essay5.txt

Java Essay Serials 1 - Java Basics - 5, & and &&
The operator && is conditional AND, and the operator & is boolean logical AND and/or Integer Bitwise operator(& has two functions).

Firstly we take a look at the simplest situation.
for boolean value expression, a && b, if a is true and b is true, the result will be true, otherwise the result will be false;
for boolean value expression, a & b, if a is true and b is true, the result will be true, otherwise the result will be false;
That means for this situation, & and && is functional equivalent.

But the evaluation process is not the same. for &, two operands should be evaluated, and for &&, if the left operand is false, the right-hand operand expression is not evaluated(Please note that it's not a compiler  optimization, it's a JLS' rule, so you can rely on it).
for example, 
		Boolean boolA = false;
		Boolean boolB = null;
		System.out.println(boolA && boolB);
with the above statements, the output is false. that's because boolA is false, so boolB is noe evaluated(otherwise it will be a NullPointerException here).
let us check what will be happened with &:
		Boolean boolA = false;
		Boolean boolB = null;
		System.out.println(boolA & boolB);
the output is:
	Exception in thread "main" java.lang.NullPointerException.
that's because for & operator, Java will evaluate both expression even if the first expression is false. this process is different from what of &&.
Programmers call && short-circuit operator.

& is also the bitwsie AND operator. showed as below:
		int ia = 135;
		int ib = 178;
		System.out.println(ia & ib);
the output will be 130.
Here is how it is evaluated:
the binary representation of 135 is 10000111, and 10110010 for 178.
   10000111
   10110010
AND----------
   10000010

the last issue is that how Java know & should be a bitwise operateor or a boolean logical operator?
the answer is depends on the operands type. The bitwise and logical operators may be used to compare two operands of numeric
type or two operands of type boolean. for numeric type, it will be considered as bitwise operator.
Keep in mind that here the numeric type only for those types which can be converted to an integral number. you can not use & for floating numbers.