chapter03-jshell_vs_java.md

Jshell vs Java

We are using jshell, jshell has mostly the same behavior as Java but because it's interactive it can do more

Re-defining records or variables

you can define a record or a variable several times, jshell will use the most recent one

record Point(int x, int y) { }
var p = new Point(2, 3);

record Point(int x, int y) {
  int distanceInX(int anotherX) {
    return Math.abs(x - anotherX);
  }
}
var p = new Point(2, 3);
System.out.println(p.distanceInX(0));

and you can also directly write a method outside a record, it will act as a static method

void hello() {
  System.out.println("hello !");
}
hello();

Special commands

There are a bunch of special commands that starts with '/' you can use /help if you want to know more

By example, you have also a list of the packages automatically imported

/import

and you can import new package dynamically

import java.util.zip.*

note that unlike import in Python or #include in C, Java import doesn't load any code, it says that is you search a name of a type, here are the packages to search. If a type appear in more than one package, an error will be reported and you will have to import the type explicitly (without any *) by example, to import a List of the package java.util

import java.util.List;

Entry Point

With jshell, the code is executed from top to bottom In Java, when executing java Hello, the launcher looks for a static method named main that

• must be visible from the outside (public)
• takes an array of String (String[]) and
• returns no value (void)

record Hello() {
  public static void main(String[] args) {
    System.out.println("Hello !");
  }
}

unlike in C, the first argument is args[0].