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Variables

int Data Type

In Java, the int datatype is used to store integer values. This means that it can store all positive and negative whole numbers and zero.

int num1 = 10; // positive value int num2 = -5; // negative value int num3 = 0; // zero value int num4 = 12.5; // not allowed

boolean Data Type

In Java, the boolean primitive data type is used to store a value, which can be either true or false.

boolean result = true; boolean isMarried = false;

char Data Type

In Java, char is used to store a single character. The character must be enclosed in single quotes.

char answer = 'y';

Primitive Data Types

Java’s most basic data types are known as primitive data types and are in the system by default.

The available types are as follows:

  • int
  • char
  • boolean
  • byte
  • long
  • short
  • double
  • float

null is another, but it can only ever store the value null.

int age = 28; char grade = 'A'; boolean late = true; byte b = 20; long num1 = 1234567; short no = 10; float k = (float)12.5; double pi = 3.14;

Strings

A String in Java is a Object that holds multiple characters. It is not a primitive datatype.

A String can be created by placing characters between a pair of double quotes (").

To compare Strings, the equals() method must be used instead of the primitive equality comparator ==.

// Creating a String variable String name = "Bob"; // The following will print "false" because strings are case-sensitive System.out.println(name.equals("bob"));

Static Typing

In Java, the type of a variable is checked at compile time. This is known as static typing. It has the advantage of catching the errors at compile time rather than at execution time.

Variables must be declared with the appropriate data type or the program will not compile.

int i = 10; // type is int char ch = 'a'; // type is char j = 20; // won't compile, no type is given char name = "Lil"; // won't compile, wrong data type

final Keyword

The value of a variable cannot be changed if the variable was declared using the final keyword.

Note that the variable must be given a value when it is declared as final. final variables cannot be changed; any attempts at doing so will result in an error message.

// Value cannot be changed: final double PI = 3.14;

double Data Type

The double primitive type is used to hold decimal values.

double PI = 3.14; double price = 5.75;

Math Operations

Basic math operations can be applied to int, double and float data types:

  • + addition
  • - subtraction
  • * multiplication
  • / division
  • % modulo (yields the remainder)

These operations are not supported for other data types.

int a = 20; int b = 10; int result; result = a + b; // 30 result = a - b; // 10 result = a * b; // 200 result = a / b; // 2 result = a % b; // 0

Comparison Operators

Comparison operators can be used to compare two values:

  • > greater than
  • < less than
  • >= greater than or equal to
  • <= less than or equal to
  • == equal to
  • != not equal to

They are supported for primitive data types and the result of a comparison is a boolean value true or false.

int a = 5; int b = 3; boolean result = a > b; // result now holds the boolean value true

Compound Assignment Operators

Compound assignment operators can be used to change and reassign the value of a variable using one line of code. Compound assignment operators include +=, -=, *=, /=, and %=.

int number = 5; number += 3; // Value is now 8 number -= 4; // Value is now 4 number *= 6; // Value is now 24 number /= 2; // Value is now 12 number %= 7; // Value is now 5

Increment and Decrement Operators

The increment operator, (++), can increase the value of a number-based variable by 1 while the decrement operator, (--), can decrease the value of a variable by 1.

int numApples = 5; numApples++; // Value is now 6 int numOranges = 5; numOranges--; // Value is now 4

Order of Operations

The order in which an expression with multiple operators is evaluated is determined by the order of operations: parentheses -> multiplication -> division -> modulo -> addition -> subtraction.

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