Here is a Java program that does nothing.

public class Test
{
}

To compile, use the javac command.

unix> javac Test.java

You will notice a new file, Test.class, which is the Java bytecode translated from your source. Here is a hex dump of it. It largely looks like gibberish with some recognizable elments.

unix> xxd Test.class
0000000: cafe babe 0000 0034 000d 0a00 0300 0a07  .......4........
0000010: 000b 0700 0c01 0006 3c69 6e69 743e 0100  ........<init>..
0000020: 0328 2956 0100 0443 6f64 6501 000f 4c69  .()V...Code...Li
0000030: 6e65 4e75 6d62 6572 5461 626c 6501 000a  neNumberTable...
0000040: 536f 7572 6365 4669 6c65 0100 0954 6573  SourceFile...Tes
0000050: 742e 6a61 7661 0c00 0400 0501 0004 5465  t.java........Te
0000060: 7374 0100 106a 6176 612f 6c61 6e67 2f4f  st...java/lang/O
0000070: 626a 6563 7400 2100 0200 0300 0000 0000  bject.!.........
0000080: 0100 0100 0400 0500 0100 0600 0000 1d00  ................
0000090: 0100 0100 0000 052a b700 01b1 0000 0001  .......*........
00000a0: 0007 0000 0006 0001 0000 0001 0001 0008  ................
00000b0: 0000 0002 0009                  

We then gave our program a main method so it would execute.

public class Test
{
    public static void main(String[] args)
    {
        System.out.println("Testing Java....");
    }
}

Since we changed the source code we must recompile. After that, we run the program.

unix> javac Test.java
unix> java Test
Testing Java....

System.out.println is just like Python's print.

Here is a cold-cocking from Java.

jshell> x = 5
|  Error:
|  cannot find symbol
|    symbol:   variable x
|  x = 5
|  ^

It turns out that variables have type in Java. Here we specify that our variable x has type int.

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

Use /vars to see what variables you have created in your session. It works like dir() in Python.

jshell> /vars
|    int x = 5

Here is a floating-point type.

jshell> double z = 4.56;
z ==> 4.56

There is a second floating-point type in Java, which we shall probably never use, float.

jshell> float f = 4.5
|  Error:
|  incompatible types: possible lossy conversion from double to float
|  float f = 4.5;
|            ^-^

You have to put an f at the end as follows.

jshell> float f = 4.5f;
f ==> 4.5

The double type is a 64 bit double-precison IEEE 754 number. The float type is a 32 bit single-precison IEEE 754 number.

Now let's make a string.

jshell> String s = "some string";
s ==> "some string"

Here is how to see all of the variables you have created.

jshell> /vars
|    int x = 5
|    double z = 4.56
|    float f = 4.5
|    String s = "some string"

Compound assignment: Just like Python.

jshell> x += 1;
$5 ==> 6

jshell> x
x ==> 6

This the autoincrement operator. It does not exist in Python.

jshell> x++;   //does not exist in Python
$7 ==> 6

jshell> x
x ==> 7

Notice that each reply in jshell is given a variable name of the form $number.

jshell> $5
$5 ==> 6

jshell> $7
$7 ==> 6

More compound assignment.

jshell> x += 2
$11 ==> 9

jshell> x *= 2
$12 ==> 18

Integer divison for integers.

jshell> 6/5
$13 ==> 1

Here is how to cast.

jshell> (double) 6/5
$14 ==> 1.2

Our old friend mod.

jshell> 365%7
$15 ==> 1

No **.

jshell> 5**3
|  Error:
|  illegal start of expression
|  5**3
|    ^

Batman says POW!

jshell> Math.pow(5,3)
$16 ==> 125.0

There are many Math functions.

jshell> Math.cos(0)
$17 ==> 1.0

No factorial.

jshell> int x = Math.factorial(5)
|  Error:
|  cannot find symbol
|    symbol:   method factorial(int)
|  int x = Math.factorial(5);
|          ^------------^

Our old pal log.

jshell> Math.log(10)
$18 ==> 2.302585092994046

jshell> Math.log10(10)
$19 ==> 1.0

Relational operators on numbers.

jshell> 4 < 5
$20 ==> true

jshell> boolean b = 4 < 5;
b ==> true

And, and or.

jshell> true && true
$22 ==> true

jshell> //and

jshell> true || true
$23 ==> true

jshell> // || is or

Bitwise operators too.

jshell> 5&7
$24 ==> 5

Watch this....

jshell> int n = 0;
n ==> 0

jshell> n = 1;
n ==> 1

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

jshell> n
n ==> -2147483648

jshell> n--;
$29 ==> -2147483648

The integers are a circle, not a line. The int type is a 32 bit integer type.

jshell> n
n ==> 2147483647

Have a byte.

jshell> byte b = 1
b ==> 1

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

jshell> b
b ==> -128

jshell> short sh = 1
sh ==> 1

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

jshell> sh
sh ==> -32768

jshell> b
b ==> -128

jshell> b = 300;
|  Error:
|  incompatible types: possible lossy conversion from int to byte
|  b = 300;
|      ^-^

jshell> b
b ==> -128

jshell> b = (byte) 300;
b ==> 44

jshell> long big = 1;
big ==> 1

jshell> for(int k = 0; k < 30; k++){big *= 2;}

jshell> big
big ==> 1073741824

jshell> for(int k = 0; k < 30; k++){big *= 2;}

jshell> big
big ==> 1152921504606846976

jshell> big *=2
$45 ==> 2305843009213693952

jshell> big *=2
$46 ==> 4611686018427387904

jshell> big *=2
$47 ==> -9223372036854775808