现实世界中有许多价值观是永远不会改变的。正方形总是有四条边，小数点后三位的圆周率总是3.142，一天总是有24小时。这些值保持不变。在编写程序时，以同样的方式表示它们是有意义的，因为这些值一旦分配给变量就不会被修改。这些变量称为常量。

将变量声明为常量

在声明变量时，我们展示了为int变量赋值很容易：

int numberOfHoursInADay = 24;

我们知道这个值在现实世界中永远不会改变，所以我们确保它在程序中不会改变。这是通过添加关键字修饰符来完成的

final final int NUMBER_OF_HOURS_IN_A_DAY = 24;

除了

finalkeyword you should have noticed that the case of the variable name has changed to be uppercase as per the standard Java naming convention

If we now try and change the value of

NUMBER_OF_HOURS_IN_A_DAY final int NUMBER_OF_HOURS_IN_A_DAY = 24; NUMBER_OF_HOURS_IN_A_DAY = 36;

we will get the following error from the compiler:

cannot assign a value to final variable NUMBER_OF_HOURS_IN_A_DAY

The same goes for any of the other primitive data type variables. To make them into constants just add the

final

Where to Declare Constants

As with normal variables you want to limit the scope of constants to where they are used. If the value of the constant is only needed in a method then declare it there: public static int calculateHoursInDays(int days) { final int NUMBER_OF_HOURS_IN_A_DAY = 24; return days * NUMBER_OF_HOURS_IN_A_DAY; }

If it’s used by more than one method then declare it at the top of the class definition:

public class AllAboutHours{ private static final int NUMBER_OF_HOURS_IN_A_DAY = 24; public int calculateHoursInDays(int days) { return days * NUMBER_OF_HOURS_IN_A_DAY; } public int calculateHoursInWeeks(int weeks) { final int NUMBER_OF_DAYS_IN_A_WEEK = 7; return weeks * NUMBER_OF_DAYS_IN_A_WEEK * NUMBER_OF_HOURS_IN_A_DAY; } }

Notice how I’ve also added the keyword modifiers

privateand staticto the variable declaration of NUMBER_OF_HOURS_IN_A_DAY. This means that the constant can only be used by its class (hence the privatescope) but you could just as easily make it a publicconstant if you want other classes to have access to it. The statickeyword is to allow the value of the constant to be shared amongst all instances of an object. As it's the same value for every object created, it only needs to have one instance

Using the Final Keyword with Objects

It’s very important to realize that when it comes to objects, Java does not support constants as you might expect. If you assign a variable to an object using the

final

A Brief Note on the Const Keyword

You may have noticed in the reserved words list that there is a keyword called const . This is not used with constants, in fact, it’s not used at all in the Java language