Chapter10.statics

BULLET POINTS

• A static method should be called using the class name rather than an object reference variable

• A static method can be invoked without any instances of the method's class on the heap.

• A static method is good for a utility method that does not(and will never) depend on a particular instance variable value.

• A static method is not associated with a particular instance--only the class--so it cannot access any instance variable values of its class.It wouldn't know which instance's values to use.

• A static method cannot access a non-static method,since non-static methods are usually associated with instance variable state.

• If you have a class with only static methods,and you do not want the class to be instantiated,you can mark the constructor private.

• A static variable is a variable shared by all members of a given class. There is only one copy of a static variable in a class,rather than one copy per each individual instance for instance variables.

• A static method can access a static variable.

• To make a constant in Java,mark a variable as both static and final.

• A final static variable must be assigned a value either at the time it is declared, or in a static initializer.

  static {
    DOG_CODE = 420;
  }

• The naming convention for constants(final static variables) is to make the name all uppercase.

• A final variable value cannot be changed once it has been assigned.

• Assigning a value to a final instance variable must be either at the time it is declared,or in the constructor.

• A final method cannot be overridden.

• A final class cannot be extended(subclassed).

要点

• 静态方法应该通过类名调用而不是通过对象引用调用

• 当堆上没有静态方法的对象实例时，静态方法依然可以被调用。

• 静态方法是一个好的通用方法，不应该依靠特别的实例变量值。

• 静态方法与类中的实例变量没有联系，它不能使用类的任何实例变量值。

• 静态方法不能调用非静态方法。因为非静态方法与实例变量有联系。

• 如果你的类只有静态方法，并且不希望类被实例化，你可以用private修饰构造函数。

• 静态变量是可以被类的所有实例共享。静态变量所有类只有一份，而不是每个类的实例各自有一份。

• 静态方法能使用静态变量。

• 在Java中创建一个常量，必须使用static和final修饰

• 一个被final和static修饰的变量必须在声明时或者静态初始化时程序时赋值。

  static{
    DOG_CODE = 420;
  }

• 常量的名字规范是全部大写。

• final变量值不能被修改。

• final实例变量必须在声明时或者在构造函数中赋值。

• final方法不能被覆盖。

• final类不能被继承。