如何检查对象是否为 null？

我正在创建一个应用程序，检索从网络图像。如果无法检索到该图像，则应使用另一个本地图像。

在试图执行以下代码时:

Drawable drawable = Common.getDrawableFromUrl(this, product.getMapPath());
if (drawable.equals(null)) {
drawable = getRandomDrawable();
}

如果可绘制值为 null，则 if (draable.equals (null))行将引发异常。

是否有人知道应该如何检查可绘制的值，以避免在异常为 null 时抛出异常，并检索本地图像(Executable = getRodd Drawable ()) ？

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小开 
Drawable drawable = Common.getDrawableFromUrl(this, product.getMapPath());
if (drawable == null) {
drawable = getRandomDrawable();
}

The equals() method checks for value equality, which means that it compares the contents of two objects. Since null is not an object, this crashes when trying to compare the contents of your object to the contents of null.

The == operator checks for reference equality, which means that it looks whether the two objects are actually the very same object. This does not require the objects to actually exist; two nonexistent objects (null references) are also equal.

小开
最佳答案

Edited Java 8 Solution:

final Drawable drawable =
Optional.ofNullable(Common.getDrawableFromUrl(this, product.getMapPath()))
.orElseGet(() -> getRandomDrawable());

You can declare drawable final in this case.

As Chasmo pointed out, Android doesn't support Java 8 at the moment. So this solution is only possible in other contexts.

小开 
drawable.equals(null)

The above line calls the "equals(...)" method on the drawable object.

So, when drawable is not null and it is a real object, then all goes well as calling the "equals(null)" method will return "false"

But when "drawable" is null, then it means calling the "equals(...)" method on null object, means calling a method on an object that doesn't exist so it throws "NullPointerException"

To check whether an object exists and it is not null, use the following

if(drawable == null) {
...
...
}

In above condition, we are checking that the reference variable "drawable" is null or contains some value (reference to its object) so it won't throw exception in case drawable is null as checking 

null == null

is valid.

小开

I use this approach:

if (null == drawable) {
//do stuff
} else {
//other things
}

This way I find improves the readability of the line - as I read quickly through a source file I can see it's a null check.

With regards to why you can't call .equals() on an object which may be null; if the object reference you have (namely 'drawable') is in fact null, it doesn't point to an object on the heap. This means there's no object on the heap on which the call to equals() can succeed.

Best of luck!

小开

It's probably slightly more efficient to catch a NullPointerException. The above methods mean that the runtime is checking for null pointers twice.

小开

if (yourObject instanceof yourClassName) will evaluate to false if yourObject is null.

小开

Use google guava libs to handle is-null-check (deamon's update)

Drawable drawable = Optional.of(Common.getDrawableFromUrl(this, product.getMapPath())).or(getRandomDrawable());
小开

DIY

private boolean isNull(Object obj) {
return obj == null;
}

Drawable drawable = Common.getDrawableFromUrl(this, product.getMapPath());
if (isNull(drawable)) {
drawable = getRandomDrawable();
}

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