I'm just debugging some code and found some expression: if (mObject != null && a || b ) { mObject.someMethod(); } which had a NullPointerException in the mObject.someMethod() line - so my question is: how is the expression evaluated? 1. a && b || c == a && (b || c) or 2. a && b || c == (a && b) || c If 1. is the case i'd probably have to look at some multithreading issues. This is probably a dumb question and i'm pretty sure it has been asked before, but i can't find the answer here. Thanks for the answers in advance.

Reputation: 2044

Logic evaluation with && and || in a statement

I'm just debugging some code and found some expression:

if (mObject != null && a || b )
{
    mObject.someMethod();
}

which had a NullPointerException in the mObject.someMethod() line - so my question is:

how is the expression evaluated?

1. a && b || c == a && (b || c)

2. a && b || c == (a && b) || c

If 1. is the case i'd probably have to look at some multithreading issues.

This is probably a dumb question and i'm pretty sure it has been asked before, but i can't find the answer here. Thanks for the answers in advance.

Upvotes: 5

Reputation: 802

If parentheses are not provided then it will be always evaluated from left to right of the equals expression.

Upvotes: 0

Reputation: 1840

According to here, it's:

((( mObject != null ) && a ) || b )

so you should be looking into b, because:

(null != null) is false
false && a is always false

everything depends on b then.

Upvotes: 2

Reputation: 394146

!= has precedence over && which has precedence over || (Source)

Therefore

if (mObject != null && a || b )

is equivalent to

if (((mObject != null) && a) || b )

To avoid the exception, use parentheses :

if (mObject != null && (a || b) )

Upvotes: 4

Reputation: 72884

It is equivalent to the following condition:

if ((mObject != null && a) || b )

&& has a higher precedence that ||.

If mObject is null, the condition may pass if b is true.

Upvotes: 7