Java泛型方法声明基础 [英] Java generic method declaration fundamentals

问题描述

我开始为 Java 学习 Generics ，我读了几个教程，但我有点困惑并且不确定如何声明泛型方法。

I'm starting to learn Genericsfor Java and I read several tutorials, but I'm a bit confused and not sure how a generic method is declared.

当我使用泛型类型时，定义方法的正确顺序是什么？我找到了这个样本，什么时候需要使用尖括号？什么时候不需要？

When I use a generic type, what is the correct order to define the method? I found this sample, when do I need to use the angle brackets and when not?

public class Box<A> {

    private A a;
    ...

    public void setA(A a) {
        this.a = a;
    }

    public <A> List<A> transform(List<A> in) {
        return null;
    }

    public static <A> A getFirstElement(List<A> list) {
        return null;
    }

    public A getA() {
        return a;
    }

推荐答案

问题是你的代码使用相同的字符A，但它在不同的地方有几个不同的含义：

The problem is that your code is using the same character A, but it has several different "meanings" in the different places:

public class Box<T> { 

需要大括号，因为您在这里说：Box使用通用类型，称为T.

braces required, because you are saying here: Box uses a generic type, called T.

用法 T没有大括号：

private T a;
public void setA(T a) {

但是

public <T2> List<T2> transform(List<T2> in) {

正在引入另一个类型参数。我把它命名为T2，以明确它 与T相同。想法是T2的范围仅 那个方法转换。其他方法不知道T2！

is introducing another type parameter. I named it T2 to make it clear that it is not the same as T. The idea is that the scope of T2 is only that one method transform. Other methods do not know about T2!

public static <A> A getFirstElement(List<A> list) {

与上面相同 - 此处为T3; - ）

Same as above - would be "T3" here ;-)

编辑您的评论：您不能使用静态方法使用类范围类型T.这根本不可能！有关原因，请参见此处！

EDIT to your comment: you can't have a static method use the class-wide type T. That is simply not possible! See here for why that is!

编辑2：通用版允许您编写泛型的代码（因为它可以处理不同的类）;但仍然给你编译时安全性。示例：

EDIT no.2: the generic allows you to write code that is generic (as it can deal with different classes); but still given you compile-time safety. Example:

 Box<String> stringBox = new Box<>();
 Box<Integer> integerBox = new Box<>();
 integerBox.add("string"); // gives a COMPILER error!

在人们使用仿制药之前，他们只能处理全部的对象地点;和手工铸造。

Before people had generics, they could only deal with Object all over the place; and manual casting.

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