Java--Generics

Type parameters

Use when defining a generic class or declaring a variable of a generic class Angle brackets to specify formal type parameters. The relationship between formal type parameters and actual type parameters is similar to the relationship between formal method parameters and actual method parameters, except that type parameters represent types rather than values.

Named type parameters

The recommended naming convention is to use uppercase, single-letter names for type parameters. This differs from the C++ convention (see Appendix A: Comparison with C++ templates), and reflects the assumption that most generic classes will have a small number of type parameters. For common generic patterns, the recommended name is:

K - key, such as a mapped key.
V —— Value, such as the contents of List and Set, or the value in Map.
E ——Exception class.
T —— Generic.

方法签名由方法名称和一个参数列表（方法的参数的顺序和类型）组成。

##1. Why use generics

1. There is no limit to the elements that can be put in. Putting in two different objects may cause exceptions.

2. Throw the object into the collection. The collection loses the state information of the object. The collection only knows that it holds Object, so after taking out the collection elements, it usually Force conversion is also required

2. What are generics

Java's parameterized types are called generics, which allow the program to specify the type of collection elements when creating a collection

3. Generic diamond Syntax

only needs to be followed by a pair of diamond brackets, no generics are required

4. Create a custom class with a generic declaration. When defining a constructor for the class, the constructor name should still be the original class name. Do not add a generic declaration.

5. Derive a subclass from a generic class. When inheriting, you must pass in the actual parameters for the parent class

public class A extends Apple{}

##All methods in it that override the parent class become the corresponding type

You can also pass in no actual parameters

##public class A extends Apple{}

Treat as Object type

6. There is no generic class

No matter what the actual type parameters of generics are, they always have the same class at runtime. No matter which type of actual parameter is passed into the generic type parameter, ( lies in the purpose of the concept of generics in Java, which causes it to only act in the code compilation phase. During the compilation process, After the generic result is correctly verified, the generic related information will be erased. That is to say, the successfully compiled class file does not contain any generic information and will not enter the runtime phase##. #.) For Java, they are still treated as the same class and occupy a memory space in the memory. Therefore, types are not allowed to be used in the declaration and initialization of static methods, static initialization blocks or static variables. Formal parameters

Explanation:

Static variables are shared by all instances of a generic class. For a class declared as MyClass, the method to access the static variables in it is still MyClass.myStaticVar. Regardless of whether the objects are created through new MyClass or new MyClass, they all share a static variable. Assume that type parameters are allowed as types of static variables. Then consider the following situation:

MyClass class1 = new MyClass();

MyClass class2 = new MyClass();

class1.myStaticVar = "hello";

class2.myStaticVar = 5;

Due to the type erasure of the generic system Except (type erasure). myStaticVar is restored to the Object type, and then when class1.myStaticVar= "hello"; is called, the compiler performs forced type conversion, that is, myStaticVar = (String)"hello"; and then when the class2.myStaticVar statement is called, the compiler continues to perform forced type conversion. , myStaticVar = (Integer)Integer.valueOf(5); At this time, myStaticVar is of type String. Of course, this statement will throw a ClassCastException at runtime, so there is a type safety issue. Therefore, the generic system does not allow static variables of a class to use type parameters as variable types.

# Generic classes will not actually be generated in the system, so the generic class cannot be used after the instanceod operator because it does not exist at all!

7. Type wildcard

Note that, If Foo is a subtype of Bar and G is a class or interface with a generic declaration, G is not a subtype of G.

##Assuming that List