如果使用有界的类型参数，Java编译器会用其界线替换通用类型的类型参数。

package com.learnfk;

public class GenericsTester {
   public static void main(String[] args) {
      Box<Integer> integerBox=new Box<Integer>();
      Box<Double> doubleBox=new Box<Double>();

      integerBox.add(new Integer(10));
      doubleBox.add(new Double(10.0));

      System.out.printf("Integer Value :%d\n", integerBox.get());
      System.out.printf("Double Value :%s\n", doubleBox.get());
   }
}

class Box<T extends Number> {
   private T t;

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

   public T get() {
      return t;
   }   
}

在这种情况下，java编译器将用Number类替换T，并且在类型擦除之后，编译器将为以下代码生成字节码。

package com.learnfk;

public class GenericsTester {
   public static void main(String[] args) {
      Box integerBox=new Box();
      Box doubleBox=new Box();

      integerBox.add(new Integer(10));
      doubleBox.add(new Double(10.0));

      System.out.printf("Integer Value :%d\n", integerBox.get());
      System.out.printf("Double Value :%s\n", doubleBox.get());
   }
}

class Box {
   private Number t;

   public void add(Number t) {
      this.t=t;
   }

   public Number get() {
      return t;
   }   
}

在两种情况下，输出都是相同的-

Integer Value :10
Double Value :10.0

参考链接

https://www.learnfk.com/java-generics/java-generics-bound-typeerasure.html