java中引入正则表达式、_引入Java表达式

迷南。 2022-12-06 15:29 124阅读 0赞

java中引入正则表达式、

Expressions are essential building blocks of any Java program, usually created to produce a new value, although sometimes an expression assigns a value to a variable. Expressions are built using values, [variables][], operators and method calls.

表达式是任何Java程序的基本构造块，通常创建它们是为了产生新值，尽管有时表达式会为变量分配值。 表达式是使用值， [变量][variables] ，运算符和方法调用构建的。

## Java语句和表达式之间的区别 **(** Difference Between Java Statements and Expressions **)** ##

In terms of the syntax of the Java language, an expression is akin to a [clause in the English language][] which portrays a specific meaning. With the right punctuation, it can sometimes stand on its own, although it can also be a part of a sentence. Some expressions equate to statements by themselves (by adding a semicolon at the end), but more commonly, they comprise part of a statement.

就Java语言的语法而言，表达类似于[英语][clause in the English language]中描述特定含义的[子句][clause in the English language] 。 使用正确的标点符号，有时它也可以独立存在，尽管它也可以作为句子的一部分。 有些表达式本身等同于语句(在末尾添加分号)，但更常见的是，它们构成语句的一部分。

For example,

例如，

(a * 2) is an expression.

b + (a * 2); is a statement. You could say that the expression is a clause, and the statement is the complete sentence since it forms the complete unit of execut

A statement doesn't have to include multiple expressions, however. You can turn a simple expression into a statement by adding a semi-colon:

但是，一条语句不必包含多个表达式。 您可以通过添加分号将简单的表达式转换为语句：

(a * 2);
     Types of Expressions 
     While an expression frequently produces a result, it doesn't always. There are three types of expressions in Java:

*  Those that produce a value, i.e., the result of
    
    那些产生价值的东西，即
    
        (1 + 1)
 *  Those that assign a variable, for example
    
    例如，那些分配变量的变量
    
        (v = 10)
 *  Those that have no result but might have a "side effect" because an expression can include a wide range of elements such as method invocations or increment operators that modify the state (i.e., memory) of a program. 
    
    那些没有结果但可能会有“副作用”的表达式，因为表达式可以包含各种元素，例如方法调用或修改程序状态(即内存)的增量运算符。

## 表达式示例 **(** Examples of Expressions **)** ##

Here are some examples of various types of expressions.

这是各种类型的表达式的一些示例。

**Expressions that Produce a Value**

**产生价值的表达**

Expressions that produce a value use a wide range of Java arithmetic, comparison or conditional operators. For example, arithmetic operators include +, \*, /, <, >, ++ and %. Some [conditional operators][] are ?, ||, and the comparison operators are <, <= and >. See the [Java specification][] for a complete list.

产生值的表达式使用各种Java算术，比较或条件运算符。 例如，算术运算符包括+，\*，/，<，>，++和％。 一些[条件运算符][conditional operators]是？，||，比较运算符是<，<=和>。 有关完整列表，请参见[Java规范][Java specification] 。

These expressions produce a value:

这些表达式产生一个值：

Note the parentheses in the last expression. This directs Java first to compute the value of the expression within the parentheses (just like the arithmetic you learned in school), then complete the rest of the computation.

注意最后一个表达式中的括号。 这将指导Java首先计算括号内的表达式的值(就像您在学校学习的算法一样)，然后完成其余的计算。

**Expressions that Assign a Variable**

**分配变量的表达式**

This program here contains plenty of expressions (shown in bold italics) that each assigns a value.

这里的程序包含大量的表达式(以粗斜体显示)，每个表达式都分配一个值。

int secondsInDay = 0;

int  
整型

daysInWeek = 7;

int  
整型

hoursInDay = 24;

int  
整型

minutesInHour = 60;

int  
整型

secondsInMinute = 60;

boolean  
布尔值

calculateWeek = true;

secondsInDay = secondsInMinute * minutesInHour * hoursInDay; //7

System.out.println(  
System.out.println(

"The number of seconds in a day is: " + secondsInDay);

if (  
如果(

calculateWeek == true)

\{  
\{  
  System.out.println(  
System.out.println(

"The number of seconds in a week is: " + secondsInDay * daysInWeek);

The expressions in the first six lines of the code above, all use the assignment operator to assign the value on the right to the variable on the left.

上面代码的前六行中的表达式全部使用赋值运算符将右侧的值分配给左侧的变量。

The line denoted with //7 is an expression that can stand on its own as a statement. It also shows that expressions can be built up through the use of more than one operator. The final value of the variable secondsInDay is the culmination of evaluating each expression in turn (i.e., secondsInMinute \* minutesInHour = 3600, followed by 3600 \* hoursInDay = 86400).

用// 7表示的行是一个可以独立作为语句的表达式。 它还显示可以通过使用多个运算符来构建表达式。 变量secondsInDay的最终值是依次评估每个表达式的结果(例如，secondsInMinute \* minutesInHour = 3600，然后是3600 \* hoursInDay = 86400)。

**Expressions with No Result**

**没有结果的表达式**

While some expressions produce no result, they can have a side effect which occurs when an expression changes the value of any of its [operands][conditional operators].

尽管某些表达式不会产生结果，但是它们可能会产生副作用，当表达式更改其任何[操作数][conditional operators]的值时，就会发生这种情况。

For example, certain operators are considered to always produce a side effect, such as the assignment, increment and decrement operators. Consider this:

例如，某些运算符被认为总是产生副作用，例如赋值，递增和递减运算符。 考虑一下：

int pro

The only variable changed in this expression is *the product*; *a* and *b* are not changed. This is called a side effect.

此表达式中唯一更改的变量是*乘积* ； *a*和*b*不变。 这称为副作用。

> 翻译自: [https://www.thoughtco.com/expression-2034097][https_www.thoughtco.com_expression-2034097]

java中引入正则表达式、

[variables]: https://www.thoughtco.com/variable-2034325
[clause in the English language]: https://www.thoughtco.com/what-is-clause-grammar-1689850
[conditional operators]: https://www.thoughtco.com/conditional-operator-2034056
[Java specification]: http://docs.oracle.com/javase/specs/jls/se8/html/jls-4.html#jls-4.2
[https_www.thoughtco.com_expression-2034097]: https://www.thoughtco.com/expression-2034097