泛型型协变逆变

by Fabian Terh

由Fabian Terh

Java泛型类型简介：协变和逆变 (An introduction to generic types in Java: covariance and contravariance)

种类 (Types)

Java is a statically typed language, which means you must first declare a variable and its type before using it.

Java是一种静态类型的语言，这意味着您必须先声明一个变量及其类型，然后才能使用它。

For example: int myInteger = 42;

例如： int myInteger = 42;

Enter generic types.

输入通用类型。

通用类型 (Generic types)

Definition: “A generic type is a generic class or interface that is parameterized over types.”

定义：“ 泛型类型是通过类型进行参数化的泛型类或接口。”

Essentially, generic types allow you to write a general, generic class (or method) that works with different types, allowing for code re-use.

本质上，泛型类型允许您编写可与不同类型一起使用的泛型通用类(或方法)，从而可以重复使用代码。

Rather than specifying obj to be of an int type, or a String type, or any other type, you define the Box class to accept a type parameter <;T>. Then, you can use T to represent that generic type in any part within your class.

您可以将Box类定义为接受类型参数< ; T>，而不是将obj指定为int类型， String类型或任何其他类型。然后，CA n用t代表在类中的任何一部分泛型类型。

Now, enter covariance and contravariance.

现在，输入协方差和自变量。

协方差和自变量 (Covariance and contravariance)

定义 (Definition)

Variance refers to how subtyping between more complex types relates to subtyping between their components (source).

差异是指更复杂类型之间的子类型如何与其组件( 源 )之间的子类型相关。

An easy-to-remember (and extremely informal) definition of covariance and contravariance is:

协方差和协方差的一个易于记忆(非常非正式)的定义是：

Covariance: accept subtypes协方差：接受子类型
Contravariance: accept supertypes矛盾：接受超类型

数组 (Arrays)

In Java, arrays are covariant, which has 2 implications.

在Java中， 数组是covariant ，有2个含义。

Firstly, an array of type T[] may contain elements of type T and its subtypes.

首先，类型为T[]的数组可以包含类型T及其子类型的元素。

Number[] nums = new Number[5];nums[0] = new Integer(1); // Oknums[1] = new Double(2.0); // Ok

Secondly, an array of type S[] is a subtype of T[] if S is a subtype of T.

其次，类型的数组S[]是的子类型T[]如果S是的子类型T 。

Integer[] intArr = new Integer[5];Number[] numArr = intArr; // Ok

However, it’s important to remember that: (1) numArr is a reference of reference type Number[] to the “actual object” intArr of “actual type” Integer[].

但是，重要的是要记住：(1) numArr是引用类型Number[]对“ actual type” Integer[]的“ actual object” intArr 。

Therefore, the following line will compile just fine, but will produce a runtime ArrayStoreException (because of heap pollution):

因此，以下行将编译得很好，但将生成运行时ArrayStoreException (由于堆污染)：

numArr[0] = 1.23; // Not ok

It produces a runtime exception, because Java knows at runtime that the “actual object” intArr is actually an array of Integer.

它产生一个运行时异常，因为Java在运行时知道“实际对象” intArr实际上是Integer的数组。

泛型 (Generics)

With generic types, Java has no way of knowing at runtime the type information of the type parameters, due to type erasure. Therefore, it cannot protect against heap pollution at runtime.

对于泛型类型，由于类型擦除，Java无法在运行时知道类型参数的类型信息。因此，它无法在运行时防止堆污染。

As such, generics are invariant.

因此，泛型是不变的。

ArrayList<Integer> intArrList = new ArrayList<>();ArrayList<Number> numArrList = intArrList; // Not okArrayList<Integer> anotherIntArrList = intArrList; // Ok

The type parameters must match exactly, to protect against heap pollution.

类型参数必须完全匹配，以防止堆污染。

But enter wildcards.

但是输入通配符。

通配符，协方差和逆方差 (Wildcards, covariance, and contravariance)

With wildcards, it’s possible for generics to support covariance and contravariance.

使用通配符，泛型有可能支持协方差和协方差。

Tweaking the previous example, we get this, which works!

调整前面的示例，我们得到了这个，它有效！

ArrayList<Integer> intArrList = new ArrayList<>();ArrayList<? super Integer> numArrList = intArrList; // Ok

The question mark “?” refers to a wildcard which represents an unknown type. It can be lower-bounded, which restricts the unknown type to be a specific type or its supertype.

问号“？” 表示代表未知类型的通配符。它可以是下界的，这将未知类型限制为特定类型或其超类型。

Therefore, in line 2, ? super Integer translates to “any type that is an Integer type or its supertype”.

因此，在第2行， ? super Integer ? super Integer转换为“任何为Integer类型或其超类型的类型”。

You could also upper-bound the wildcard, which restricts the unknown type to be a specific type or its subtype, by using ? extends Integer.

您还可以使用? extends Integer来限制通配符的范围，通配符将未知类型限制为特定类型或其子类型? extends Integer ? extends Integer 。

只读和只写 (Read-only and write-only)

Covariance and contravariance produce some interesting outcomes. Covariant types are read-only, while contravariant types are write-only.

协方差和自变量产生一些有趣的结果。 协变类型是只读的，而协变类型是只写的。

Remember that covariant types accept subtypes, so ArrayList<? extends Number> can contain any object that is either of a Number type or its subtype.

还记得协变类型接受子类型ArrayList<? extends Numb ，所以ArrayList<? extends Numb ArrayList<? extends Numb ER>可以包含任何对象，或者是of a数字类型或子类型。

In this example, line 9 works, because we can be certain that whatever we get from the ArrayList can be upcasted to a Number type (because if it extends Number, by definition, it is a Number).

在此示例中，第9行有效，因为我们可以确定从ArrayList中获得的任何内容都可以转换为Number类型(因为如果它扩展Number ，顾名思义，它就是 Number )。

But nums.add() doesn’t work, because we cannot be sure of the “actual type” of the object. All we know is that it must be a Number or its subtypes (e.g. Integer, Double, Long, etc.).

但是nums.add()不起作用，因为我们不能确定对象的“实际类型”。我们所知道的是它必须是一个Number或其子类型(例如Integer，Double，Long等)。

With contravariance, the converse is true.

在相反的情况下，反之亦然。

Line 9 works, because we can be certain that whatever the “actual type” of the object is, it must be Integer or its supertype, and thus accept an Integer object.

第9行行得通，因为我们可以确定，无论对象的“实际类型”是什么，它都必须是Integer或它的超类型，因此可以接受Integer对象。

But line 10 doesn’t work, because we cannot be sure that we will get an Integer. For instance, nums could be referencing an ArrayList of Objects.

但是第10行不起作用，因为我们无法确定是否会获得Integer 。例如， nums可以引用Objects的ArrayList。

应用领域 (Applications)

Therefore, since covariant types are read-only and contravariant types are write-only (loosely speaking), we can derive the following rule of thumb: “Producer extends, consumer super”.

因此，由于协变量类型是只读的，而协变量类型是只写的(松散地说)，我们可以得出以下经验法则： “生产者扩展，消费者超级” 。

A producer-like object that produces objects of type T can be of type parameter <? extends T>, while a consumer-like object that consumes objects of type T can be of type parameter <? super T>.

产生类型T对象的类似生产者的对象可以是参数<? extends <? extends T>，而消费类对象消费 T型可以是参数meter <? 超级T>。

翻译自: https://www.freecodecamp.org/news/understanding-java-generic-types-covariance-and-contravariance-88f4c19763d2/