Advance Programming Practice Using Java

A method is a block of code which only runs when it is called.

You can pass data, known as parameters, into a method.

Methods are used to perform certain actions, and they are also known as functions.

Why use methods? To reuse code: define the code once, and use it many times.

Create a Method

A method must be declared within a class. It is defined with the name of the method, followed by parentheses (). Java provides some pre-defined methods, such as System.out.println(), but you can also create your own methods to perform certain actions:

Type Description No parameters, no return value Just performs an action With parameters, no return value Takes input, no result back No parameters, with return value Returns a result, but no input With parameters and return value Takes input and returns result
class Demo {

    void show() {

        System.out.println("No parameters");

    }

    void show(int a) {

        System.out.println("One parameter: " + a);

    }

    void show(String s, int a) {

        System.out.println("String and int: " + s + ", " + a);

    }

}

Encapsulation

The meaning of Encapsulation, is to make sure that "sensitive" data is hidden from users. To achieve this, you must:

• declare class variables/attributes as private

• provide public get and set methods to access and update the value of a private variable

Get and Set

You learned from the previous chapter that private variables can only be accessed within the same class (an outside class has no access to it). However, it is possible to access them if we provide public get and set methods.

The get method returns the variable value, and the set method sets the value.

Syntax for both is that they start with either get or set, followed by the name of the variable, with the first letter in upper case:

public class Person {

  private String name; // private = restricted access

  // Getter

  public String getName() {

    return name;

  }

  // Setter

  public void setName(String newName) {

    this.name = newName;

  }

}

Encapsulation in Java is one of the fundamental principles of object-oriented programming (OOP). It refers to the concept of wrapping the data (variables) and the code (methods) that operates on the data into a single unit, typically a class. By making variables private and providing public getter and setter methods, Java allows controlled access to the fields of a class, ensuring data security and integrity. This mechanism helps protect the internal state of an object from unintended interference and misuse. Encapsulation promotes modularity and maintainability, as it hides the implementation details and exposes only what is necessary for the outside world. This makes it easier to make changes to a class without affecting other parts of the program that use the class. Overall, encapsulation helps in building robust and secure Java applications.

Java Conditions and If Statements

You already know that Java supports the usual logical conditions from mathematics:

• Less than: a < b

• Less than or equal to: a <= b

• Greater than: a > b

• Greater than or equal to: a >= b

• Equal to a == b

• Not Equal to: a != b

You can use these conditions to perform different actions for different decisions.

Java has the following conditional statements:

• Use if to specify a block of code to be executed, if a specified condition is true

• Use else to specify a block of code to be executed, if the same condition is false

• Use else if to specify a new condition to test, if the first condition is false

• Use switch to specify many alternative blocks of code to be executed

Java Switch Statements

Instead of writing many if..else statements, you can use the switch statement.

The switch statement selects one of many code blocks to be executed:
switch(expression) {

  case x:

    // code block

    break;

  case y:

    // code block

    break;

  default:

    // code block

}

Java Data Types

As explained in the previous chapter, a variable in Java must be a specified data type:
int myNum = 5;               // Integer (whole number)

float myFloatNum = 5.99f;    // Floating point number

char myLetter = 'D';         // Character

boolean myBool = true;       // Boolean

String myText = "Hello";     // String
Data types are divided into two groups:

• Primitive data types - includes byte, short, int, long, float, double, boolean and char

• Non-primitive data types - such as String, Arrays and Classes (you will learn more about these in a later chapter)

Primitive Data Types

A primitive data type specifies the type of a variable and the kind of values it can hold.

There are eight primitive data types in Java:

Data TypeDescriptionbyteStores whole numbers from -128 to 127shortStores whole numbers from -32,768 to 32,767intStores whole numbers from -2,147,483,648 to 2,147,483,647longStores whole numbers from -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807floatStores fractional numbers. Sufficient for storing 6 to 7 decimal digitsdoubleStores fractional numbers. Sufficient for storing 15 to 16 decimal digitsbooleanStores true or false valuescharStores a single character/letter or ASCII values