In Java, numbers are an important data type that is used to represent numeric values. There are several types of numbers in Java, including integers, floating-point numbers, and complex numbers. 

Integers: 
Integers are whole numbers that can be positive, negative, or zero. Java provides several types of integer variables, including byte, short, int, and long. The sizes are: 
● byte: 8-bit signed integer (-128 to 127) 
● short: 16-bit signed integer (-32768 to 32767) 
● int: 32-bit signed integer (-2147483648 to 2147483647) 
● long: 64-bit signed integer (-9223372036854775808 to 9223372036854775807) 

Example: 

byte b = 100; 
short s = 20000; 
int i = 1000000000; 
long l = 1000000000000000000L; 

Floating-Point Numbers 
Floating-point numbers are numbers that have a decimal point. The two types of floating point numbers in java are: 
● float: 32-bit single-precision floating-point number 
● double: 64-bit double-precision floating-point number 
Example: 

float f = 3.14f; 
double d = 3.14; 

Java also provides a class called BigDecimal, which can be used to represent arbitrary-precision decimal numbers. 

BigDecimal bd = new BigDecimal("3.14"); 

Java also provides several built-in functions, such as Math. abs(), Math. ceil(), Math. floor(), Math. round() and Math. max(), Math. min(), Math. sqrt() etc.