The keyword static indicates that the main method is is a class methd, not a method for an instance.

Three methods are handy for printing.

• System.out.println puts to stdout with a newline
• System.out.print puts to stdout, but no newline
• System.err.print(ln) - puts to stderr, which by default is the screen.

A python program that dos the same thing looks like this.

print("Hello, World!")

Python has classes too. Let's make one that looks like our Java program

class Hello(object):
    def speak(self):
        return "Hello, World!"

def main():
    h = Hello()    #make new Hello object
    print(h.speak())
main()

Java has three types of comments which you can see here.

public class Hello
{
    //this is a one-line comment.  It si the same as Python's #
    /*
     * This is a multiline comment
     */
    /**
     * This is a javadoc comment
     */
    //  def main(): This is your main routine
    public static void main(String[] args)
    {
        System.out.println("Hello, World!");
    }

Types

Python has these types of objects.

dictionary
string
list
bool
int
float
complex

Python variables have NO type.

In contrast Java variables AND data have types. Let us begin with Java's integer types. The ++ and -- postfix operators are "primitive" in the sense that they act on integer types as raw binary numbers.

If you do arithmetic on shorts or bytes, the result is promoted automatically to an int. This does not happen with the autoincrement and autodecrement operators.

Using jshell Invoke this Java REPL by typing jshell in a command window.

When defining a Java variable, you must specify the variable's type.

(base) MAC:Mon Oct 12:09:57:~> jshell
|  Welcome to JShell -- Version 14.0.1
|  For an introduction type: /help intro

jshell> int x = 5;
x ==> 5

This varible is an integer variable with value 5. The type of a variable is fixed for the variables lifetime and cannot be changed. Assignment, as it is in Python, is a worker statement. Worker statements in Java must end with a semicolon or the compiler will shriek at you and abort.

OK, break the rules and receive a nasty ululation.

jshell> x = "cowabunga"
|  Error:
|  incompatible types: java.lang.String cannot be converted to int
|  x = "cowabunga"
|      ^---------^

Compound assignment is identical to Python.

jshell> x += 4;
$2 ==> 9

No surprises here.

jshell> 4*5
$3 ==> 20

jshell> 4 + 5
$4 ==> 9

jshell> 4 - 5
$5 ==> -1

Integer division here.

jshell> 4/5
$6 ==> 0

Mod works but ** does not.

jshell> 4%5
$7 ==> 4

jshell> 4**5
|  Error:
|  illegal start of expression
|  4**5
|    ^

jshell> // you got POW for using pow.

jshell> Math.pow(4,5)
$8 ==> 1024.0

I seldom take more than a 40 lb byte.

jshell> byte b = 6;
b ==> 6

jshell> while(b > 0){ b++;}

jshell> b
b ==> -128

jshell> b--;
$12 ==> -128

jshell> b
b ==> 127

A byte is an 8-bit integer stored in two's complement notation. Notice how type overflow introduces doughnutting.

Now you will get short shrift.

jshell> short s = 0;
s ==> 0

jshell> s
s ==> 0

jshell> while(s >= 0){s++;}

jshell> s
s ==> -32768

jshell> s--;
$19 ==> -32768

jshell> s
s ==> 32767

Let us now inspect int

jshell> int i = 0;
i ==> 0

jshell> while(i >= 0){i++;}

jshell> i
i ==> -2147483648

jshell> i--;
$24 ==> -2147483648

jshell> i
i ==> 2147483647

The long type will take too long to do this. So.... punt!

jshell> Long.MAX_VALUE
$27 ==> 9223372036854775807

jshell> Long.MIN_VALUE
$28 ==> -9223372036854775808

Someone asked, "What's with the capital letters?" All-caps in an identifier indicates that this identifier is a constant. This is a universally-observed convention in Java.

Here is a constant we all know and love.

jshell> Math.PI
$29 ==> 3.141592653589793

More Types

Creating a double is a simple matter

jshell> double x = 5.6;
x ==> 5.6

Casting About

This should come as no surprise.

jshell> (int) x
$31 ==> 5

jshell> (double) 4
$32 ==> 4.0

Truthiness does not manifest itself here.

jshell> (boolean) 5
|  Error:
|  incompatible types: int cannot be converted to boolean
|  (boolean) 5
|            ^

Punish-mint. Not spearmint, peppermint, or experiment. You can see the boolean constants here.

jshell> true
$33 ==> true

jshell> false
$34 ==> false

Java has the same relational operators as Python.

< <= >= > == !=

No suprise here. It's just like Python or JavaScript.

jshell> 4 < 5
$35 ==> true

Here we see Java's char type. It has some fluidity with the int type, which is quite useful.

jshell> char c = 'a'
c ==> 'a'

jshell> (int) c
$37 ==> 97

jshell> (char) 945
$38 ==> 'α'

Java also has a String type as you see here.

jshell> String foo = "bar";
foo ==> "bar"

And..... we are out of time. :(

jshell> /exit
|  Goodbye