Java order of OR comparison

does the following snippet throw NPE when argument is null?

public void doSomething(String string) {
    if (string.trim().equals("") || string==null) {
    [...]
    }
}

I've found this in someone else's code (someone else who should be more experienced than me). Since I've been facing difficulties with this code, I want to ask if the comparison 开发者_如何学JAVAshould be inverted or the Java compiler is someway smart enough to swap the operands. I don't have direct control over this code, nor this throws me NPE because of many catch blocks.

Thank you

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Yes. That snippet of code will throw a NullPointerException when string is null. Changing it to the following is advisable:

public void doSomething(String string) {
    if (string==null || string.trim().equals("")) {
        // ...
    }
}

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It will throw a NullPointerException because if string is null and you try to trim a null it will throw the exception. Try putting the null check before you try to trim().

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Yeah, that looks dodgy to me. It should almost certainly be the other way round. Java logical operators (like in C and C++) have a "short-circuit" facility, whereby the left-hand operand is evaluated first, and then the right-hand operand is only evaluated if needed.

[Note: Couldn't you have just tried running this to find out whether it throws an exception?]

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Yes, every sensible IDE will complain about this code, because the right half can never be evaluated.

 if (string.trim().equals("") || string==null) {

if the string is null, the left part throws a NPE, so the right part never gets evaluated

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try that with if(!"".equals(someString)) which enables to avoid an explicit null-check

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binary operators of equal precedence are always evaluated from left to right, apart from assignment (in just about every language I can think of) This is important in this case, because || is a short cut operator. If the result is know i.e. the first expression is true, the second expression will not be evaluated. So the correct way to use || and && to check for null is like the following.

if(text == null || text.method())

or

if(text != null && text.method2())

The order is this way as we read left to right (and top to bottom) in English. c.f. In Japanese you read from top to bottom then right to left.