The actual implementation in a given environment depends on the programming language and target environment, and might differ from the implementation shown here. Here we declare a variable of type String and with the identifier name that points to the string "Bob". More on this later. The memory addresses in the above picture are chosen arbitrarily and are irrelevant for our discussion. As we can see, the string "Bob" is stored at address B and occupies 4 memory cells.
Variable name is located at address A0A1. The content of A0A1 is B, which is the starting memory location of the string "Bob".
That's why we say: The variable name points to "Bob". We can see that nothing has changed for the string "Bob" which is still stored in memory. Note: The memory needed to store the string "Bob" might later be released if there is a garbage collector and no other reference points to "Bob" , but this is irrelevant in our discussion. So, variable name doesn't point to "Bob" anymore.
The value 0 all bits at zero is a typical value used in memory to denote null. It means that there is no value associated with name. You can also think of it as the absence of data or simply no data. Note: The actual memory value used to denote null is implementation-specific. For example the Java Virtual Machine Specification states at the end of section 2. If a reference points to null , it simply means that there is no value associated with it. Technically speaking, the memory location assigned to the reference contains the value 0 all bits at zero , or any other value that denotes null in the given environment.
As we learned in the previous section, operations involving null are extremely fast and easy to perform at run-time. So far we assumed working with reference types. The reason for this is simple: null doesn't exist for value types. As we have seen previously, a reference is a pointer to a memory-address that stores a value e. If a reference points to null , then no value is associated with it.
On the other hand, a value is, by definition, the value itself. There is no pointer involved. A value type is stored as the value itself. Therefore the concept of null doesn't exist for value types.
The following picture demonstrates the difference. On the left side you can see again the memory in case of variable name being a reference pointing to "Bob". The right side shows the memory in case of variable name being a value type. As we can see, in case of a value type, the value itself is directly stored at the address A0A1 which is associated with variable name. There would be much more to say about reference versus value types, but this is out of the scope of this article.
Please note also that some programming languages support only reference types, others support only value types, and some e. C and Java support both of them. The concept of null exists only for reference types.
It doesn't exist for value types. Suppose we have a type person with a field emailAddress. Suppose also that, for a given person which we will call Alice, emailAddress points to null. As we have seen already, what we can assert is that no value is associated with emailAddress. But why is there no value? What is the reason of emailAddress pointing to null? If we don't know the context and history, then we can only speculate.
The reason for null could be:. In practice we often know the application and context. We intuitively associate a precise meaning to null. In a simple and flawless world, null would simply mean that Alice actually doesn't have an email address. When we write code, the reason why a reference points to null is often irrelevant.
We just check for null and take appropriate actions. For example, suppose that we have to write a loop that sends emails for a list of persons. The code in Java could look like this:. We just acknowledge the fact that there is no email address, log a warning, and continue. If a reference points to null then it always means that there is no value associated with it. In most cases, null has a more specific meaning that depends on the context. In this case, returning null or an empty list is ambiguous.
Does it mean that the patient doesn't have allergies, or does it mean that an allergy test has not yet been performed? These are two semantically very different cases that must be handled differently. Or else the outcome might be life-threatening. Just suppose that the patient has allergies, but an allergy test has not yet been done and the software tells the doctor that 'there are no allergies'. Hence we need additional information. We need to know why the function returns null.
Thus, a law that is unjust is morally null and void, and must be defied until it is legally null and void as well. Journals aren't interested in null results or "yup, we replicated that other study"; they want new studies with new findings. It also declared the acts of any partnership into which such spiritual person had been introduced to be null and void.
Everything which the revolutionary party had done since August 18,, was declared null and void. In either case, according to the learned Dr. Sicklewit, the ceremony is utterly null and void of effect. He had alleged that she was a woman of no character, and he had further alleged that their marriage was null and void. There is no qualification that if we do not deem them legal we can treat them as null and void.
New Word List Word List. Save This Word! There are clear exceptions to those alternate hypotheses, so if you test the wrong plants, you could reach the wrong conclusion.
The null hypothesis is a general statement that can be used to develop an alternate hypothesis, which may or may not be correct. Actively scan device characteristics for identification. Use precise geolocation data. Select personalised content. Create a personalised content profile. Measure ad performance. Select basic ads. Create a personalised ads profile. Select personalised ads. Apply market research to generate audience insights. Measure content performance.
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