Java Cheat Sheet

This is a simple cheat sheet for Java.

Startup

public class Main {
  public static void main(String[] args) {
    System.out.println("Hello World");
  }
}

Variables Types

Type	Description
String	Stores text, such as “Hello”. String values are surrounded by double quotes
int	stores integers (whole numbers), without decimals, such as 123 or -123
float	stores floating point numbers, with decimals, such as 19.99 or -19.99
char	Stores single characters, such as ‘a’ or ‘B’. Char values are surrounded by single quotes
boolean	stores values with two states: true or false

Examples

    int myNum = 5;               // Integer (whole number)
    float myFloatNum = 5.99f;    // Floating point number
    char myLetter = 'D';         // Character
    boolean myBool = true;       // Boolean
    String myText = "Hello";     // String

Arithmetic Operators

Operator	Name	Description	Example
+	Addition	Adds together 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 the value of a variable by 1	++x
--	Decrement	Decreases the value of a variable by 1	--x

Assignment Operators

Operator	Example	Same As	Description
=	x = 5	x = 5
+=	x += 3	x = x + 3	Adds and assign
-=	x -= 3	x = x - 3	Subtract and assign
*=	x *= 3	x = x * 3	Multiply and assign
/=	x /= 3	x = x / 3	Divide and assign
%=	x %= 3	x = x % 3	Modulo and assign remainder
&=	x &= 3	x = x & 3	Bitwise AND (0101(5) & 0011(3) = 0001(1))
|=	x |= 3	x = x | 3	Bitwise OR (0101(5) | 0011(3) = 0111(7))
^=	x ^= 3	x = x ^ 3	Bitwise XOR (0101(5) ^ 0011(3) = 0110(6))
>>=	x >>= 3	x = x >> 3	Shift bits right (0100(4) >>= 2 = 0001(1))
<<=	x <<= 3	x = x << 3	Shift bits left (0100(4) <<= 1 = 1000(8))

Comparison Operators

Operator	Name	Example	Description
==	Equal to	x == y	Returns true if x equals y.
!=	Not equal	x != y	Returns true if x is not equal to y.
>	Greater than	x > y	Returns true if x is greater than y.
<	Less than	x < y	Returns true if x is less than y.
>=	Greater than or equal to	x >= y	Returns true if x is greater than or equal to y.
<=	Less than or equal to	x <= y	Returns true if x is less than or equal to y.

Logical Operator

Operator	Name	Description	Example
&&	Logical AND	Returns true if both conditions are true	x < 5 && x < 10
||	Logical OR	Returns true if at least one condition is true	x < 5 || x < 4
!	Logical NOT	Reverses the result: returns false if the expression is true	!(x < 5 && x < 10)

If…Else

Else If

Syntax

if (condition1) {
  // block of code to be executed if condition1 is true
} else if (condition2) {
  // block of code to be executed if the condition1 is false and condition2 is true
} else {
  // block of code to be executed if the condition1 is false and condition2 is false
}

Example

int time = 22;
if (time < 10) {
  System.out.println("Good morning.");
} else if (time < 18) {
  System.out.println("Good day.");
} else {
  System.out.println("Good evening.");
}
// Outputs "Good evening."

Short hand

Syntax

variable = (condition) ? expressionTrue :  expressionFalse;

Example

String result = (time < 18) ? "Good day." : "Good evening.";

Switch Case

Syntax

switch(expression) {
  case x:
    // code block
    break;
  case y:
    // code block
    break;
  default:
    // code block
}

Example

int day = 4;
switch (day) {
  case 1:
    System.out.println("Monday");
    break;
  case 2:
    System.out.println("Tuesday");
    break;
  case 3:
    System.out.println("Wednesday");
    break;
  case 4:
    System.out.println("Thursday");
    break;
  case 5:
    System.out.println("Friday");
    break;
  case 6:
    System.out.println("Saturday");
    break;
  case 7:
    System.out.println("Sunday");
    break;
}
// Outputs "Thursday" (day 4)

While Loop

While

Syntax

while (condition) {
  // code block to be executed
}

Example

int dice = 1;
while (dice <= 6) {
  if (dice < 6) {
    System.out.println("No Yatzy.");
  } else {
    System.out.println("Yatzy!");
  }
  dice = dice + 1;
}

Do/While

Syntax

do {
  // code block to be executed
}
while (condition);

Example

int i = 0;
do {
    System.out.println(i);
    i++;
}
while (i < 5);

For Loops

For Loop

Syntax

for (statement 1; statement 2; statement 3) {
  // code block to be executed
}

Example

for (int i = 0; i < 5; i++) {
  System.out.println(i);
}

Nested

// Outer loop
for (int i = 1; i <= 2; i++) {
    System.out.println("Outer: " + i); // Executes 2 times
  
    // Inner loop
    for (int j = 1; j <= 3; j++) {
        System.out.println(" Inner: " + j); // Executes 6 times (2 * 3)
    }
} 

For Each

Syntax

for (type variableName : arrayName) {
  // code block to be executed
}

Example

String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
for (String i : cars) {
  System.out.println(i);
}