Java 中带有泛型的有界类型
原文:https://www.geeksforgeeks.org/bounded-types-generics-java/
有时，您可能希望限制可在参数化类型中用作类型参数的类型。例如，对数字进行操作的方法可能只想接受数字或其子类的实例。这就是有界类型参数的用途。
- Sometimes we don't want the whole class to be parameterized. In this case, we can create a Java generic method. Because the constructor is a special method, we can also use generic types in the constructor.
- Suppose we want to limit the object types that can be used in parameterized types. For example, in a method of comparing two objects, we want to ensure that the accepted objects are comparable.
- These methods are called similarly to unbounded methods, except that if we try to use any incomparable classes, it will throw a compile-time error.
如何在 Java 中声明有界类型参数？
- Lists the names of type parameters,
- Unicom extends keyword
- And arranged according to its upper limit. (In the following example, c is a . )
语法
<T extends superClassName>
注意，在这个上下文中，extends 一般用于表示“extends”(如在类中)。此外，这指定 T 只能被超类名或超类名的子类替换。因此，超类定义了一个包含性的上限。
让我们举一个如何用泛型实现有界类型(扩展超类)的例子。
Java
// This class only accepts type parametes as any class
// which extends class A or class A itself.
// Passing any other type will cause compiler time error
class Bound<T extends A>
{
private T objRef;
public Bound(T obj){
this.objRef = obj;
}
public void doRunTest(){
this.objRef.displayClass();
}
}
class A
{
public void displayClass()
{
System.out.println("Inside super class A");
}
}
class B extends A
{
public void displayClass()
{
System.out.println("Inside sub class B");
}
}
class C extends A
{
public void displayClass()
{
System.out.println("Inside sub class C");
}
}
public class BoundedClass
{
public static void main(String a[])
{
// Creating object of sub class C and
// passing it to Bound as a type parameter.
Bound<C> bec = new Bound<C>(new C());
bec.doRunTest();
// Creating object of sub class B and
// passing it to Bound as a type parameter.
Bound<B> beb = new Bound<B>(new B());
beb.doRunTest();
// similarly passing super class A
Bound<A> bea = new Bound<A>(new A());
bea.doRunTest();
}
}
输出
Inside sub class C
Inside sub class B
Inside super class A
现在，我们只限于类型 A 和它的子类，所以它会为任何其他类型的子类抛出一个错误。
爪哇
// This class only accepts type parametes as any class
// which extends class A or class A itself.
// Passing any other type will cause compiler time error
class Bound<T extends A>
{
private T objRef;
public Bound(T obj){
this.objRef = obj;
}
public void doRunTest(){
this.objRef.displayClass();
}
}
class A
{
public void displayClass()
{
System.out.println("Inside super class A");
}
}
class B extends A
{
public void displayClass()
{
System.out.println("Inside sub class B");
}
}
class C extends A
{
public void displayClass()
{
System.out.println("Inside sub class C");
}
}
public class BoundedClass
{
public static void main(String a[])
{
// Creating object of sub class C and
// passing it to Bound as a type parameter.
Bound<C> bec = new Bound<C>(new C());
bec.doRunTest();
// Creating object of sub class B and
// passing it to Bound as a type parameter.
Bound<B> beb = new Bound<B>(new B());
beb.doRunTest();
// similarly passing super class A
Bound<A> bea = new Bound<A>(new A());
bea.doRunTest();
Bound<String> bes = new Bound<String>(new String());
bea.doRunTest();
}
}
输出:
error: type argument String is not within bounds of type-variable T
多重界限
有界类型参数可以与方法以及类和接口一起使用。
Java 泛型也支持多边界，也就是说，在这种情况下，A 可以是一个接口或类。如果 A 是类，那么 B 和 C 应该是接口。我们不能在多个范围内有一个以上的类。
语法:
<T extends superClassName & Interface>
Java
class Bound<T extends A & B>
{
private T objRef;
public Bound(T obj){
this.objRef = obj;
}
public void doRunTest(){
this.objRef.displayClass();
}
}
interface B
{
public void displayClass();
}
class A implements B
{
public void displayClass()
{
System.out.println("Inside super class A");
}
}
public class BoundedClass
{
public static void main(String a[])
{
//Creating object of sub class A and
//passing it to Bound as a type parameter.
Bound<A> bea = new Bound<A>(new A());
bea.doRunTest();
}
}
输出
Inside super class A
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