泛型中的有界类型参数

一般来说泛型编程中只要指定泛型的名字就可以了，但是如果我们想对泛型中的参数进行限制，怎么办？

比如一个泛型class A，其中E是没有限制的，如果相对E限制其必须是String的子类。

这时候要使用有界类型参数（Bounded Type Parameter），语法如下

extends superClassName>

使用例子

// This class only accepts type parameters as any class
// which extends class A or class A itself.
// Passing any other type will cause compiler time error
 
class Boundextends 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 bec = new Bound(new C());
		bec.doRunTest();
		
		// Creating object of sub class B and
		// passing it to Bound as a type parameter.
		Bound beb = new Bound(new B());
		beb.doRunTest();
		
		// similarly passing super class A
		Bound bea = new Bound(new A());
		bea.doRunTest();
		
	}
}

Inside sub class C
Inside sub class B
Inside super class A

Bound bes = new Bound(new String());
 
error: type argument String is not within bounds of type-variable T

extends superClassName & Interface>

class Boundextends 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 bea = new Bound(new A());
		bea.doRunTest();
		
	}
}

Inside super class A