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This article mainly explains "what is the role of null in Java". The content of the explanation is simple and clear, and it is easy to learn and understand. Please follow the editor's train of thought to study and learn "what is the role of null in Java"?

First, let's take a look at an example. Readers can read and judge the output results first:

Public class TestClass {private static void testMethod () {System.out.println ("testMethod");} public static void main (String [] args) {(TestClass) null) .testMethod ();}}

The above example will output correctly: testMethod

Analysis: the first thing to understand is that this is a call to a method for a class, not a call to a method by an object

Second, testMethod methods are statici static methods, and you can use "class methods" directly, because the use of static methods does not depend on whether the object is created. Null can be cast to any type (not any type of object), so it can be used to execute static methods

Finally, non-static methods are called in the way of "object. method" and can only be used after the object is created. If the static before the testmethod () method is removed, a null pointer exception will be reported. The above point is also verified here. Of course, whether it is static or not, it already exists, but the access method is different.

In-depth understanding: first, null represents an uncertain object

In Java, null is a keyword that identifies an uncertain object. Therefore, you can assign null to reference type variables, but you cannot assign null to base type variables. For example: int is null; it is wrong; and Ojbect o=null is correct. In Java, the application of variables follows the principle that they are defined and initialized before they can be used. We can't print the value of an after int a without assigning a value. The same applies to reference type variables.

Sometimes, we define a reference type variable, at the beginning, can not give a definite value, but do not specify a value, the program may initialize the value in the try statement block. At this point, we will report an error when we use variables next. Therefore, you can first assign a null value to the variable, and the problem is solved. For example:

/ / java project source code www.1b23.comConnection conn=null; try {conn=Drivermanager get Connection ("url", user "," password ");} catch (SQLException e) {e printstacktrace ();} String catalog=conn. Getcatalog ()

If you do not specify conn=null at the beginning, the last sentence will get an error.

Null itself is not an object, nor is it an instance of Objet

Null itself can represent an uncertain object, but as far as null itself is concerned, it is not an object, it does not know what type, and it is not an instance of java.lang.Objecte. You can make a simple example:

/ / is the java project source code www.1b23.com//nul an object? Is it of Object type? if (null instanceof java.lang.Object) Systen.Out. Println ("null belongs to java.lang. Object type"); elseSystem.Out. Println ("null is not of type Java.lang.Object")

The result is output:

Null is not of type Java.lang.Object

3. Java assigns values to variables by default

When defining a variable, if the variable is not assigned a value after the definition, Java automatically assigns a value to the variable at run time. The assignment principle is that integer types int, byte, short and long are automatically assigned to 0; float and double with decimal point are automatically assigned to 0, and boolean is automatically assigned to null for other referenced type variables of false;. This can be seen through debugging.

IV. Container types and null

List: allows repeating elements, and can add as many nullO as you like

Set: duplicate elements are not allowed. You can join null at most

The key of Map:Map can be added to a maximum of null,value fields without limit.

Array: an array of primitive types that, after definition, is automatically given by the java runtime if no initial value is given; if an array of reference types is not given, all element values are null.

Other functions of null

1. Judge whether a reference type data is null or not, and use = = to judge.

2. Free memory and let a non-null reference type variable point to null, so that the object will no longer be applied by any object, waiting for the JVM garbage collection mechanism to collect.

Thank you for your reading, the above is the content of "what is the role of null in Java". After the study of this article, I believe you have a deeper understanding of the role of null in Java, and the specific use needs to be verified in practice. Here is, the editor will push for you more related knowledge points of the article, welcome to follow!

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