Generics : 限制泛型可用類型

class GenericNumber2 <T extends Integer>{
 T number;
 public void setNumber(T number){
  this.number = number;
 }
 
 public T getNumber(){
  return number;
 }
}
public class GenericsList {

 public static void main(String[] args) {
  GenericNumber2 <integer> i = new GenericNumber2 <integer>();
  i.setNumber(10);
  System.out.println("Number value : "+i.getNumber());

 }

}

執行結果

若資料型態想改用Double定義，例如： GenericNumber2 < Double > d = new GenericNumber2 < Double > (); 那麼在編譯時，就會指出下列錯誤

Exception in thread "main" java.lang.Error: Unresolved compilation problems: 
 Bound mismatch: The type Double is not a valid substitute for the bounded parameter  of the type GenericNumber2
 Bound mismatch: The type Double is not a valid substitute for the bounded parameter  of the type GenericNumber2

 at GenericsList.main(GenericsList.java:18)

另，定義泛型類別

public class GenericClass <T> {
 ...
}

就如同以下列方式定義

public class GenericClass <T extends Object > {
 ...
}