Detailed explanation of Java generics (summary of knowledge points)

Concept

Source: Java was not universal until the design started, until it was introduced in JDK 1.5. Java generics are implemented through erasure. Do you know what erasure is? Look down.

Concept: Regular classes and methods can only use specific types; basic types or custom classes. If you want to write code that can be applied to many types of code, this strict restriction will impose a lot of constraints on the code. Generics implement the concept of parameterized types, allowing code to be applied to multiple types. When generics emerged in programming languages, their original purpose was to enable classes and methods to have broad expressive power.

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Simple generics

There are many reasons why generics appear, one of the most important reasons is Create container class. We don't specify the type yet, but decide what type to use later. To achieve this, type parameters need to be used after the class name, enclosed in angle brackets. Then, when this class is used, parameters of this type will be replaced with the actual type. In the example below, t is a type parameter. The code is as follows:

# However, in many source codes, some generic classes have multiple generic parameters, such as Java. UTIL function. Double function, with three types of parameters t, u, r.

Generic methods

Generic methods make methods independent of classes. When writing generic code, the basic guideline is to use generic methods whenever possible. This means that if you can use generic methods instead of generic methods for the entire class, then you can use generic methods because they make things clearer. Additionally, for static methods, the type parameters of a generic class cannot be accessed, so if static methods need to use generalization capabilities, they must be made into generic methods.

Generic erasure

When I read the "Mysteries of Deletion" section in the general chapter of Java Programming Thoughts, I was particularly dizzy, and then when I looked down, I became increasingly confused. Especially when you look at boundaries and wildcards, it's a little confusing. First let’s look at what erasure is. In generic code, information about generic parameter types is not available. Java generics are implemented through erasure, which means that when using generics, any specific type is erased and the only thing you know about it is using objects. Since generics were not originally introduced in Java, it was designed to be compatible with older versions of the JDK. ErAcess is a compromise between Java's generic implementations. So when you run, list is the same as list. Note that it is running, but at compile time, list represents a list container of this string type, and list represents a list container of the current integer type. For example, examples from Java programming ideas.

Boundaries of generics

Generics in Java, when compiled, T represents a type. If no boundary is specified, it is equal to the object. . We can specify boundaries for generics using the extends keyword. To be able to call f(), we can help the generic class given a generic class boundary, telling the compiler that it must accept types behind the boundary. Extended keywords are used here. Change the above code to

Summary

Java generics features are widely used in many open source frameworks. This needs to be understood in depth. I think as the years of coding begin, I will have a different understanding in the future. Now I can only know so much through reading books.

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