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This article mainly introduces "what is the knowledge of Null in Java". In daily operation, I believe that many people have doubts about the knowledge of Null in Java. The editor consulted all kinds of materials and sorted out simple and easy-to-use methods of operation. I hope it will be helpful to answer the questions of "what is the knowledge of Null in Java?" Next, please follow the editor to study!

What is Null in Java?

As I said, null is a very important concept in Java. Null is designed to represent something missing, such as a missing user, resource, or something else. However, a year later, the troublesome null pointer exception caused a lot of harassment to Java programmers. In this material, we will learn the basic details of the null keyword in Java and explore techniques to minimize null checking and how to avoid disgusting null pointer exceptions.

1) first of all, null is a keyword in Java, such as public, static, final. It is case sensitive, you cannot write null as Null or NULL, the compiler will not be able to recognize them and report an error.

Object obj = NULL; / / Not Ok Object obj1 = null / / Ok

Programmers who use other languages may have this problem, but now the use of IDE has made the problem trivial. Now, when you type the code, IDE like Eclipse and Netbeans can correct this error. But using other tools such as notepad, Vim, Emacs, this question will waste your valuable time.

2) just as every primitive type has a default value, for example, the default value of int is 0. the default value of false,null Boolean is the default value of any reference type, not strictly speaking, the default value of all object types. Just like you create a Boolean variable, it takes false as its default value, and any reference variable in Java has null as the default value. This applies to all variables, such as member variables, local variables, instance variables, static variables (but when you use an uninitialized local variable, the compiler warns you). To prove this fact, you can observe the reference variable by creating a variable and printing its value, as shown in the following figure code:

Private static Object myObj; public static void main (String args []) {System.out.println ("What is value of myObjc:" + myObj);} What is value of myObjc: null

This is true for both static and non-static object. As you can see here, I define myObj as a static reference, so I can use it directly in the main method. Note that the main method is static and cannot use non-static variables.

3) to clarify some misunderstandings, null is neither an object nor a type, it is only a special value, you can assign it to any reference type, or you can convert null to any type, take a look at the following code:

String str = null; / / null can be assigned to String Integer itr = null; / / you can assign null to Integer also Double dbl = null; / / null can also be assigned to Double String myStr = (String) null; / / null can be type cast to String Integer myItr = (Integer) null; / / it can also be type casted to Integer Double myDbl = (Double) null; / / yes it's possible, no error

You can see that it is possible to cast null to any reference type at compile and run time, and null pointer exceptions are not thrown at run time.

4) null can be assigned to reference variables, but you cannot assign null to basic type variables, such as int, double, float, boolean. If you do that, the compiler will report an error, as follows:

Int I = null; / / type mismatch: cannot convert from null to int short s = null; / / type mismatch: cannot convert from null to short byte b = null: / / type mismatch: cannot convert from null to byte double d = null; / / type mismatch: cannot convert from null to double Integer itr = null; / / this is ok int j = itr; / / this is also ok, but NullPointerException at runtime

As you can see, a compilation error occurs when you assign null directly to the base type. But if you assign null to the wrapper class object and then assign object to their respective primitive types, the compiler will not report it, but you will encounter a null pointer exception at run time. This is caused by automatic unpacking in Java, which we will see in the next point.

5) any wrapper class that contains a null value will throw a null pointer exception when Java unboxing generates a basic data type. Some programmers make the mistake of thinking that autoboxing converts null to default values of their own basic types, such as converting int to 0 and Boolean types to false, but that's not true, as shown below:

Integer iAmNull = null; int i = iAmNull; / / Remember-No Compilation Error

But when you run the code snippet above, you will see the main thread throwing a null pointer exception on the console. Many of these errors occur when using the HashMap and Integer keys. An error occurs when you run the following code.

Import java.util.HashMap; import java.util.Map; / * * An example of Autoboxing and NullPointerExcpetion * * @ author WINDOWS 8 * / public class Test {public static void main (String args []) throws InterruptedException {Map numberAndCount = new HashMap (); int [] numbers = {3,5,7,9,11,13,17,19,2,3,5,33,12,5} For (int I: numbers) {int count = numberAndCount.get (I); numberAndCount.put (I, count++); / / NullPointerException here}

Output:

Exception in thread "main" java.lang.NullPointerException at Test.main (Test.java:25)

This code looks very simple and error-free. All you do is find out how many times a number appears in an array, which is a typical technique for finding repetition in Java arrays. The developer first gets the previous value, then adds one, and * * puts the value back into the Map. The programmer might think that when calling the put method, autoboxing will take care of boxing int into Interger, but he forgets that when a number has no count, the get () method of HashMap will return null instead of 0, because the default value of Integer is null instead of 0. Auto-boxing will return a null pointer exception when the null value is passed to an int variable. Imagine if this code is in an if nesting and does not run in QA, but once you put it in a production environment, BOOM:-)

6) if a reference type variable with a null value is used, the instanceof operation will return false:

Integer iAmNull = null; if (iAmNull instanceof Integer) {System.out.println ("iAmNull is instanceof Integer");} else {System.out.println ("iAmNull is NOT an instanceof Integer");}

Output:

IAmNull is NOT an instance of Integer

This is an important feature of instanceof operations, which makes it useful for type casting checking

7) you probably know that you cannot call non-static methods to use a reference type variable with a value of null. It will throw a null pointer exception, but you may not know that you can use a static method to use a reference type variable with a value of null. Because static methods use static binding, null pointer exceptions are not thrown. Here is an example:

Public class Testing {public static void main (String args []) {Testing myObject = null; myObject.iAmStaticMethod (); myObject.iAmNonStaticMethod ();} private static void iAmStaticMethod () {System.out.println ("I am static method, can be called by null reference") } private void iAmNonStaticMethod () {System.out.println ("I am NON static method, don't date to call me by null");}

Output:

I am static method, can be called by null reference Exception in thread "main" java.lang.NullPointerException at Testing.main (Testing.java:11)

8) you can pass null to the method, where the method can receive any reference type, for example, public void print (Object obj) can call print (null). This is OK from a compilation point of view, but the result depends entirely on the method. Null-safe methods, such as the print method in this example, do not throw null pointer exceptions, but simply exit gracefully. The null secure approach is recommended if the business logic allows.

9) you can use the = = or! = operation to compare null values, but you cannot use other algorithms or logical operations, such as less or greater than. Unlike SQL, null==null returns true in Java, as shown below:

Public class Test {public static void main (String args []) throws InterruptedException {String abc = null; String cde = null; if (abc = = cde) {System.out.println ("null = = null is true in Java");} if (null! = null) {System.out.println ("null! = null is false in Java") } / / classical null check if (abc = = null) {/ / do something} / / not ok, compile time error if (abc > null) {}

Output:

Null = = null is true in Java

This is all about null in Java. With some experience in Java programming and using simple techniques to avoid null pointer exceptions, you can make your code null safe. Because null is often used as a null or uninitialized value, it is a source of confusion. For methods, it is also important to record how the method behaves when null is used as a parameter. All in all, remember that null is the default value for any reference type variable, and you cannot use a null reference to call any instance method or instance variable in java.

At this point, the study of "what is the knowledge of Null in Java" is over. I hope to be able to solve your doubts. The collocation of theory and practice can better help you learn, go and try it! If you want to continue to learn more related knowledge, please continue to follow the website, the editor will continue to work hard to bring you more practical articles!

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