What are variables in Java

In JAVA we describe variables through three elements: variable type, variable name and variable value.

String love="imooc";

Variable type variable name value (String is immutable, a new String object will be generated after reassignment, love variable name is actually a reference to the object address, "imooc" for a specific value).

love="I love imooc";

The variable is reassigned and points to a new object, with the object value being "i love imooc".

Variables are divided into class variables (static variables), instance variables and local variables (local variables).

Local variables are local variables,

are used in methods, constructors or blocks, and are created when the method, constructor or block enters. The variable will be destroyed when exiting

Local variables have no default value, so local variables must be declared and assigned a value before using them for the first time

Instance variables:

Declared in a class, but outside a method, constructor, or block, the location of each instance variable is created when a space is allocated for an object in the heap.

The instance variable is created when the object is created using the keyword "new", and it is destroyed when the object is destroyed.

The value of an instance variable must be referenced by more than one method, constructor, or block, or a significant part of the object's state that must appear in the class

Instance variables can be accessed using access descriptors

Instance variables have default values. The default value for numbers is 0, the default value for boolean is false, and the default value for object references is NULL

Instance variables can be accessed directly by calling names in the class. However, fully qualified names should be used in static methods and in different classes. ObjectReference.VariableName

Instance variables are variables belonging to instances of a class, also known as member variables, and have not been static modified. Instance variables can only be manipulated through the instance dot operator variable of the class name.

Class variables:

Also called static variables, they are declared with the static keyword in the class, but they are outside the method, constructor or block.

Each class has only one class variable, no matter how many objects the class has.

Except for being declared as constants, class variables are rarely used. Constants are variables declared as public, private, final and static. The initial value of the instance variable will not be changed

Static variables are stored in static memory. Static variables are rarely used instead of being declared at the end or using one of the constants public or private.

Static variables start and end with the start and end of the program.

Visibility is similar to instance variables. However, most static variables are declared public because they must be available to users of the class.

The default value is similar to the instance variable, except that it can be assigned in a special static initialization area

Static variables can be accessed with the name of the class ClassName.VariableName

When When a static variable is declared as public static final, the variable (constant) name must be in uppercase letters. If a static variable is not public and final, its naming method is the same as instance variables and local variables.

Class variables can not only be operated directly through the class name dot operator variable name, but also through the instance dot operator variable of the class. In most cases, the former operation method is not effective. Use the variable properly, and secondly, it can indicate that the variable is a class variable.

The above is the detailed content of What are variables in Java. For more information, please follow other related articles on the PHP Chinese website!