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The boolean Type and boolean Values

Terrence White
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Joined: Jun 26, 2002
Posts: 39
This passage is from the JLS:

An integer x can be converted to a
boolean, following the C language convention that any nonzero value is true, by the expression x!=0. An object reference obj can be converted to a boolean, following the C language convention that any reference other than null is true, by the expression obj!=null.

What does this mean? I thought a Boolean could not be converted or casted to anything.
JLS-4.2.5
Valentin Crettaz
Gold Digger
Sheriff

Joined: Aug 26, 2001
Posts: 7610
Like this for instance:
int x = 1;
int y = 0;
boolean b1 = (x!=0); // b1 is true
boolean b2 = (y!=0); // b2 is false
Object o1 = new Object();
Object o2 = null;
boolean b3 = (o1!=null); // b3 is true
boolean b4 = (o2!=null); // b4 is false


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Jessica Sant
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Joined: Oct 17, 2001
Posts: 4313

correct a boolean can not be cast to anything -- but notice they're not casting it --
basically its just an expression just like any other, the result of which is of the type boolean, try typing it into a lil class and run it. It's not really magic.
Take the first expression:
int x = 1; boolean y;
y = x != 0; //returns true
it simply assigns the result of the expression x!=0 to the value of y.
int x = 0; boolean y;
y = x != 0; //returns false


- Jess
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Terrence White
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Joined: Jun 26, 2002
Posts: 39
Hmm.
Your example has two integers: x and y.
x is used in the boolean expression x!=0 .
y is used in the boolean expression y!=0 .
Both expressions return a boolean and
are assigned to boolean values.
At what point are the integers x and y converted to booleans?
Valentin Crettaz
Gold Digger
Sheriff

Joined: Aug 26, 2001
Posts: 7610
The integers are not converted to boolean values, but the comparison of the integer value with the value 0 yields a boolean which is assigned to a boolean primitive.
In this respect I must admit that the JLS terminology is erroneous since an integer cannot be converted to a boolean. But you get the idea, don't you?
[ July 10, 2002: Message edited by: Valentin Crettaz ]
Terrence White
Ranch Hand

Joined: Jun 26, 2002
Posts: 39
I took the words from the JLS passage literally:
"...An integer x can be converted to a boolean..."
My confusion was really about those words, not the code itself.
Thanks for your help.
Corey McGlone
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Joined: Dec 20, 2001
Posts: 3271
You're right - a integer can not be transformed into a boolean in Java. The reason this is shown is because this was a trick often used in C++:

You see, in C and C++, an int could be used wherever boolean was expected. The value was interpreted as 0 = false and anything else = true.
All the JLS is talking about is how to build an expression that allows you to do something similar in Java in spite of the fact that you can't actually an integer where a boolean in expected. Rather, you can use a boolean expression involving an integer where a boolean is expected.
I hope that helps,
Corey


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Terrence White
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Joined: Jun 26, 2002
Posts: 39
Wow! That did clear it up for me.
This forum stuff is cool...
 
 
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