AND and OR in IF statements

Question

I have the following code     if  partialHits get req nr  containsKey z     partialHits get req nr  get z   lt  tmpmap get z          partialHits get z  put z  tmpmap get z          where partialHits is a HashMap  What will happen if the first statement is true  Will Java still check the second statement  Because in order the first statement to be true  the HashMap should not contain the given key  so if the second statement is checked  I will get NullPointerException  So in simple words  if we have the following code    if a  amp  amp  b    if a    b    would Java check b if a is false in the first case and if a is true in the second case

User · Answer

No  if a is true  in a or test   b will not be tested  as the result of the test will always be true  whatever is the value of the b expression   Make a simple test   if  true      String  null  equals  foobar                  will not throw a NullPointerException

User · Answer

No it won t  Java will short-circuit and stop evaluating once it knows the result

User · Answer

No it will not be checked  This behaviour is called short-circuit evaluation and is a feature in many languages including Java

User · Answer

Yes  the short-circuit evaluation for boolean expressions is the default behaviour in all the C-like family   An interesting fact is that Java also uses the  amp  and   as logic operands  they are overloaded  with int types they are the expected bitwise operations  to evaluate all the terms in the expression  which is also useful when you need the side-effects

User · Answer

This goes back to the basic difference between  amp  and  amp  amp     and      BTW you perform the same tasks many times  Not sure if efficiency is an issue  You could remove some of the duplication   Z z2   partialHits get req nr  get z      assuming a value cannout be null  Z z3   tmpmap get z      assuming z3 cannot be null  if z2    null    z2  lt  z3          partialHits get z  put z  z3

User · Answer

Short circuit here means that the second condition won t be evaluated   If   A  amp  amp  B   will result in short circuit if A is False   If   A  amp  amp  B   will not result in short Circuit if A is True   If   A    B   will result in short circuit if A is True   If   A    B   will not result in short circuit if A is False

User · Answer

Java has 5 different boolean compare operators   amp    amp  amp              amp  and  amp  amp  are  and  operators    and     or  operators    is  xor   The single ones will check every parameter  regardless of the values  before checking the values of the parameters  The double ones will first check the left parameter and its value and if true      or false   amp  amp   leave the second one untouched  Sound compilcated  An easy example should make it clear   Given for all examples    String aString   null    AND    if  aString    null  amp  aString equals  lala      Both parameters are checked before the evaluation is done and a NullPointerException will be thrown for the second parameter    if  aString    null  amp  amp  aString equals  lala      The first parameter is checked and it returns false  so the second paramter won t be checked  because the result is false anyway   The same for OR    if  aString    null    aString equals  lala      Will raise NullPointerException  too    if  aString    null     aString equals  lala      The first parameter is checked and it returns true  so the second paramter won t be checked  because the result is true anyway   XOR can t be optimized  because it depends on both parameters

User · Answer

All the answers here are great but  just to illustrate where this comes from  for questions like this it s good to go to the source  the Java Language Specification   Section 15 23  Conditional-And operator   amp  amp    says      The  amp  amp  operator is like  amp     15 22 2   but evaluates its right-hand operand only if the value of its left-hand operand is true        At run time  the left-hand operand expression is evaluated first       if the resulting value is false  the value of the conditional-and expression is false and the right-hand operand expression is not evaluated  If the value of the left-hand operand is true  then the right-hand expression is evaluated       the resulting value becomes the value of the conditional-and expression  Thus   amp  amp  computes the same result as  amp  on boolean operands  It differs only in that the right-hand operand expression is evaluated conditionally rather than always    And similarly  Section 15 24  Conditional-Or operator       says      The    operator is like      15 22 2   but evaluates its right-hand operand only if the value of its left-hand operand is false        At run time  the left-hand operand expression is evaluated first        if the resulting value is true  the value of the conditional-or expression is true and the right-hand operand expression is not evaluated  If the value of the left-hand operand is false  then the right-hand expression is evaluated        the resulting value becomes the value of the conditional-or expression  Thus     computes the same result as   on boolean or Boolean operands  It differs only in that the right-hand operand expression is evaluated conditionally rather than always    A little repetitive  maybe  but the best confirmation of exactly how they work  Similarly the conditional operator      only evaluates the appropriate  half   left half if the value is true  right half if it s false   allowing the use of expressions like   int x    y    null    0   y getFoo      without a NullPointerException

User · Answer

No  it will not be evaluated  And this is very useful  For example  if you need to test whether a String is not null or empty  you can write   if  str    null  amp  amp   str isEmpty        doSomethingWith str charAt 0        or  the other way around  if  str    null    str isEmpty        complainAboutUnusableString      else     doSomethingWith str charAt 0        If we didn t have  short-circuits  in Java  we d receive a lot of NullPointerExceptions in the above lines of code

[java] && (AND) and || (OR) in IF statements

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