How to use JShell in Java 9 for interactive programming

How to use JShell in Java 9 for interactive programming

Since the introduction of JShell in Java 9, developers can perform interactive Java programming through JShell. JShell is a REPL (Read-Eval-Print Loop) tool for interactive code execution. It allows developers to enter and execute Java code directly on the command line without writing a complete Java class file. This article will introduce how to use JShell for interactive programming and provide some code examples.

1. Install Java 9 and start JShell
   First, make sure you have installed the Java 9 development environment. Enter java -version on the command line to confirm that the Java version is 9 or higher. Next, run the jshell command on the command line to start JShell.
2. Execute simple Java expressions
   An important feature of JShell is the ability to directly execute Java expressions. Take the input 1 1 as an example, press Enter, JShell will immediately calculate and output the result 2. This method can be used to check some simple Java code, such as calculating expressions, verifying output, etc.
3. Declaring and using variables
   In JShell, you can declare and use variables. Use the var keyword to declare a variable, for example: var x = 10;. You can then use this variable directly, for example: x 5 will return 15. Variables declared in JShell do not need to specify a type. JShell will automatically infer the type of the variable based on the context.
4. Define and call methods
   JShell also supports defining and calling methods. Use the /method command to define a method. For example: /method int add(int a, int b) { return a b; } defines a method add, which accepts two parameters of type int , and return their sum. You can call this method, for example: add(3, 5) will return 8.
5. Importing and using external classes
   In JShell, you can import and use external Java classes. Taking importing java.util.List as an example, use the /import command to import this class, and then use it directly to create List objects and call methods. For example: List<String> list = new ArrayList<>(); and list.add("Hello");.
6. Writing multi-line code blocks
   JShell supports writing multi-line code blocks. After entering a line of code, press Enter and start with /, then enter a new line of code again. JShell will recognize these multiple lines of code and execute them. This is useful for writing complex logic or how to define a class.

The following is an example of using JShell for interactive programming:

// 打印Hello World!
System.out.println("Hello World!");

// 定义一个名为Person的类
class Person {
    private String name;
    
    Person(String name) {
        this.name = name;
    }
    
    public String getName() {
        return name;
    }
}

// 创建一个Person对象并输出其名称
Person p = new Person("John");
System.out.println(p.getName());

This is only an example of some of the functions of JShell. It also has more powerful functions, such as: auto-completion , history records, exception handling, etc. By using JShell, you can quickly verify the correctness of code snippets without having to write complete Java class files. It is a useful tool for learning Java and debugging code.

Summary
This article introduces how to use JShell in Java 9 for interactive programming. We learned about the basic usage of JShell, such as executing expressions, declaring variables, defining methods, etc., and provided corresponding code examples. I hope this article can help you use JShell for interactive programming more efficiently.

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