> For the complete documentation index, see [llms.txt](https://eric3wade.gitbook.io/java/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://eric3wade.gitbook.io/java/learning-the-java-language/generics/bounded-type-parameters.md).

# Bounded Type Parameters

有时我们想要限制参数化类型中所使用的类型参数。比如，一个操作数字的函数可能只接受`Number`类及其子类的实例。这就是我们所说的`Bounded Type Parameters`（有界类型参数）。

在定义有界类型参数时，先列出类型参数名，后面紧跟`extends`关键字，最后是参数的`upper bound`(上界)，例子中我们用的是`Number`。请注意，在这语境中，`extends`被用在一个普通场景表示对类的`extends`（扩展）或者对接口的`implements`（实现）。

```java
public class Box<T> {

    private T t;          

    public void set(T t) {
        this.t = t;
    }

    public T get() {
        return t;
    }

    public <U extends Number> void inspect(U u){
        System.out.println("T: " + t.getClass().getName());
        System.out.println("U: " + u.getClass().getName());
    }

    public static void main(String[] args) {
        Box<Integer> integerBox = new Box<Integer>();
        integerBox.set(new Integer(10));
        integerBox.inspect("some text"); // error: this is still String!
    }
}
```

修改`Box`类中的泛型函数使得它包含有界类型参数，编译就会失败，因为我们在调用`inspect`时任然使用了`String`类型：

```java
Box.java:21: <U>inspect(U) in Box<java.lang.Integer> cannot
  be applied to (java.lang.String)
                        integerBox.inspect("10");
                                  ^
1 error
```

除了限制可用于实例化泛型类型的类型之外，有界类型参数还允许我们调用边界类型中定义的方法：

```java
public class NaturalNumber<T extends Integer> {

    private T n;

    public NaturalNumber(T n)  { this.n = n; }

    public boolean isEven() {
        return n.intValue() % 2 == 0;
    }

    // ...
}
```

`isEven`函数中通过`n`调用`Integer`类中定义的`intValue`函数。

## Multiple Bounds

上面的例子说明了单个上界类型的类型参数的使用方式，然而一个类型参数可以有`Multiple Bounds`(多个上界类型)：

```java
<T extends B1 & B2 & B3>
```

一个有多个上界类型的类型变量即为所有上界类型的子类型。如果其中一个上界类型是一个累的话，他应该出现在第一位，比如：

```
Class A { /* ... */ }
interface B { /* ... */ }
interface C { /* ... */ }

class D <T extends A & B & C> { /* ... */ }
```

如果类`A`没有出现在第一位，在编译过程中将会报错：

```
class D <T extends B & A & C> { /* ... */ }  // compile-time error
```


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://eric3wade.gitbook.io/java/learning-the-java-language/generics/bounded-type-parameters.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.