Variables and Types

Overview

Teaching: 0 min
Exercises: 0 min

Questions

• What is a variable?

• What is a type?

Objectives

• Use variables to store values and do calculations

In computer programming a variable is a symbolic name for a specific area in the computer’s memory where a value is stored. In Java we have to define a variable before we can use it and when we define it, we have to specify the type of information we will be storing in the variable.

In Java we have the following types available which we can use to define what information will be stored in a variable:

• byte: stores whole numbers from -128 to 127
• short: stores whole numbers from -32768 to 32767
• int: stores whole numbers from -2³¹ to 2³¹-1.
• long stores whole numbers from -2⁶³ to 2⁶³-1.
• float: stores fractional numbers with 6 to 7 decimal digits
• double: stores fractional numbers with 15 decimal digits
• boolean: stores true or false
• char: stores a single character from UNICODE

In addition to these types, Java provides special support for strings with the String type. Note the capitalisation of the String type. We’ll just mention the reason for the capitalisation but we won’t discuss it in more detail. String is a class whereas the other types we mentioned are known as primitives.

Each of these types use a different amount of memory.

Create a new repl and call it 03.Variables. Enter the following code and compile and run it:

1  // Define the class
2  class Main {
3   // Define the main method
4   public static void main(String[] args) {
5     // Define variables and initialise them with values;
6     String stringVariable = "Print ten plus ten: ";
7     int integerValue = 10;
8     // You can also define a variable without initialising it;
9     int total;
10    System.out.println(stringVariable);
11    System.out.println(stringVariable + integerValue + 10);
12    System.out.println(stringVariable + integerValue + integerValue);
13    System.out.println(stringVariable + (integerValue + integerValue));
14    // Do some multiplication and store the value in the variable total
15    // Storing a value in a variable is called assingment
16    total = integerValue * integerValue * 10;
17    System.out.println("Print the value of total: " + total);
18  }
19}

Print ten plus ten: 
Print ten plus ten: 1010
Print ten plus ten: 1010
Print ten plus ten: 20
Print the value of total: 1000

Let’s look at the program line by line:

• Note: By convention variable names start with lower-case.

1. The first line is just a comment.
2. Define the beginning of a class named Main.
3. Another comment.
4. Declaring the main method (the front door to the program).
5. Yet another comment.
6. String specifies the type of value that will be stored in the variable called stringVariable. The = is an assignment operator. The variable stringVariable will be assigned the value specified to the right of the =, which in this case is Print ten plus ten:.
7. int specifies the type of value that will be stored in integerValue. The variable integerValue will be assigned the value specified to the right of the =, which in this case is 10.
8. Another comment.
9. On this line we declare yet another integer, called total, using the int keyword but we don’t assign a value to the variable.
10. Print the content of the variable stringVariable to the console.
11. Convert the number 10 and the integer value stored in integerValue to strings and then concatenate them to the string value stored in stringValue. Print the final string to the console.
12. Convert the integer value stored in integerValue to a string and then concatenate that twice to the string value stored in stringValue. Print the final string to the console.
13. Add the integer value stored in integerValue to itself, convert the result to a string and then concatenate that to the string stored in stringAviable. Print the final string to the console.
14. Another comment
15. … and yet another comment.
16. The variable total is assigned the value of the calculation integerValue * integerValue * 10
17. Convert the integer value stored in total to a string and concatenate it to the string Print the value of total:. Print the final string to the console.
18. Curly brace to close the main method.
19. Curly brace to close the class called Main.

Operators

Before leaving you to experiment with what you have learnt so far we are going to mention a couple more things to make it more interesting.

• Operators: Operators are characters that are used to perform actions on values.
• Operators have precedence. That means that some operators have higher precedence than others. Operators with higher precedence are evaluated before operators with lower precedance. Operators on the same level of precedence are evaluated from left to right. Below is a table of operators. The subheadings in the table groups the operators with the same precedence together. The groups are listed according to the precedence order. The closer to the top of the table a group appears, the higher its precedence.

Challenge

Take the next few minutes and see if you can write some code to do the calculations below.

1. Converting fahrenheit to celsius. The formula used for this is: ((fahrenheit - 32) * (5/9))
2. Converting celsius to fahrenheit. The formula used for this is: ((celsius * 9/5) + 32)
3. Converting celsius to kelvin: The formula used for this is: (celsius + 273.15)
4. Converting fahrenheit to kelvin: Can you work this one out yourself by using what you have already done?

Steps:

• make four additional methods in Main; one for each of the calculations that a convert a value into another value. Go ahead and use the existing one as a skeleton (copy-paste) to be modified if you want.

Hints:

• your methods can return the converted value.
• if methods with the void keyword terminates in a void, what might other variable types as method keywords enable the methods to do?
• append f for float to 273.15 or java won’t compute.

Suggestions:

• add more System.out.println() statements to explain the conversion by calling your methods with values of your choosing.

• make one method and the accompanying printed explanation until you have made all four calculations.

Solution

class Main {
  public static void main(String[] args) {
    System.out.println("Calculate to convert!");
    System.out.println(50 + " degrees Fahrenheit is " + fahrenheitToCelcius(50) + " degrees Celcius");
    System.out.println(20 + " degrees Celcius is " + celciusToFahrenheit(20) + " degrees Fahrenheit");
    System.out.println(10 + " degrees Celcius is " + celciusToKelvin(10) + " degrees Kelvin");
    System.out.println(500 + " degrees Fahrenheit is " + fahrenheitToKelvin(500) + " degrees Kelvin");
    System.out.println(0 + " degrees Fahrenheit is " + fahrenheitToKelvin(0) + " degrees Kelvin");
  }

  public static float fahrenheitToCelcius(float fahrenheit) {
    float celcius = (fahrenheit - 32) * 5 / 9;
    return celcius;
  }

  public static float celciusToFahrenheit(float celcius) {
    float fahrenheit = (celcius * 9 / 5) + 32;
    return fahrenheit;
  }

  public static float celciusToKelvin(float celcius) {
    float kelvin = celcius + 273.15f;
    return kelvin;
  }

  public static float fahrenheitToKelvin(float fahrenheit) {
    float celcius = fahrenheitToCelcius(fahrenheit);
    float kelvin = celciusToKelvin(celcius);
    return kelvin;
  }
}

Press the green Run button.

Calculate to convert!
50 degrees Fahrenheit is 10.0 degrees Celcius
20 degrees Celcius is 68.0 degrees Fahrenheit
10 degrees Celcius is 283.15 degrees Kelvin
500 degrees Fahrenheit is 533.15 degrees Kelvin
0 degrees Fahrenheit is 255.37222 degrees Kelvin

Key Points