Java中的声明和定义有什么区别？ [英] What is the difference between declaration and definition in Java?

问题描述

我对这两个术语感到很困惑。我检查了stackoverflow，对于C ++有一个类似的问题，但对于java则没有。

I'm very confused between the two terms. I checked on stackoverflow and there's a similar question for C++ but not for java.

有人可以解释java的两个术语之间的区别吗？

Can someone explain the difference between the two terms for java?

推荐答案

概念上的区别很简单：

• 声明：你声明存在某种东西，例如类，函数或变量。你没有说出那个那个类或函数的样子，你只是说它存在。

• Declaration: You are declaring that something exists, such as a class, function or variable. You don't say anything about what that class or function looks like, you just say that it exists.

定义：你定义如何实现某些东西，例如类，函数或变量，即你说它实际上是什么

Definition: You define how something is implemented, such as a class, function or variable, i.e. you say what it actually is.

在Java 中，两者之间几乎没有区别，正式来说，声明不仅包括标识符，还有它的定义。以下是我个人对这些术语的详细解释：

In Java, there is little difference between the two, and formally speaking, a declaration includes not only the identifier, but also it's definition. Here is how I personally interpret the terms in detail:

• 类：Java并不是真的与C / C ++一样的单独声明和定义（在头文件和cpp文件中）。您可以在声明它们的位置定义它们。

• Classes: Java doesn't really separate declarations and definitions as C/C++ does (in header and cpp files). You define them at the point where you declare them.

函数：当您编写接口（或抽象类）时），你可以说你在宣布一个函数而没有定义它。但是，普通函数总是在声明它们的地方定义。如果您愿意，请将函数体看作其定义。

Functions: When you're writing an interface (or an abstract class), you could say that you're declaring a function, without defining it. Ordinary functions however, are always defined right where they are declared. See the body of the function as its definition if you like.

变量：变量声明看起来像这样：

Variables: A variable declaration could look like this:

int x;

（您声明变量 x 存在且类型为 int ），如果它是局部变量或成员字段。在Java中，没有关于 x 到 define 的信息，除了它应该保留的值，可能由它的赋值决定。

(you're declaring that a variable x exists and has type int) either if it's a local variable or member field. In Java, there's no information left about x to define, except possible what values it shall hold, which is determined by the assignments to it.

以下是我如何使用这些条款的粗略摘要：

Here's a rough summary of how I use the terms:

abstract class SomeClass {                // class decl.
                                          //                           \
    int x;                                // variable decl.            |
                                          //                           |
    public abstract void someMethod();    // function decl.            |
                                          //                           |
    public int someOtherMethod() {        // function decl.            |
                                          //                           | class
        if (Math.random() > .5)           // \                         | def.
            return x;                     //  |  function definition   |
        else                              //  |                        |
            return -x;                    // /                         |
                                          //                           |
    }                                     //                           |
}                                         //                          /

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