Java Note--java keyword static, final usage summary

Source: Internet
Author: User

Static 1. Static variablesThere are two ways to classify a class member variable statically: A variable that is statically modified, called a static variable or a class variable, and another variable that is not modified by static, called an instance variable. The difference is that there is only one copy of the static variable in memory (memory saving), the JVM allocates only one memory at a time, completes the memory allocation of the static variable during the loading of the class, can be accessed directly (conveniently) by the class name, and, of course, is accessible through the object (but this is not recommended). For instance variables, when an instance is not created, the instance variable is allocated one memory, and the instance variable can have multiple copies in memory without compromising (flexibility). 2. Static code blockThe static code block is automatically executed when the class is loaded. If there are multiple static blocks of code, the JVM executes them sequentially in the order in which they appear in the class, and each code block is executed only once. 3. Static methodThe static method can be called directly from the class name, and any instance can also be called, so the static method cannot use the This and Super keywords, and cannot directly access the instance variables and instance methods of the owning class (that is, member variables and member methods without static). Only static member variables and member methods of the owning class can be accessed.    Because the static method is independent of any instance, the static method must be implemented, not abstract. The static method can only access static variables and methods, because non-static variables and methods are required to create an object to access, and static variables/methods do not need to create any objects. Static data or methods that belong to the entire class and not to an object are not associated with any object instances of the class. Therefore, there can be a static method name between the subclass and the parent class with the same name, which does not involve overloading. Therefore, you cannot declare a variable in any method body as static, for example: Fun () {static int i=0;//illegal. In fact, understand that static is only a storage place, and use the direct use, do not need to create objects, you can understand the above considerations. In addition, the generic class is not static, only the inner class can be added static to represent the nested class.     When final declares properties, methods, and classes in Java, it can be decorated with the keyword final. The final variable is a constant, which can only be assigned once; The final method cannot be overridden by the quilt class; The final class cannot be inherited. 1. Final variableDeclaring a final field helps the optimizer make better optimization decisions, because if the compiler knows that the value of the field does not change, it can safely cache the value in the register. The final field also provides an additional level of security by having the compiler force the field to be read-only.its initialization can be in two places, one is its definition, that is, when the final variable is defined directly assigned to it, and the second is in the constructor。    These two places can only be selected, either in the definition of the value, or in the constructor to give a value, not both in the definition of the value, but also in the constructor to give another value. Once initialized, it cannot be changed, meaning it cannot be changed here.for a base type, its value is immutable, whileits reference cannot be changed for object variables。    When the function parameter is the final type, you can read it using the parameter, but you cannot change the value of the parameter. In addition, when an inner class in a method is used with a parameter variable in a method, this parameter must also be declared final for use. in Java, the normal variable system is automatically initialized, the numeric variable is automatically initialized to 0, and the remaining type variables are automatically initialized to null. However, a variable of the final type must show initialization, and the method of initialization must be assigned directly at the time of declaration or in the constructor method, and cannot be assigned by calling the function. 2. Final MethodIf a class does not allow its subclasses to overwrite a method, you can declare this method as the final method.    The final method is used for two reasons: first, locking the method, preventing any inherited classes from modifying its meaning and implementation. Second, efficient. When the compiler encounters a call to the final method, it moves into the inline inline mechanism, which greatly improves execution efficiency. Note that all private methods in a class are implicitly final. Because the private method cannot be taken, it cannot be overloaded. 3. Final classThe final class cannot be inherited, so the member methods of the final class have no chance of being overwritten, and the default is final. In the design class, if the class does not need to have subclasses, the implementation details of the class are not allowed to change, and it is certain that the class will not be extended, then it is designed as the final class. -----------------particular note: For instance constants that are static and final modified, the instance itself can no longer be changed, but for instance variables of some container types (for example, ArrayList, HashMap), the container variable itself cannot be changed , but you can modify the objects stored in the container, which is a lot of programming. Look at an example:
1 Importjava.util.ArrayList;2 3  Public classteststaticfinal {4         Private Static FinalString Strstaticfinalvar = "AAA";5         Private StaticString Strstaticvar =NULL;6         Private FinalString Strfinalvar =NULL;7         Private Static Final intIntstaticfinalvar = 0;8         Private Static FinalInteger Integerstaticfinalvar =NewInteger (8);9         Private Static FinalArraylist<string> Alstaticfinalvar =NewArraylist<string>();Ten  One         Private voidTest () { ASystem.out.println ("----------before-------------value processing"); -System.out.println ("strstaticfinalvar=" + Strstaticfinalvar + ""); -System.out.println ("strstaticvar=" + Strstaticvar + ""); theSystem.out.println ("strfinalvar=" + Strfinalvar + ""); -System.out.println ("intstaticfinalvar=" + Intstaticfinalvar + ""); -System.out.println ("integerstaticfinalvar=" + Integerstaticfinalvar + ""); -System.out.println ("alstaticfinalvar=" + Alstaticfinalvar + ""); +  -  +                 //strstaticfinalvar= "haha haha";//error, final indicates an end state, and the variable itself cannot be changed. AStrstaticvar = "haha haha";//correct, static represents a class variable, and the value can be changed. at                 //strfinalvar= "hehe hehe"; //Error, final represents the final state, the initial value (even if given a null) at the time of definition, and cannot be changed once given.  -                 //intstaticfinalvar=2; //Error, final represents the final state, the initial value (even if given a null) at the time of definition, and cannot be changed once given.  -                 //integerstaticfinalvar=new Integer (8); //Error, final represents the final state, the initial value (even if given a null) at the time of definition, and cannot be changed once given.  -Alstaticfinalvar.add ("AAA");//correctly, the container variable itself has not changed, but the storage content has changed. This rule is very common and has many uses.  -Alstaticfinalvar.add ("BBB");//correctly, the container variable itself has not changed, but the storage content has changed. This rule is very common and has many uses.  -  inSystem.out.println ("----------after-------------value processing"); -System.out.println ("strstaticfinalvar=" + Strstaticfinalvar + ""); toSystem.out.println ("strstaticvar=" + Strstaticvar + ""); +System.out.println ("strfinalvar=" + Strfinalvar + ""); -System.out.println ("intstaticfinalvar=" + Intstaticfinalvar + ""); theSystem.out.println ("integerstaticfinalvar=" + Integerstaticfinalvar + ""); *System.out.println ("alstaticfinalvar=" + Alstaticfinalvar + ""); $         }Panax Notoginseng  -          Public Static voidMain (String args[]) { the                 Newteststaticfinal (). Test (); +         } A}
Output result-------------value before processing----------strstaticfinalvar=aaastrstaticvar=nullstrfinalvar=nullintstaticfinalvar= 0integerstaticfinalvar=8alstaticfinalvar=[]----------After-------------value is processed strstaticfinalvar=aaastrstaticvar= Haha strfinalvar=nullintstaticfinalvar=0integerstaticfinalvar=8alstaticfinalvar=[aaa, BBB] The above content is referenced in the Java keyword final, Static Usage Summary. Copy: http://android.blog.51cto.com/268543/384614/

Java Note--java keyword static, final usage summary

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