Java logical operator short-circuiting

Question

Which set is short-circuiting  and what exactly does it mean that the complex conditional expression is short-circuiting   public static void main String   args      int x  y  z     x   10    y   20    z   30        T T      T F      F T      F F      SET A   boolean a    x  lt  z   amp  amp   x    x     boolean b    x  lt  z   amp  amp   x    z     boolean c    x    z   amp  amp   x  lt  z     boolean d    x    z   amp  amp   x  gt  z       SET B       boolean aa    x  lt  z   amp   x    x     boolean bb    x  lt  z   amp   x    z     boolean cc    x    z   amp   x  lt  z     boolean dd    x    z   amp   x  gt  z

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In plain terms  short-circuiting means stopping evaluation once you know that the answer can no longer change  For example  if you are evaluating a chain of logical ANDs and you discover a FALSE in the middle of that chain  you know the result is going to be false  no matter what are the values of the rest of the expressions in the chain  Same goes for a chain of ORs  once you discover a TRUE  you know the answer right away  and so you can skip evaluating the rest of the expressions   You indicate to Java that you want short-circuiting by using  amp  amp  instead of  amp  and    instead of    The first set in your post is short-circuiting   Note that this is more than an attempt at saving a few CPU cycles  in expressions like this  if  mystring    null  amp  amp  mystring indexOf       gt  0                short-circuiting means a difference between correct operation and a crash  in the case where mystring is null

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There are a couple of differences between the  amp  and  amp  amp  operators  The same differences apply to   and     The most important thing to keep in mind is that  amp  amp  is a logical operator that only applies to boolean operands  while  amp  is a bitwise operator that applies to integer types as well as booleans   With a logical operation  you can do short circuiting because in certain cases  like the first operand of  amp  amp  being false  or the first operand of    being true   you do not need to evaluate the rest of the expression  This is very useful for doing things like checking for null before accessing a filed or method  and checking for potential zeros before dividing by them  For a complex expression  each part of the expression is evaluated recursively in the same manner  For example  in the following case     7    8       1    3      4    4     Only the emphasized portions will evaluated  To compute the     first check if 7    8 is true  If it were  the right hand side would be skipped entirely  The right hand side only checks if 1    3 is false  Since it is  4    4 does not need to be checked  and the whole expression evaluates to false  If the left hand side were true  e g  7    7 instead of 7    8  the entire right hand side would be skipped because the whole    expression would be true regardless   With a bitwise operation  you need to evaluate all the operands because you are really just combining the bits  Booleans are effectively a one-bit integer in Java  regardless of how the internals work out   and it is just a coincidence that you can do short circuiting for bitwise operators in that one special case  The reason that you can not short-circuit a general integer  amp  or   operation is that some bits may be on and some may be off in either operand  Something like 1  amp  2 yields zero  but you have no way of knowing that without evaluating both operands

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Java provides two interesting Boolean operators not found in most other computer languages  These secondary versions of AND and OR are known as short-circuit logical operators  As you can see from the preceding table  the OR operator results in true when A is true  no matter what B is   Similarly  the AND operator results in false when A is false  no matter what B is  If you use the    and  amp  amp  forms  rather than the   and  amp  forms of these operators  Java will not bother to evaluate the right-hand operand alone  This is very useful when the right-hand operand depends on the left one being true or false in order to function properly   For example  the following code fragment shows how you can take advantage of short-circuit logical evaluation to be sure that a division operation will be valid before evaluating it   if   denom    0  amp  amp  num   denom  gt 10    Since the short-circuit form of AND   amp  amp   is used  there is no risk of causing a run-time exception from dividing by zero  If this line of code were written using the single  amp  version of AND  both sides would have to be evaluated  causing a run-time exception when denom is zero   It is standard practice to use the short-circuit forms of AND and OR in cases involving Boolean logic  leaving the single-character versions exclusively for bitwise operations  However  there are exceptions to this rule  For example  consider the following statement    if   c  1  amp  e    lt  100   d   100    Here  using a single  amp  ensures that the increment operation will be applied to e whether c is equal to 1 or not

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boolean a    x  lt  z   amp  amp   x    x     This kind will short-circuit  meaning if  x  lt  z  evaluates to false then the latter is not evaluated  a will be false  otherwise  amp  amp  will also evaluate  x    x     amp  is a bitwise operator  but also a boolean AND operator which does not short-circuit   You can test them by something as follows  see how many times the method is called in each case    public static boolean getFalse         System out println  Method        return false     public static void main String   args        if getFalse    amp  amp  getFalse                    System out println                                               if getFalse    amp  getFalse

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SET A uses short-circuiting boolean operators   What  short-circuiting  means in the context of boolean operators is that for a set of booleans b1  b2       bn  the short circuit versions will cease evaluation as soon as the first of these booleans is true      or false   amp  amp     For example      2    2 will never get evaluated because it is already clear from evaluating    1    1 that the result will be false   1    1   amp  amp   2    2      2    2 will never get evaluated because it is already clear from evaluating    1    1 that the result will be true   1    1      2    2

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Logical OR  - returns true if at least one of the operands evaluate to true  Both operands are evaluated before apply the OR operator   Short Circuit OR  - if left hand side operand returns true  it returns true without evaluating the right hand side operand

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Short circuiting means the second operator will not be checked if the first operator decides the final outcome   E g  Expression is  True    False  In case of     all we need is one of the side to be True  So if the left hand side is true  there is no point in checking the right hand side  and hence that will not be checked at all   Similarly  False  amp  amp  True  In case of  amp  amp   we need both sides to be True  So if the left hand side is False  there is no point in checking the right hand side  the answer has to be False  And hence that will not be checked at all

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The  amp  amp  and    operators  short-circuit   meaning they don t evaluate the right-hand side if it isn t necessary   The  amp  and   operators  when used as logical operators  always evaluate both sides   There is only one case of short-circuiting for each operator  and they are    false  amp  amp      - it is not necessary to know what the right-hand side is because the result can only be false regardless of the value there true        - it is not necessary to know what the right-hand side is because the result can only be true regardless of the value there   Let s compare the behaviour in a simple example   public boolean longerThan String input  int length        return input    null  amp  amp  input length    gt  length     public boolean longerThan String input  int length        return input    null  amp  input length    gt  length      The 2nd version uses the non-short-circuiting operator  amp  and will throw a NullPointerException if input is null  but the 1st version will return false without an exception

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if demon  0 amp  amp  num demon gt 10    Since the short-circuit form of AND  amp  amp   is used  there is no risk of causing a run-time exception when demon is zero   Ref  Java 2 Fifth Edition by Herbert Schildt

[java] Java logical operator short-circuiting

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