我们如何将泛型（类型参数）限制为Java中特定类的子类？
每当您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。如果仅将类型（类）指定为有界参数，则当前泛型类仅接受该特定类的子类型。
您可以通过在角括号内将类型扩展为所需的类来声明绑定参数-
class Sample <T extends Number>
示例
在下面的Java示例中，泛型类Sample将类型参数限制为使用有界参数的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();
}
}输出结果
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），则将生成编译时错误。
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多重界限
您还可以使用extend关键字将多个类型作为有界参数列出，并使用“＆”将它们分隔开。传递给此类的(type)参数应该是所有指定类的子类型。
示例
在下面的示例中，我们设置了上限的Number和Comparable类型，即该类接受属于这两个类的子类的类型。
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();
}
}输出结果
Data value is: 22 Data value is: 20.22
如果将AtomicInteger类作为参数传递给Sample类，因为它不是可比较类的子类型，则将生成编译时错误
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