Java中的变量类型

1,Local variables: 

    **Local variables are declared in methods,constructors, or blocks.

   **Local variables are created when the method,constructor or block id entered and the variable will be destroyed once it exits the method,consturctor, or blocks.

    **Access modifiers(访问修饰符) cannot be used for local variables.    

    **Local variables are visible only within the declared methods,constructor, or block.

    **There is no default value for local variables, so local variables should be declared and an initial value should be assigned before the first use.

2,Instance variables: 

    **Instance variables are declared in a class, but outside a method, constructor or any block.

    **When a space is allocated for an object in the heap, a slot(跟踪) for each instance variable value is created.

    **Instance variables are created when the object is created with the use of keyword 'new' and destroyed when the object is destroyed.

    **Instance variables hold values that must be referenced by more than one method, constructor or blocks,or essential parts of an object`s state that must be present throughtout the class.

    **Instance variables can be declared in class level before or after use.

    **Access modifiers can be given for instance variables.

    **The instance variables are visible for all methods, constructors and blocks in the class.Normally, it is recommended to make these variables private(access level). However ,visiblility for subclass can be given for these variables with the use of access modifiers.

    **Instance variables have default value.For numbers,the default value is 0,for Booleans it it false,and for object reference itit null. Values can be assigned during the declaration or within the constructor.

    **Instance variables can be accessed directly by calling the variable name inside the class.However,within static methods (when instance variables are given accessibility), they should be called using the fully qualified name 

.ObjectReference.VariableName.

3,Class/static variables:

    **Class variables also known as static variables are declared with the static keyword in a class,but outside a mehod, constructor or a block.

    **There would only be one copy of class variable per class,regardless of how many objects are created from it.

    **Static variables are rarely used other than being declared as constants.Constants are variables never change from there initial value.

    **Static variables are stored in the static memory. It is rare to use static variables other than declared final and used as either public or private constants.

    **Static variables are created when the program starts and destroyed when the program stops.

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

    **Default values are same as instance variables.For numbers,the default value is 0; for Booleans,it is false; and for object references, it is null. Values can be assigned during the declaration or within the constructor.Additionally,values can be assigned in special static initializer blocks.

    **Static variables can be accessed by calling with the class name  ClassName.VariableName.

    **When declaring class variables as public static final,then variable names(constants) are all in upper case.If the static variables are not public and final,the naming syntax is the same as instance and local variables.