Shulou(Shulou.com)06/01 Report--

This article mainly introduces the relevant knowledge of "case analysis of the use of new features of Java", the editor shows you the operation process through the actual case, the operation method is simple, fast and practical. I hope this article "case analysis of the use of new features of Java" can help you solve the problem.

Enumeration: although enumeration types have been added in JDK 5, the CONSTANT_Class_info type constants of the Class file constant pool have not undergone any semantic changes, they are still symbolic references that represent a class or interface, no enumerations have been added, and "enumeration symbol references" constants such as "CONSTANT_Enum_info" have not been added. So using the enum keyword to define constants is syntactically the same as using the class keyword to define the class and the interface keyword to define the interface, but in fact this is an illusion made by the Javac compiler. From the bytecode point of view, enumerations are just ordinary Java classes that inherit from java.lang.Enum and automatically generate values () and valueOf () methods, so enumerations are classified as reference types.

However, when it comes to JDK 10:00, we have a new choice. JDK 10 adds var local variable inference, with which we can happy forget about data types, so how does it work? Next, let's take a look.

1. Use comparison

Next, we will use the way of comparison to experience the role of var.

Scenario 1: define a string

Old way of writing:

String str = "Hello, Java."

The new way of writing:

Var s = "Hello, Java."

PS: the old way of writing here refers to the version before JDK 10, while the new method refers to the later version of JDK 10 (including JDK 10).

Scenario 2: numerical addition

Old way of writing:

Int num1 = 111bot double num2 = 555.666d TX double num3 = num1 + num2;System.out.println (num3)

PS: the upward transformation of data types occurs when different types are added (int+ double), so num3 is upgraded to double types.

The new way of writing:

Var N1 = 111L *

Old way of writing:

List list = new ArrayList (); list.add ("Hello"); list.add ("Java")

The new way of writing:

Var list = new ArrayList (); list.add ("Hello"); list.add ("Java"); scenario 4: loop

Old way of writing:

For (Object item: list) {System.out.println ("item:" + item);} for (int I = 0; I

< 10; i++) { // do something...} 新写法： for (var item : list) { System.out.println("item:" + item);}for (var i = 0; i < 10; i++) { // do something...}场景五：配合 Lambda 使用 旧写法： List flist = list.stream().filter(v ->

V.equals ("Java"). Collectors.toList (); System.out.println (flist)

The new way of writing:

Var flist = list.stream (). Filter (v-> v.equals ("Java")). Advantages (Collectors.toList ()); System.out.println (flist); 2. Advantage analysis

From the above example, we can see that var has two obvious advantages: improved code readability and naming alignment.

① improves readability

Before we use var, this happens if the name of the type is very long:

InternationalCustomerOrderProcessor orderProcessor = createInternationalOrderProcessor (customer, order)

When you limit each line to no more than 150 characters, the variable name is pushed to the next line, making the whole code less readable. But when we use var, the code looks like this:

Var orderProcessor = createInternationalOrderProcessor (customer, order)

As you can see from the above code, the longer the type, the greater the value of var.

② naming alignment

When you are not using var, the code looks like this when you encounter the following situations:

/ / explicit type No no = new No (); AmountIncrease more = new BigDecimalAmountIncrease (); HorizontalConnection jumping = new HorizontalLinePositionConnection (); Variable variable = new Constant (6); List names = List.of ("Java", "Chinese community")

After using var, the code looks like this:

Var no = new No (); var more = new BigDecimalAmountIncrease (); var jumping = new HorizontalLinePositionConnection (); var variable = new Constant (6); var names = List.of ("Java", "Chinese Community")

From the above code, you can see that after using var, the naming is aligned and the whole code becomes more elegant.

3. Rules of use & counterexamples

The implementation of var is from JEP 286 (improvement proposal 286), details: http://openjdk.java.net/jeps/286

As can be seen from the title "Local variable Type inference" of JEP 286, var can only be used for local variable declarations, which means that var must meet the following conditions:

It can only be used on local variables.

Must be initialized when declaring

Cannot be used as method parameters and global variables (class variables).

PS: because the implementation of var must infer the type from the code on the right, it cannot be assigned null or not initialized.

Counterexample 1: uninitialized and assigned null counterexample 2: midway type change counterexample 3: global variable counterexample 4: as return value 4, principle analysis

After the previous use, we have a preliminary understanding of var, but what is the implementation principle of var?

To figure out how it works, we compiled the following code (using the command javac MainTest.java):

Then we use the decompiler to open the compiled class and find that var has been replaced with a certain data type, as shown in the following figure:

From this we can draw a conclusion: the implementation of the var keyword is closely related to its name, var is only local type inference, it will only be valid in the Java coding period and compilation time, when the class is compiled into a class file, the var will become a certain data type (inferred). So we can think of var as the syntax sugar of Java, which allows us to implement business code quickly and elegantly, but var does not exist at the bytecode level.

This is the end of the introduction to the "case study of the use of Java's new features". Thank you for reading. If you want to know more about the industry, you can follow the industry information channel. The editor will update different knowledge points for you every day.

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