问题:

I come from a c++ background and I find myself constantly doing this in java:

SomeClass sc=new SomeClass();  if(null!=sc) {     sc.doSomething(); }

What I want to know is what will be in the variable sc if the constructor fails for some reason (like maybe not enough memory). I can' t find a straight answer, and I am worried that I am just wasting my time because maybe if the new operator fails would the program just crash anyway?

回答1:

The Java Specification Language 3rd Edition covers your question thoroughly:

12.5 Creation of New Class Instances

Whenever a new class instance is created, memory space is allocated for it with room for all the instance variables declared in the class type and all the instance variables declared in each superclass of the class type, including all the instance variables that may be hidden. If there is not sufficient space available to allocate memory for the object, then creation of the class instance completes abruptly with an OutOfMemoryError. Otherwise, all the instance variables in the new object, including those declared in superclasses, are initialized to their default values.

Just before a reference to the newly created object is returned as the result, the indicated constructor is processed to initialize the new object using the following procedure: [...]

So it's simply not possible for a new expression to return null. Whatever is returned, if the execution completes normally, will always be a valid instanceof whatever class was instantiated.

Handling exceptions

Generally speaking, possible exceptions are usually handled with a try-catch block:

String someString = askFromUser(); try {    int num = Integer.parseInt(someString);    doSomethingWith(num); } catch (NumberFormatException e) {    complainAboutIt(); }

In your case, you may consider putting new SomeClass() in a try block with a corresponding catch (OutOfMemoryError e), but this is highly atypical. Unless you plan to do something meaningful when this happens, in most typical scenarios it's best to not catch any Error that may occur during your program execution.

From the documentation:

An Error is a subclass of Throwable that indicates serious problems that a reasonable application should not try to catch. Most such errors are abnormal conditions.

A method is not required to declare in its throws clause any subclasses of Error that might be thrown during the execution of the method but not caught, since these errors are abnormal conditions that should never occur.

回答2:

If the constructor failed, then it would throw an exception or an error and you wouldn't get any further in the program. Specifically, if it ran out of memory, you would get an OutOfMemoryError.

回答3:

Yes you are wasting your time :-). sc is guaranteed to be non-null after the constructor is called. If the constructor failed an exception would be thrown and the following code would never be run. Hence the following is safe in Java:

SomeClass sc = new SomeClass(); sc.doSomething();

without throwing a NullPointerException.

回答4:

That if check is not needed, and some IDEs will even complain that it is unnecessary as it always evaluates to true. The only reason that new would fail is if the constructor threw an exception, and in that case your if check will be bypassed anyways.

回答5:

SomeClass sc=new SomeClass();  if(null!=sc) { sc.doSomething(); }

An object reference is never null after creating an object, so the null check is unnecessary because the condition will always be true.

If, for example, there is not enough memory at the point in the constructor, an OutOfMemoryError will be thrown and the constructor will not return normally.