CMUQ 15-121 Notes: Intro to Java

Source Code -> Execution

Source Code -> Byte Code -> Execution

Comments in Java are as follows:

// This is a single line comment.
// Everything after // on a line is ignored.

/* This is a multi-line comment.
   You can write whatever you want after the /* and it will all
   be ignored until the comment closes as follows...
 */

 /* You can use the multiline comment on a single line if you want. */

In Python, all errors occur at runtime. In Java, some errors occur at compile time and others at runtime.

• Syntax errors (such as misspelling whlie in a while loop) occur at compile time.
• Runtime errors (such as divide by 0) always cause a crash.
• Logical errors (such as writing a program based on a bad idea) don’t trigger errors from Java. (Or any language, really…)

In Java, you must explicitly declare the type of a variable and you can’t change it later. This is called static typing.

int a = 5; // a is an integer and its initial value is 5
a = 10; // This is no problem
a = "Ten"; // This won't work, because a is an integer and you can't change it later to a String

Java has eight primitive types, four of which you will use on a regular basis.

Type	Size	Example	Other Note
int	4 bytes	1567	Max value: Integer.MAX_VALUE Min value: Integer.MIN_VALUE
double	8 bytes	3.141592653589793	Max value: Double.MAX_VALUE Min value: Double.MIN_VALUE
boolean	unspecified	true	2 values: true or false
char	2 bytes	'g'
byte	1 byte	125	Not commonly used
short	2 bytes	2036	Not commonly used
long	8 bytes	9223372036854775801	Like an int, but longer
float	4 bytes	3.1415927	Like a double, but less precise

For integers, longs, bytes and shorts, n-bits can be used to store \(2^n\) values. (8-bits can store 256 values)

• Unsigned values range from \(0\) to \(2^n -1\)
• Signed integers and longs range from \(-2^{n-1}\) through \(2^{n-1}-1\).
  So, 4-byte (32-bit) integers range from -2,147,483,648 to 2,147,483,647.

Operator	Description
+, -, *	Basic math operations
/	Integer division
%	Modulus
++	Increment
--	Decrement
=	Assignment
==	Equivalence

+, -, *	Basic math operations
/	Real division
=	Assignment
==	Equivalence (But never do this...)
Math.pow()	Power
Math.sqrt()	Square Root

==	Equality
!=	Not equal
<, >, <=, >=	Relational operators
!	Logical: Not
&&	Logical: And
||	Logical: Or

Many of the same operators that work on integers work on characters. But make sure you know what you are doing before using them…