Why is the output different in case of &&, &,

2018-06-26 10:10:35

Here is the code segments

Can you explain why outputs are varying

public static ShortCkt {
    public static void main(String args[]) {
        int i = 0;
        boolean t = true;
        boolean f = false, b;
        b = (t && ((i++) == 0));
        b = (f && ((i+=2) > 0));
        System.out.println(i);      
    }
}

output in this case is 1

public static ShortCkt {
    public static void main(String args[]) {
        int i = 0;
        boolean t = true;
        boolean f = false, b;
        b = (t & ((i++) == 0));
        b = (f & ((i+=2) > 0));
        System.out.println(i);      
    }
}

output in this case is 3

public static ShortCkt {
    public static void main(String args[]) {
        int i = 0;
        boolean t = true;
        boolean f = false, b;
        b = (t || ((i++) == 0));
        b = (f || ((i+=2) > 0));
        System.out.println(i);      
    }
}

output in this case is 2

public static ShortCkt {
    public static void main(String args[]) {
        int i = 0;
        boolean t = true;
        boolean f = false, b;
        b = (t | ((i++) == 0));
        b = (f | ((i+=2) > 0));
        System.out.println(i);      
    }
}

output in this case is 3

Why is the output different in case of &&, &, || ?

Just as in C/C++ && is evaluated "lazily" while & is not.

If a is false then a && b will return false without even evaluating b .

Same goes for a || b a || b : If the first operand, a is true, the whole expression is true and the second operand, b is never evaluated. For a | b a | b however, both a and b will be evaluated.

This has consequences if the operand that's not being evaluated when using && (or || ) has side effects, as in your examples.

Side note: Few java-programmers know that ^ (xor) works for booleans as well. (A ^^ version does not exist simply because it would be redundant.)

There are 4 boolean binary operators that we're concerned with here:

&& is the conditional and operator

& is the logical and operator

|| is the conditional or operator

| is the logical or operator

Here's the key point:

"conditional" means it short-circuits: it does not evaluate the right operand if it will not affect the result of the operation

"logical" does not short-circuit: it evaluates both operands, left first, then right.

The result of "and" is true only if both operands are true

If the left operand is false , the result is false regardless of right operand

The result of "or" is true only if at least one operand is true

If the left operand is true , the result is true regardless of right operand

In other words, assuming no exception etc:

& and | always evaluate both operands

&& and || evaluate the right operand conditionally; the right operand is evaluated only if its value could affect the result of the binary operation. That means that the right operand is NOT evaluated when:

The left operand of && evaluates to false

(because no matter what the right operand evaluates to, the entire expression is false )

The left operand of || evaluates to true

(because no matter what the right operand evaluates to, the entire expression is true )

References

JLS 15.22.2 Boolean Logical Operators & , ^ , and |

For & , the result value is true if both operand values are true ; otherwise, the result is false .

For | , the result value is false if both operand values are false ; otherwise, the result is true .

JLS 15.23 Conditional-And Operator &&

The && operator is like & , but evaluates its right-hand operand only if the value of its left-hand operand is true .

JLS 15.24 15.24 Conditional-Or Operator ||

The || operator is like | , but evaluates its right-hand operand only if the value of its left-hand operand is false .

Why is the output different in case of &&, &,

References

See also

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