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This article introduces the knowledge of "what are the tips for using Java". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

Using stream to realize list to map

Normal: for list to map, we often use traversal, and then take out the value and put it in the result, as follows

Magic trick: implemented with stream, through Collectors.groupingBy

Stream has many very intuitive and useful methods, such as sum,count,distinct and so on.

Reduce non-null judgment and use Objects tools gracefully

Usually use! = null to make judgment, which is less elegant.

Jdk1.7 provides Objects tools, which are intuitive and easy to use. Many methods help us to make a non-empty judgment and reduce the code of! = null.

Random numbers can use seeds to ensure random rules.

Sometimes in order to save a fixed value, we often need to save a database or file, which is very troublesome. We can calculate a specific value through the seed of a random number.

For example, the values of the five cycles from the second print must be equal. Just make sure that the value of seed is fixed, and the value calculated by random number must be fixed.

Double curly braces syntax

In many cases, you need to initialize some list data. If you do something like the following, it will be tedious.

Magic trick: use the {{}} syntax, as follows

Note: many students mentioned that there is a problem here, it is not recommended to use a lot of programs like this, because this method is an anonymous inner class initialization method, it will produce a lot of xxx$1.class,xxx$2.class. There will be a memory leak problem.

Goto to be used in special scenarios

It is well known that java does not have goto syntax, but it provides methods similar to goto syntax effects, as follows

Try-with-resource automatically shuts down resources

JVM cannot automatically collect references to external resources, such as database connections, network connections, input and output IO streams, and so on. These connections need to be closed manually, otherwise it will lead to external resource leakage, connection pool overflow and abnormal occupation of files.

Therefore, in the IO operation, we often need to write the following code to manually release resources

Jdk1.7 provides us with try-with-resource syntax, so that we no longer have to call the close method in finally, jvm automatically calls.

Use MultiValueMap to build Map

If you need to build Map

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