Java Variable type

Declaring variables:

Variable type: (Declaration (position, initial value), Create & Destroy, visible range, how to access)

1. Local Variables:

1. 1. Declared in a method, a construction method, or a block of statements ;
   2. is visible only in the method, construction method, or block of statements that declares it;
   3. Created when methods, construction methods, or block of statements are executed, and when they are executed , the variables are destroyed ;
   4. access Modifiers (public,private) cannot be used for local variables;
   5. Local variables are allocated on the stack .

2. member variables (instance variables)

• Instance variables are declared in a class, but outside of methods, construction methods, and statement blocks;
• When an object is instantiated , the value of each instance variable is then determined ;
• Instance variables are created when the object is created and destroyed when the object is destroyed ;
• The value of the instance variable should be referenced by at least one method, construct method, or statement block , so that the external can obtain instance variable information through these methods;
• Instance variables can be declared before use or after use ;
• An access modifier can modify an instance variable;
• Instance variables are visible to methods, construction methods, or block statements in a class. In general, you should set the instance variable to private. The instance variable can be visible to the child class by using the access modifier (public);
• The instance variable has a default value . The default value for a numeric variable is 0, the default value for a Boolean variable is false, and the default value for the reference type variable is null. The value of a variable can be specified at the time of declaration, or it can be specified in a constructor method;
• Instance variables can be accessed directly from the variable name in the class. However, in static methods and other classes, you should use the fully qualified name: Obejectreference.variablename.

3. Class variables (static variables)

The
• class variable is also known as a static variable that is declared in the class with static keyword, but must be constructed in method and statement block .
• No matter how many objects a class creates, the class has only the one copy of the class variable.
• Static variables are seldom used except when declared as constants. refers to the declaration as public/private+final+static Types of variables. Constants cannot be changed after initialization.
  • Final:
    • final class cannot be inherited , there are no subclasses, the method in the final class is final by default. The
    • final method , but can be inherited
    • Final member variable means constant , which can only be assigned once and the value no longer changes.
    • final cannot be used to decorate a construction method.

• Static variables are stored in a static storage area . are often declared as constants , and static declaration variables are seldom used alone.
• Static variables are created at the beginning of the program and are destroyed at the end of the program .
• has similar visibility to instance variables. However, in order to be visible to the consumer of a class, Most static variables are declared as public types.
• The default value is similar to the instance variable. The default value for numeric variables is 0, the Boolean default is False, and the reference type default value is null. The value of the variable can be specified at 1 declaration, or in the 2 constructor method. In addition, static variables can be initialized in 3 static statement blocks.
• accessed directly through the variable name in the class , in other classes: Static variables can be accessed by:classname.variablename .
• When a class variable is declared as public static final type, the class variable name must use uppercase letters . If the static variable is not public and final, it is named in the same way as the instance variable and the local variable.

Java Variable type