Null example in java

Connect and share knowledge within a single location that is structured and easy to search. At run time, the result of the instanceof operator is true if the value of the RelationalExpression is not null and the reference could be cast to the ReferenceType without raising a ClassCastException.

Otherwise the result is false. There is also a special null type, the type of the expression null , which has no name. Because the null type has no name, it is impossible to declare a variable of the null type or to cast to the null type. The null reference is the only possible value of an expression of null type. The null reference can always be cast to any reference type.

In practice, the programmer can ignore the null type and just pretend that null is merely a special literal that can be of any reference type. As the JLS quote above says, in practice you can simply pretend that it's "merely a special literal that can be of any reference type". Note also that by contract, it also has this special property from java.

Object :. For any non- null reference value x , x. How this is used varies. You can use it to enable what is called lazy initialization of fields, where a field would have its initial value of null until it's actually used, where it's replaced by the "real" value which may be expensive to compute. There are also other uses. Let's take a real example from java. System :. This is a very common use pattern: null is used to denote non-existence of an object.

Here's another usage example, this time from java. BufferedReader :. Returns : A String containing the contents of the line, not including any line-termination characters, or null if the end of the stream has been reached. So here, readLine would return instanceof String for each line, until it finally returns a null to signify the end.

This allows you to process each line as follows:. One can design the API so that the termination condition doesn't depend on readLine returning null , but one can see that this design has the benefit of making things concise. Note that there is no problem with empty lines, because an empty line ""! Let's take another example, this time from java.

Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key. If this map permits null values, then a return value of null does not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key to null. The containsKey operation may be used to distinguish these two cases. Here we start to see how using null can complicate things.

The first statement says that if the key isn't mapped, null is returned. The second statement says that even if the key is mapped, null can also be returned. In contrast, java. Hashtable keeps things simpler by not permitting null keys and values; its V get Object key , if returns null , unambiguously means that the key isn't mapped. You can read through the rest of the APIs and find where and how null is used.

Do keep in mind that they aren't always the best practice examples. This is now borderline subjective. Some people say that null causes many programmer errors that could've been avoided.

Some say that in a language that catches NullPointerException like Java, it's good to use it because you will fail-fast on programmer errors. Some people avoid null by using Null object pattern , etc. I will end this with a quote from the inventor of null himself, C.

R Hoare of quicksort fame :. I call it my billion-dollar mistake. It was the invention of the null reference in My goal was to ensure that all use of references should be absolutely safe, with checking performed automatically by the compiler. But I couldn't resist the temptation to put in a null reference, simply because it was so easy to implement.

This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years. The video of this presentation goes deeper; it's a recommended watch. That is why null instanceof X will return false for all classes X. Don't be fooled by the fact that you can assign null to a variable whose type is an object type. Strictly speaking, the assignment involves an implicit type conversion; see below.

See above. My goal is to provide high quality but simple to understand Java tutorials and examples for free. If you like my website, follow me on Facebook and Twitter. Your email address will not be published. Leave a reply. How to check if the String is null or empty in Java? String is null. Exception in thread "main" java. String is empty. String1 is null or empty. String2 is null or empty. You may also like.

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