the Java keywords: final ,static &public

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10. the Java keywords: final &static&public

10.1 the keyword "final"

​ only for classes, methods and variables:

• for classes: the classes cannot be inherited.
• for methods: the methods cannot be overwrite.

• for variable: the variable can be assigned only once. And after then the variable will be treated as a constant , be stocked at the static area(constant area)

  Note: if a pointer is modified by “final”, it’s address cannot be changed. but the attributes of object it point can be assigned.

10.2 the keyword “static”

• only for methods and variables, cannot be used on classes.
• The static methods or variables will be shared by all of the class’s objects.
• The static methods or variables are only related to the class, not the object. then all the object don’t need to apply heap’s memory for these attributes.
• when to use static? all the objects are share a same attributes or method.
  • for common function extraction from class. eg:

  public static boolean isNull(sting [] args){// judge a string Null or not
      if(args!=Null){
          return ture;
      }else
      return false;    
  }
             

• the static method and variable are stocked in the “Method Area”, not in heap. And if the class is public , other class’s objects can access it,too. So it can be used directly by "class.method&variable or interface.method " , do not need to new an object in heap, saving the memory.

Note: Static method can only access the static contexts, cannot access the non-static contexts. The non-static contexts must be instantiated to a object, then it can be accessed by static method

Keyword package: 指定类的路径。同一个包表示路径相同。

​ 作用：1、区分同名的类

​ 2、 通过包名（路径）找到对应的类

10.3 Privilege modifier：

public>protected>(default)>private.

可以应用修饰符的对象有三种：类(Class)、方法(Method)、变量(Variable)

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修饰类成员：能访问？1 ：0	public	protected	default	private
同一个类中：	1	1	1	1
同一个包中：	1	1	1
不同包，子类中：	1	1
不同包，非子类中：	1