What is the difference between T and ? in java generics?

1. T represents an unknown type, used in parameters in methods or generics of classes

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public class ExampleA {
　　public <T> void f(T x) {
　　      System.out.println(x.getClass().getName());
　　}
　　
　　public static void main(String[] args) {
　　   ExampleA ea = new ExampleA();
　　   ea.f(" ");
　　   ea.f(10);
　　   ea.f(&#39;a&#39;);
　　   ea.f(ea);
　　}
}

2. ? means a general reference in a generic class. It is a placeholder and data cannot be added to the container.

 
// 注意ArrayList中不能加<?>
List<?> list = new ArrayList();
list.add(123);// 错误

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