类< T>的通用性质是什么？意思？什么是T？ [英] What does the generic nature of the class Class<T> mean? What is T?

问题描述

当涉及到集合时，我理解泛型。但是在 Class< T> 类的情况下是什么意思？当你实例化一个 Class 对象时，只有一个对象。那么为什么 T 参数？它指定什么？

类型参数< T>

code>已添加到 java.lang.Class 以启用一个特定的成语 ¹ - 使用类对象作为类型安全对象工厂。基本上，添加< T> 可以以类型安全的方式实例化类，例如：

  T instance = myClass.newInstance（）; 
  

类型参数< T> 类本身，通过在通用类中存储类< T> 或将其作为参数传递给通用方法，可以避免类型擦除的不愉快影响。注意 T 本身不足以完成此任务 ² ： T 被擦除，所以它变成 java.lang.Object 。

这是一个经典的例子其中< T> 类的参数变得重要。在下面的示例中，Java编译器能够确保类型安全性，允许您从SQL字符串和类< T> 的实例生成类型化集合。请注意，该类用作工厂，并且其类型安全性可以在编译时验证：

  public static< T>集合< T> select（Class< T> c，String sqlStatement）{
 Collection< T> result = new ArrayList< T>（）; 
 / *运行sql查询使用jdbc * / 
 for（/ * iterate over jdbc results * /）{
 T item = c.newInstance（）; 
 / *使用反射，并从sql结果中设置所有项目的字段* / 
 result.add（item）; 
} 
 return result;因为Java会删除类型参数，使它成为一个 
} 
  


$ b < java.lang.Object 或指定为泛型上限​​的类别，访问 select 方法中的 Class< T> 对象很重要。由于 newInstance 返回类型< T> 的对象，编译器可以执行类型检查， / p>

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¹ SUN Oracle已发布一篇解释所有这一切的好文章。

² 这不同于没有类型擦除的泛型的实现，例如.NET中的一种。

³ Oracle的泛型教程。

I understand generics when it comes to collections. But what does it mean in the case of the Class<T> class? When you instantiate a Class object, there's only one object. So why the T parameter? What is it specifying? And why is it necessary (if it is)?

解决方案

Type parameter <T> has been added to java.lang.Class to enable one specific idiom¹ - use of Class objects as type-safe object factories. Essentially, the addition of <T> lets you instantiate classes in a type-safe manner, like this:

T instance = myClass.newInstance();

Type parameter <T> represents the class itself, enabling you to avoid unpleasant effects of type erasure by storing Class<T> in a generic class or passing it in as a parameter to a generic method. Note that T by itself would not be sufficient to complete this task²: the type of T is erased, so it becomes java.lang.Object under the hood.

Here is a classic example where <T> parameter of the class becomes important. In the example below, Java compiler is able to ensure type safety, letting you produce a typed collection from a SQL string and an instance of Class<T>. Note that the class is used as a factory, and that its type safety can be verified at compile time:

public static <T> Collection<T> select(Class<T> c, String sqlStatement) {
    Collection<T> result = new ArrayList<T>();
    /* run sql query using jdbc */
    for ( /* iterate over jdbc results */ ) {
        T item = c.newInstance();
        /* use reflection and set all of item’s fields from sql results */
        result.add(item);
    }
    return result;
}

Since Java erases the type parameter, making it a java.lang.Object or a class specified as the generic's upper bound, it is important to have access to the Class<T> object inside the select method. Since newInstance returns an object of type <T>, the compiler can perform type checking, eliminating a cast.

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¹ SUN Oracle has published a good article explaining all this.
² This is different from implementations of generics without type erasure, such as one in .NET.
³ Java Generics tutorial by Oracle.

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