various Short.equals( short )

[Harry Henriques]

Something interesting to look at if you have a minute. This is the output of an actual java program with conditionals.


C:\TEMP>java Dump
Integer I1 = 127
Integer I2 = 128

Short S1 = 127
Short S2 = 128

int i2 = 128

short s1 = 127
short s2 = 128

S1.equals(127) is false
S1.equals((short) 127 ) is true
S1.equals( s1 ) is true

S2.equals(128) is false
S2.equals((short) 128 ) is true
S2.equals( s2 ) is true

S2.equals( I2 ) is false
I2.equals( S2 ) is false

I1.equals((byte) 127 ) is false
I1.equals( 127 ) is true
I2.equals( i2 ) is true

C:\TEMP>

[Lucas Smith]

I cannot see anything strange...

[Harry Henriques]

The following may make what I've done a little clearer.

C:\TEMP>java Dump
Byte B1 = new Byte((byte)127);
Byte B2 = 127;

Integer I1 = new Integer( 127 );
Integer I2 = new Integer( 128 );

Short S1 = new Short((short)127);
Short S2 = new Short((short)128);

int i2 = 128;

short s1 = 127;
short s2 = 128;

B1.equals(127) is false
B2.equals(127) is false
B2.equals((byte)127) is true

S1.equals(127) is false
S1.equals((short) 127 ) is true
S1.equals( s1 ) is true

S2.equals(128) is false
S2.equals((short) 128 ) is true
S2.equals( s2 ) is true

S2.equals( I2 ) is false
I2.equals( S2 ) is false

I1.equals((byte) 127 ) is false
I1.equals( 127 ) is true
I2.equals( i2 ) is true

C:\TEMP>

[Harry Henriques]

It isn't that the results are "strange", Lucas. It's that the results are interesting. I don't think that I could have predicted these results before hand. That's why I'm still studying for the SCJP. -Harry

[Lucas Smith]

OK, so a few clues for you.
Remember the concept of equals method.
public boolean equals(Object o);
It should be checked if 2 classes are in the same inheritance tree.

From Integer class:

 
 public boolean equals(Object obj) {
	if (obj instanceof Integer) {
	    return value == ((Integer)obj).intValue();
	}
	return false;
    }

So new Integer(10).equals((short)10) will be false, because short is boxed to Short and Short IS-NOT-A(n) Integer.

Moreover between -128 and 127 Wrapper classes are buffered (but it is not requested for Long), when you instantiate the objects without "new" operator, so "==" will return true, because they are in the same place in memory.