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Java Variables

Variables are containers for storing data values.

In Java, there are different types of variables, for example:

• 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

• Declaring (Creating) Variables

• To create a variable, you must specify the type and assign it a value:

• Syntax

  type variable = value;
  

  Where type is one of Java's types (such as int or String), and variable is the name of the variable (such as x or name). The equal sign is used to assign values to the variable.

  To create a variable that should store text, look at the following example.

• Example

  Create a variable called name of type String and assign it the value "John":

  String name = "John";
  System.out.println(name);
  

  To create a variable that should store a number, look at the following example:

  Example

  Create a variable called myNum of type int and assign it the value 15.

• int myNum = 15;
  System.out.println(myNum);
  

  You can also declare a variable without assigning the value, and assign the value later:

  Example

  int myNum;
  myNum = 15;
  System.out.println(myNum);
  

  Note that if you assign a new value to an existing variable, it will overwrite the previous value:

  Example

  Change the value of myNum from 15 to 20:

  int myNum = 15;
  myNum = 20;  // myNum is now 20
  System.out.println(myNum);

• Final Variables

  However, you can add the final keyword if you don't want others (or yourself) to overwrite existing values (this will declare the variable as "final" or "constant", which means unchangeable and read-only):

  Example

  final int myNum = 15; myNum = 20; // will generate an error: cannot assign a value to a final variable

• Other Types

  A demonstration of how to declare variables of other types:

  Example

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

• Display Variables

  The println() method is often used to display variables.

  To combine both text and a variable, use the + character:

  Example

  String name = "John"; System.out.println("Hello " + name);

• You can also use the + character to add a variable to another variable:

  Example

  String firstName = "John ";
  String lastName = "Doe";
  String fullName = firstName + lastName;
  System.out.println(fullName); 
  

• For numeric values, the + character works as a mathematical operator (notice that we use int (integer) variables here):

  Example

  int x = 5;
  int y = 6;
  System.out.println(x + y); // Print the value of x + y
  

  From the example above, you can expect:

  • x stores the value 5
  • y stores the value 6
  • Then we use the println() method to display the value of x + y, which is 11

  • Declare Many Variables

    To declare more than one variable of the same type, use a comma-separated list:

    Example

    int x = 5, y = 6, z = 50;
    System.out.println(x + y + z);

  • Java Identifiers

    All Java variables must be identified with unique names.

    These unique names are called identifiers.

    Identifiers can be short names (like x and y) or more descriptive names (age, sum, totalVolume).

    Note: It is recommended to use descriptive names in order to create understandable and maintainable code:

    Example

    // Good int minutesPerHour = 60; // OK, but not so easy to understand what m actually is int m = 60;

  • The general rules for constructing names for variables (unique identifiers) are:

  • Names can contain letters, digits, underscores, and dollar signs
  • Names must begin with a letter
  • Names should start with a lowercase letter and it cannot contain whitespace
  • Names can also begin with $ and _ (but we will not use it in this tutorial)
  • Names are case sensitive ("myVar" and "myvar" are different variables)
  • Reserved words (like Java keywords, such as int or boolean) cannot be used as names