在Java中检查引用是否为null的最佳方法

Question

I was wondring what the best way to check if we have a valid reference in java. I know that this syntax works, but its a mouth full.

if (myObj == null) {
  // Do something knowing we have an object
}

I'm coming from some other languages that allow you to just check a pointer like in c++.

char* prt = null;
if (ptr) {
  // We know we have a valid c-string 
}

Is there any equivocate or similar syntax in java? I would be okay using compiler extensions or a preprocessor.

Follow up before. Before some one jumps in a starts talking about why I should just use the java syntax because you can forget an = sign please don't.

if (myObj = null) 

Will be caught by the compiler/linter.

Answer 1

Alas Java does not have an implicit conversion of the analogue of a nullptr_t or a pointer type to bool , so you have to use the somewhat more long-winded notation

if (myObj == null)

whereas in C++ we can write

if (myObj)

In this respect, Java is less terse, and arguably clearer.

Answer 2

There is no shortcut syntax for dealing with null checks in Java, not even a null coalesce or null propagation operators available in other languages. There are no user-defined conversion operators either, so you wouldn't be able to use the C++ idiom that lets you write loops on expressions returning objects, eg while (cin >> x) { ... } .

However, a powerful alternative exists in Java 8 to avoid null checks altogether: wrap your nullable objects in Optional<T> , and use its methods to hide null checks.

Here is a short example:

String s = "Hello";
Optional<String> os = Optional.ofNullable(s);
os.ifPresent(x -> { System.out.println(x); });

The above prints "Hello" . If you set s to null , the code would print nothing.

Oracle's article on using Optional<T> .

Answer 3

if(x == null) {
    doSomething();
}

... is the general idiom in Java. Java's designers made the decision not to allow treating non-boolean variables as implicit "truthy" values.

Another common idiom is to use x == null in a ternary statement:

return x == null ? "not found" : x;

Or to use a standard method to throw an exception early on nulls:

Objects.requireNonNull(x);

---

More generally, try to adopt a programming style in which you never expect null to be passed, and therefore don't have to code for the possibility.

On non-public APIs, since you never pass a null, you never need to test for null (if a NullPointerException occurs, whoever passed the null can take responsibility for the mess themselves).

In public APIs, it may be a courtesy to the caller to validate non-nulls at the point they are passed, but it's by no means essential in every case.

A reasonable goal is to always expect inputs to be non-null, and to never return a null (since Java 8, use Optional instead if necessary, or adopt the Null Object Pattern ).