Java Basics — Variables, Types, Operators, Strings, Math

Java Basics

Printing

System.out.println("Hello world");
 
// println for print in new line
// print just keeps printing on the same

Variables

int myNum = 5;
float myFloatNum = 5.99f;
char myLetter = 'D';
boolean myBool = true;
String myText = "Hello";

Data types

Choose based on the range you need — saves memory.

• byte — whole numbers from -128 to 127. Use instead of int when value is guaranteed within that range, to save memory.
• short — whole numbers from -32 768 to 32 767.
• int — whole numbers from -2 147 483 648 to 2 147 483 647. Default choice for numeric variables.
• long — whole numbers from -9.2×10¹⁸ to 9.2×10¹⁸. Used when int isn’t large enough. End the literal with "L".
• float / double — fractional numbers. Suffix with "f" (float) or "d" (double). float has ~6–7 decimal digits of precision; double has ~16. Prefer double for most calculations.

Casting

• Widening (automatic): byte → short → char → int → long → float → double
• Narrowing (manual): double → float → long → int → char → short → byte

Arithmetic operators

Operator	Name	Description	Example
+	Addition	Adds two values	x + y
-	Subtraction	Subtracts one value from another	x - y
*	Multiplication	Multiplies two values	x * y
/	Division	Divides one value by another	x / y
%	Modulus	Returns the division remainder	x % y
++	Increment	Increases value by 1	++x
--	Decrement	Decreases value by 1	--x

Assignment operators

Operator	Example	Same As
=	x = 5	x = 5
+=	x += 3	x = x + 3
-=	x -= 3	x = x - 3
*=	x *= 3	x = x * 3
/=	x /= 3	x = x / 3
%=	x %= 3	x = x % 3
&=	x &= 3	x = x & 3
|=	x |= 3	x = x | 3
^=	x ^= 3	x = x ^ 3
>>=	x >>= 3	x = x >> 3
<<=	x <<= 3	x = x << 3

Comparison operators

Operator	Name	Example
==	Equal to	x == y
!=	Not equal	x != y
>	Greater than	x > y
<	Less than	x < y
>=	Greater than or equal to	x >= y
<=	Less than or equal to	x <= y

Logical operators

Operator	Name	Description	Example
&&	Logical AND	True if both statements are true	x < 5 && x < 10
||	Logical OR	True if one statement is true	x < 5 || x < 4
!	Logical NOT	Reverses the result	!(x < 5 && x < 10)

Strings

text.length()
text.toUpperCase()
text.toLowerCase()
text.indexOf("")
text1.concat(text2)

Escape characters

Escape	Result	Description
\'	'	Single quote
\"	"	Double quote
\\	\	Backslash

String control codes

Code	Result
\n	New line
\r	Carriage return
\t	Tab
\b	Backspace
\f	Form feed

Math

Math.max(x, y)   // returns highest value
Math.min(x, y)   // returns lowest value
Math.sqrt(x)
Math.abs(x)
Math.random()    // returns a random number between 0.0 (inclusive) and 1.0 (exclusive)
int randomNum = (int)(Math.random() * 101);  // 0 to 100

See next

• Java-Control-Flow — if/else, switch, loops, arrays
• Java-OOP — classes, objects, methods