通用类型解析 - 编译器步骤

问题描述

假设以下方法（Java 8）：

  public< T>列表与LT;类< T>> getList（）{
返回新的ArrayList<>（）; 
} 
 
 public< T>列表与LT; T> getList2（）{
返回新的ArrayList<>（）; 
} 
  

以下代码使用这些方法：

  @Test 
 public void testLists（）{
 getList（）。add（String.class）; //不会编译
 this。< String> getList（）。add（String.class）; //编译
 getList2（）。add（String.class）; //编译
} 
  

第二次和第三次调用编译正常， ：

---

I do not fully understand the resolution steps here, maybe someone could explain why I need the type witness and the compiler does not infer the type automatically?

解决方案

Because the compiler doesn't/cannot infer generic arguments based on future method calls.

In your case, when you call getList() or getList2() the type T is already inferred as Object. This means getList() will return you an List<Class<Object>> object and you cannot add a Class<String> object into it.

In your third statement, you have List<Object> so you are able to add a Class<String> to it.

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