[concept]什么是闭包

最新推荐文章于 2026-01-01 15:39:09 发布

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本文深入探讨了Java 8中引入的Lambda表达式，包括其用途、聚合操作、流的概念、通道以及一个实际应用的例子。同时介绍了Groovy Closure、JavaScript Closure的基本概念和用法，并提供了相关代码示例。

Lambda Expressions@The Java™ Tutorials
https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html
 
Java 8里面正式推出了Lambda表达式。Lambda出现的背景，
One issue with anonymous classes is that if the implementation of your anonymous class is very simple, such as an interface that contains only one method, then the syntax of anonymous classes may seem unwieldy and unclear. In these cases, you're usually trying to pass functionality as an argument to another method, such as what action should be taken when someone clicks a button. Lambda expressions enable you to do this, to treat functionality as method argument, or code as data.
 
聚合操作
The operations filter, map, and forEach are aggregate operations. Aggregate operations process elements from a stream, not directly from a collection (which is the reason why the first method invoked in this example is stream).
 
流
A stream is a sequence of elements. Unlike a collection, it is not a data structure that stores elements. Instead, a stream carries values from a source, such as collection, through a pipeline.
 
通道
A pipeline is a sequence of stream operations, which in this example is filter- map-forEach.
 
一个例子，连起来使用，效率非常好，和脚本语言一致了。
In addition, aggregate operations typically accept lambda expressions as parameters, enabling you to customize how they behave.
roster
    .stream()
    .filter(
        p -> p.getGender() == Person.Sex.MALE
            && p.getAge() >= 18
            && p.getAge() <= 25)
    .map(p -> p.getEmailAddress())
    .forEach(email -> System.out.println(email));
 
lambda表达式语法
A comma-separated list of formal parameters enclosed in parentheses.
The arrow token
A body, which consists of a single expression or a statement block.  

Groovy closure
http://groovy.codehaus.org/Closures

 
定义
A Groovy Closure is like a "code block" or a method pointer. It is a piece of code that is defined and then executed at a later point. It has some special properties like implicit variables, support for currying and support for free variables
 
语法
A closure definition follows this syntax: { [closureArguments->] statements }
 
样例
def list = ['a','b','c','d']
def newList = []
def clos = { it.toUpperCase() }
list.collect( newList, clos )
assert newList == ["A", "B", "C", "D"]
 
JavaScript closure
http://jibbering.com/faq/notes/closures/
http://www.javascriptkit.com/javatutors/closures.shtml
 
定义
A "closure" is an expression (typically a function) that can have free variables together with an environment that binds those variables (that "closes" the expression).
a closure is the local variables for a function - kept alive after the function has returned, or
a closure is a stack-frame which is not deallocated when the function returns. (as if a 'stack-frame' were malloc'ed instead of being on the stack!)
 
 
 
 
 
 