = Math.max(i, 127);  
13:          // Maximum array size is Integer.MAX_VALUE  
14:          h = Math.min(i, Integer.MAX_VALUE - -low);  
15:        }  
16:        high = h;  
17:        cache = new Integer[(high - low) + 1];  
18:        int j = low;  
19:        for(int k = 0; k < cache.length; k++)  
20:          cache[k] = new Integer(j++);  
21:      }  
22:      private IntegerCache() {}  
23:  }  

* Line 17 initialises the cache array to 256 in size, assuming 127 is the highest value
* Line 19 to 21 create the integer objects from -128 to 127 and stores them into "cache" variable

From the statements above, we see that Integers are cached starting from -128 to 127 and that suggests why a1/a2 and b1/b2 equality operator comparison works but not for c1/c2 and d1/d2.