在 Java 中，我们如何将泛型(类型参数)限制为特定类的子类?
java 8object oriented programmingprogramming更新于 2025/4/15 9:52:17
每当您想要将类型参数限制为特定类的子类型时，都可以使用有界类型参数。如果您仅将类型(类)指定为有界参数，则当前泛型类只接受该特定类的子类型。
您只需使用类型参数扩展所需类即可声明绑定参数，在尖括号内为 −
class Sample <T extends Number>
示例
在下面的 Java 示例中，泛型类 Sample 使用有界参数将类型参数限制为 Number 类的子类。
class Sample <T extends Number>{
T data;
Sample(T data){
this.data = data;
}
public void display() {
System.out.println("Data value is: "+this.data);
}
}
public class BoundsExample {
public static void main(String args[]) {
Sample<Integer> obj1 = new Sample<Integer>(20);
obj1.display();
Sample<Double> obj2 = new Sample<Double>(20.22d);
obj2.display();
Sample<Float> obj3 = new Sample<Float>(125.332f);
obj3.display();
}
}
输出
Data value is: 20 Data value is: 20.22 Data value is: 125.332
现在，如果将其他类型作为参数传递给此类(例如 String)，将会产生编译时错误。
示例
public class BoundsExample {
public static void main(String args[]) {
Sample<Integer> obj1 = new Sample<Integer>(20);
obj1.display();
Sample<Double> obj2 = new Sample<Double>(20.22d);
obj2.display();
Sample<String> obj3 = new Sample<String>("Krishna");
obj3.display();
}
}
编译时错误
BoundsExample.java:16: error: type argument String is not within bounds of type-variable T
Sample<String> obj3 = new Sample<String>("Krishna");
^
where T is a type-variable:
T extends Number declared in class Sample
BoundsExample.java:16: error: type argument String is not within bounds of type-variable T
Sample<String> obj3 = new Sample<String>("Krishna");
^
where T is a type-variable:
T extends Number declared in class Sample
2 errors
多个界限
您还可以使用 extends 关键字将多个类型列为有界参数，并使用 “&” 将它们分开。传递给此类的 (type) 参数应为所有指定类的子类型。
示例
在下面的示例中，我们设置了 Number 和 Comparable 类型的上限，即此类接受两个类的子类类型。
class Sample <T extends Number & Comparable<T> >{
T data;
Sample(T data){
this.data = data;
}
public void display() {
System.out.println("Data value is: "+this.data);
}
}
public class BoundsExample {
public static void main(String args[]) {
Sample<Integer> obj1 = new Sample<Integer>(22);
obj1.display();
Sample<Double> obj2 = new Sample<Double>(20.22d);
obj2.display();
}
}
输出
Data value is: 22 Data value is: 20.22
如果将 AtomicInteger 类作为参数传递给 Sample 类，由于它不是可比较类的子类型，因此会产生编译时错误
示例
import java.util.concurrent.atomic.AtomicInteger;
class Sample <T extends Number & Comparable<T> >{
T data;
Sample(T data){
this.data = data;
}
public void display() {
System.out.println("Data value is: "+this.data);
}
}
public class BoundsExample {
public static void main(String args[]) {
Sample<Integer> obj1 = new Sample<Integer>(22);
obj1.display();
Sample<Double> obj2 = new Sample<Double>(20.22d);
obj2.display();
Sample<AtomicInteger> obj3 = new Sample<AtomicInteger>(124);
obj3.display();
}
}
编译时错误
BoundsExample.java:16: error: type argument String is not within bounds of type-variable T
Sample<String> obj3 = new Sample<String>("Krishna");
^
where T is a type-variable:
T extends Number declared in class Sample
BoundsExample.java:16: error: type argument String is not within bounds of type-variable T
Sample<String> obj3 = new Sample<String>("Krishna");
^
where T is a type-variable:
T extends Number declared in class Sample
2 errors
