泛型物件可以例項化嗎？

不可以，T t=new T()是不可以的，編譯器會報錯。由於泛型擦除，編譯器在編譯時無法確定泛型所對應的真實型別

解決方法

使用反射新建例項

Type superclass = getClass().getGenericSuperclass();
ParameterizedType parameterizedType = null;
if (superclass instanceof ParameterizedType) {
  parameterizedType = (ParameterizedType) superclass;
  Type[] typeArray = parameterizedType.getActualTypeArguments();
  if (typeArray != null && typeArray.length > 0) {
   clazz = (Class<T>) typeArray[0];
        
  }
}

執行結果

控制檯輸出

完整程式碼

package com.learn.genericinstance;


import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;

public class GenericInstanceLearn {
  public static void main(String[] args) throws InstantiationException,IllegalAccessException {
    UserDao userDao = new UserDao();
    System.out.println(userDao.toString());
  }
}


class Dao<T> {
  public Class<T> clazz;
  public T user;
  public Dao() throws IllegalAccessException,InstantiationException {
    Type superclass = getClass().getGenericSuperclass();
    ParameterizedType parameterizedType = null;
    if (superclass instanceof ParameterizedType) {
      parameterizedType = (ParameterizedType) superclass;
      Type[] typeArray = parameterizedType.getActualTypeArguments();
      if (typeArray != null && typeArray.length > 0) {
        clazz = (Class<T>) typeArray[0];
        user= clazz.newInstance();
      }
    }
  }

  @Override
  public String toString() {
    return "Dao{" +
        "user=" + user.toString() +
        '}';
  }
}

class UserDao extends Dao<User> {

  public UserDao() throws IllegalAccessException,InstantiationException {
  }

  @Override
  public String toString() {
    return super.toString();
  }
}

class User {
  String name;

  public User() {
    this.name = "default name";
  }

  @Override
  public String toString() {
    return "User{" +
        "name='" + name + '\'' +
        '}';
  }
}

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