Clojure Functional Programming & JVM Backend Development · 课时

从 Clojure 调用 Java

了解如何直接从 Clojure 代码调用 Java 方法、访问字段并实例化对象。

从 Clojure 调用 Java 是 CoddyKit 上的免费 Clojure Functional Programming & JVM Backend Development 课时。这是第 1 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Clojure Functional Programming & JVM Backend Development 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Clojure Functional Programming & JVM Backend Development 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

Clojure & Java: Best Friends!

Clojure runs on the Java Virtual Machine (JVM), which means it can use all the amazing Java libraries directly! This is called Java Interop.

Access Java classes and methods.
Leverage the vast Java ecosystem.
Combine Clojure's power with Java's stability.

This lesson will show you how to call Java code from Clojure.

Creating Java Objects

To use a Java class, you often need to create an instance of it. In Clojure, you can do this using a special syntax.

Use (new ClassName args) or the shorthand (ClassName. args).
The . after the class name is a convenient way to call a constructor.

Example: (java.util.Date.) creates a new Date object.

Demo: New Date Object

Try creating a new java.util.Date object. It will represent the current date and time.

(ns my-app.core
  (:gen-class))

(defn -main
  "Creates and prints a new Java Date object."
  [& args]
  (let [current-date (java.util.Date.)]
    (println "Current date:" current-date)))

Calling Static Java Methods

Static methods belong to the class itself, not to a specific object instance. You can call them directly on the class.

Syntax: (ClassName/staticMethodName args)
The / separates the class name from the static method name.

Example: (java.lang.Math/random) calls the random static method of the Math class.

Demo: Math.random()

Let's get a random number using Java's Math/random static method. It returns a double between 0.0 (inclusive) and 1.0 (exclusive).

(ns my-app.core
  (:gen-class))

(defn -main
  "Generates and prints a random number using Java's Math/random."
  [& args]
  (let [rand-num (java.lang.Math/random)]
    (println "Random number:" rand-num)))

Calling Instance Java Methods

Once you have an object, you can call its instance methods. These methods operate on that specific object's data.

Syntax: (.methodName instance args) or (. instance methodName args).
The . before the method name indicates an instance method call.

Example: (.getTime my-date) calls the getTime method on the my-date object.

Demo: Date.getTime()

Create a Date object and then call its getTime() method. This method returns the number of milliseconds since January 1, 1970, 00:00:00 GMT.

(ns my-app.core
  (:gen-class))

(defn -main
  "Creates a Date object and calls its getTime() method."
  [& args]
  (let [current-date (java.util.Date.)
        time-in-millis (.getTime current-date)]
    (println "Current date:" current-date)
    (println "Milliseconds since epoch:" time-in-millis)))

Accessing Java Fields

Java objects can have fields (variables) that store data. You can access these fields directly from Clojure.

Syntax: (.-fieldName instance) for getting a field's value.
For static fields, use (.-staticFieldName ClassName).

Example: (.-out System) accesses the out static field of the System class, which is a PrintStream.

Demo: System.out Field

The java.lang.System class has a static field named out, which is a PrintStream object. We can access it to print to the console.

(ns my-app.core
  (:gen-class))

(defn -main
  "Accesses System.out and prints a message using its println method."
  [& args]
  (let [out-stream (.-out java.lang.System)]
    (.println out-stream "Hello from System.out!")))

Quick Check: Java Interop

You've learned how to interact with Java classes. Which Clojure expression uses the ClassName. shorthand to instantiate a java.io.File object for the path "myfile.txt"?

Recap: Java Interop Basics

Great job! You've learned the fundamentals of interacting with Java from Clojure:

Instantiate objects: Use (ClassName. args) or (new ClassName args).
Call static methods: Use (ClassName/staticMethodName args).
Call instance methods: Use (.methodName instance args) or (. instance methodName args).
Access fields: Use (.-fieldName instance).

This powerful capability allows you to leverage the entire Java ecosystem within your Clojure projects. Next, we'll explore implementing Java interfaces.

常见问题解答

「从 Clojure 调用 Java」课时是免费的吗？

是的 — 「从 Clojure 调用 Java」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Clojure Functional Programming & JVM Backend Development 课程的其余内容，请升级到 CoddyKit PRO。 Clojure Functional Programming & JVM Backend Development 课程共包含 4 节课。

「从 Clojure 调用 Java」这节课中我会学到什么？

了解如何直接从 Clojure 代码调用 Java 方法、访问字段并实例化对象。你通过在浏览器中直接运行的动手代码来练习 Clojure Functional Programming & JVM Backend Development，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 Clojure Functional Programming & JVM Backend Development 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Clojure Functional Programming & JVM Backend Development 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 1 节课，共 4 节。

「从 Clojure 调用 Java」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 Clojure Functional Programming & JVM Backend Development 课中编写并运行代码吗？

能。每节 Clojure Functional Programming & JVM Backend Development 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

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