Java Series Part 2: Java Variables

Variables in Java are containers for storing data values.

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

  • String – stores text, such as “Hello World”. String values are surrounded by double quotes
  • int – stores integers (whole numbers), without decimals, such as 15 or -15
  • float – stores floating point numbers, with decimals, such as 1.99 or -1.99
  • char – stores single characters, such as ‘a’ or ‘Z’. Char values are surrounded by single quotes
  • boolean – stores values with two states: true or false
package com.java.basic;

public class VariablesExamples {

	public static void main(String[] args) {
		
		//create a variable that should store text
		String name = "John";
		System.out.println(name);
		
		//create a variable that should store a number
		int myNum = 15;
		System.out.println(myNum);
		
		//declare a variable without assigning the value, and assign the value later
		int myNum2;
		myNum2 = 15;
		System.out.println(myNum2);
		
		//change the value of myNum from 15 to 20
		int myNum3 = 15;
		myNum3 = 20;  // myNum is now 20
		System.out.println(myNum3);
		
		/*
		 * If you don't want others (or yourself) to overwrite existing values,
		 *  use the final keyword (this will declare the variable as "final" or
		 *   "constant", which means unchangeable and read-only)
		 */

		final int myNum4 = 15;
		//myNum4 = 20;  // will generate an error: cannot assign a value to a final variable
	
	
		//other types
		int myNum5 = 5;
		float myFloatNum = 5.99f;
		char myLetter = 'D';
		boolean myBool = true;
		String myText = "Hello";
		
		//Declare multiple variables
		int x = 5, y = 6, z = 50;
		System.out.println(x + y + z);
		
		//one value to multiple variables
		
		int a, b, c;
		a = b = c = 50;
		System.out.println(a + b + c);
	}

}

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).

The general rules for naming variables 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
// Good -camelCase
int minutesPerHour = 60;

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

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