如何使用Java 8和lambdas打印JVM的系统属性？ [英] How would I print the JVM's system properties using Java 8 and lambdas?

问题描述

您可以使用属性实例的java /郎/ System.html＃的GetProperties - > System.getProperties（） ;你将如何使用Java 8代码将所有属性打印到控制台？

You can obtain a Properties instance of the JVM properties using System.getProperties(); how would you go about using Java 8 code to print all properties to the console?

推荐答案

一个解决方案：

public final class Foo
{
    private static void printProperty(final Object key, final Object value)
    {
        System.out.println(key + ": " + value);
    }

    public static void main(final String... args)
    {
        System.getProperties().forEach(Foo::printProperty);
    }
}

破旧：

• 属性 extends Hashtable< Object，Object> 本身实现地图<对象，对象> ;


• 地图有 .forEach（） 方法，其参数为 BiConsumer ;


• BiConsumer 是功能界面;


• 静态方法 printProperty（）类 Foo 碰巧有相同的签名为 BiConsumer< Object，Object> ：其返回值为 void ，其第一个参数为对象，它的第二个参数是 Object ;


• 因此我们可以使用 Foo :: printProperty 作为方法参考。

• Properties extends Hashtable<Object, Object> which itself implements Map<Object, Object>;
• Map has a .forEach() method whose argument is a BiConsumer;
• BiConsumer is a functional interface;
• static method printProperty() of class Foo happens to have the same signature as a BiConsumer<Object, Object>: its "return value" is void, its first argument is Object, its second argument is Object;
• we can therefore use Foo::printProperty as a method reference.

更短的版本是：

public final class ShorterFoo
{
    public static void main(final String... args)
    {
        System.getProperties()
            .forEach((key, value) -> System.out.println(key + ": " + value));
    }
}

在运行时，这不会产生任何影响。请注意第二个示例中的类型推断：编译器可以推断键和值的类型为对象。编写这个匿名lambda的另一种方法是：

At runtime, this would not make a difference. Note the type inference in the second example: the compiler can infer that key and value are of type Object. Another way to write this "anonymous lambda" would have been:

(Object key, Object value) -> System.out.println(key + ": " + value)

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（不是这样）旁注：即使它有点过时，你真的想看这个视频（是的，这是一个小时的长度;是的，值得一看。）

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(not so) Side note: even though it is a little outdated, you really want to watch this video (yes, it's one hour long; yes, it is worth watching it all).

（不是这样）旁注2：您可能已经注意到 Map 的 .forEach（）提到默认实现;这意味着您的自定义 Map 实现或外部库的其他实现将能够使用 .forEach（） （例如，Guava的 ImmutableMap s）。存在许多关于Java集合的此类方法;不要犹豫在老狗上使用这些新方法。

(not so) Side note 2: you may have noticed that Map's .forEach() mentions a default implementation; this means that your custom Map implementations, or other implementations from external libraries, will be able to use .forEach() (for instance, Guava's ImmutableMaps). Many such methods on Java collections exist; do not hesitate to use these "new methods" on "old dogs".

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