NullPointer异常处理

Question

I have been going through some site where they talk about how to prevent NPE. There I saw people say use null != object but I don't understand why. What is the difference between doing the below in java?

if(null != object) 

vs

if(object != null)

Answer 1

There is no difference between the two in the effect.

However there is a school of thinking where it is recommended to use un-assignable (constant) values on the left hand side of a operator. Because this reduces the risk of unintended assignment (this is from the time where C compilers have been not warning about it).

// this is dangerous when misstyped
if (object = null) {

The argument that writing the null first really reduces the effect of misstyping is however pretty weak. Especially when not using "==" but "!=" or "<". So I would say, ignore those recommendations.

There are however some situations where order is helpfull to prevent NPE:

if ("string".equals(object))

In this case you dont get an NPE when "object" is null.

Answer 2

What is the difference between doing the below in java?

• There is no difference in terms of what the code means.

• There is no difference in terms of code safety / robustness.

In some languages, the if (null == object) pattern helps to avoid this mistake:

 if (object = null)

... which accidentally trashes the value of the object variable.

But in Java, an if statement requires a condition expression whose type is boolean , and the type of object == null is NOT boolean. Hence that particular mistake / typo is guaranteed to give a compilation error.

---

Digression ...

The case where you plausibly could get into trouble with '==' versus '=' in classic Java is this:

 if (flag == true)

or

 if (flag == false)

However, no experienced Java programmer would write that. Instead they would write:

 if (flag)

and

 if (!flag)

With Java 5 and onwards, autoboxing of Boolean can get you into trouble. However, in that case you are doing something that is inadvisable anyway. Consider this:

 Boolean TRUE = true;
 Boolean TOO_TRUE = new Boolean(true);  // generates a new instance ...

 if (TRUE == TOO_TRUE) {  // should be TRUE.equals(TRUE)
     ....
 }

Again, an experienced programmer would avoid using boxed types unnecessarily, and avoid testing them with == . Besides, you would probably get a warning for this from an IDE or from a static anaylser like FindBugs or PMD.