Shulou(Shulou.com)06/03 Report--

This article mainly introduces the example analysis of Java generics, which has a certain reference value, interested friends can refer to, I hope you can learn a lot after reading this article, let the editor take you to understand it.

The origin of generics

Why generics are needed

Generally speaking, the data type of Java needs to be determined when it is defined. The advantage of this enforcement is type safety. It is not like getting a ClassCastException data to jvm, so the class executed by data security will be very stable. But if I don't know what type of parameter to pass, because the company's requirements are changing, if it is written dead, it can only be changed once with this requirement, which is very troublesome. Sun also noticed this problem, which made the code less flexible, and they developed generics.

First acquaintance of generics

What is a generic, it can be taken literally as a general type, it is uncertain, it will not go wrong when compiling Java code, but it will report an error at run time, it is unreasonable to say that you are such a first. That's why. Because in order to improve flexibility, the conditions will be relaxed at compile time, but generics must tell jvm at run time what type of data you gave me, otherwise jvm will be confused. So the advantage of generics is to increase the flexibility of types, only on the basis of Java syntax, but generics are still more practical.

When to use generics

The application scenario of generics is applied to the model (which can be understood as a box for storing data). In order to apply more places to this box, I will use a generic representation of the data that needs to be stored, of course, you can pass in multiple values. If it is the same type of objects, it is better to use a generic array. Friends who have studied collections should know that you should make up lessons for those who have not learned.

Syntax of generics

Public class A {T t;}

Disadvantages of generics or why upper and lower boundaries are needed

Generics are powerful, but everything in the east of the world is perfect. It also has its flaws. For example, I have a box that I want to hold apples, but I may also want to hold bananas. It's hard to say. Just give one parameter. What about ten or twenty? Em.... Yes, it is. Let's say everything we want to install belongs to the same class and can be installed as long as it is a subclass of that class. Sun has come up with this idea for us. That is to use upper boundary wildcards. The syntax is that T is generic and M is the parent of T. Let's define a fruit (Fruit), the container is a Dish, and now we can hold any fruit, not bad!

Upper bound Java code

Public class Dish {private T fruitChild; public Dish (T fruitChild) {this.fruitChild = fruitChild;} public T getFruitChild () {return fruitChild;} public void setFruitChild (TF) {this.fruitChild = f;} public static void main (String [] args) {Dish dish = new Dish (); Apple apple = new apple (); / / apple must be Fruit child; dish.setFruitChild (apple); system.out.printf (dish.getFruitChild);}}

Lower bound Java code

Public class Dish {private T appleFather; public Dish (T appleFather) {this.appleFather = appleFather;} public T getAppleFather () {return appleFather;} public void setAppleFather (TF) {this.appleFather = f;} public static void main (String [] args) {Dish dish = new Dish (); Fruit fruit = new Fruit (); / / fruit must be apple son; dish.setAppleFather (fruit); system.out.printf (dish.getAppleFather);}}

What is the upper boundary wildcard?

When generic T gives a restricted field of type A such as type A to any subclass of type A, it can match any type in this restricted field. This representation is called upper boundary wildcard.

Upper bound wildcard understanding

What is a lower boundary wildcard?

When a generic T gives a restricted field of type A to any parent of type A, it can match any type in this restricted field, which is called a lower boundary wildcard.

Lower boundary wildcard understanding #

Shortcomings of upper and lower boundary wildcards

Upper bound

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