Java Basics #3

 

Java Constructors

A constructor in Java is a special method that is used to initialise objects. The constructor is called when an object of a class is created. It can be used to set initial values for object attributes.

// Create a Main class
public class Main {
  int x;  // Create a class attribute
  // Create a class constructor for the Main class
  public Main() {
    x = 5;  // Set the initial value for the class attribute x
  }

  public static void main(String[] args) {
    Main myObj = new Main(); // Create an object of class Main (This will call the constructor)
    System.out.println(myObj.x); // Print the value of x
  }
}
// Outputs 5

Note that the constructor name must match the class name, and it cannot have a return type (like void).

Also note that the constructor is called when the object is created.

All classes have constructors by default: if you do not create a class constructor yourself, Java creates one for you. However, then you are not able to set initial values for object attributes.


Constructor Parameters

Constructors can also take parameters, which is used to initialize attributes.

The following example adds an int y parameter to the constructor. Inside the constructor we set x to y (x=y). When we call the constructor, we pass a parameter to the constructor (5), which will set the value of x to 5.


public class Main {
  int x;
  public Main(int y) {
    x = y;
  }
  public static void main(String[] args) {
    Main myObj = new Main(5);
    System.out.println(myObj.x);
  }
}
// Outputs 5

Parameters and Arguments

Information can be passed to methods as parameter. Parameters act as variables inside the method.

Parameters are specified after the method name, inside the parentheses. You can add as many parameters as you want, just separate them with a comma.

The following example has a method that takes a String called fname as parameter. When the method is called, we pass along a first name, which is used inside the method to print the full name.

public class Main {
  static void myMethod(String fname) {
    System.out.println(fname + " Refsnes");
  }
  public static void main(String[] args) {
    myMethod("Liam");
    myMethod("Jenny");
    myMethod("Anja");
  }
}
// Liam Refsnes
// Jenny Refsnes
// Anja Refsnes

Multiple Parameters

You can have as many parameters as you like.

public class Main {
  static void myMethod(String fname, int age) {
    System.out.println(fname + " is " + age);
  }
  public static void main(String[] args) {
    myMethod("Liam", 5);
    myMethod("Jenny", 8);
    myMethod("Anja", 31);
  }
}
// Liam is 5
// Jenny is 8
// Anja is 31

Return Values

The void keyword, used in the examples above, indicates that the method should not return a value. If you want the method to return a value, you can use a primitive data type (such as int, char, etc.) instead of void, and use the return keyword inside the method.

public class Main {
  static int myMethod(int x) {
    return 5 + x;
  }
  public static void main(String[] args) {
    System.out.println(myMethod(3));
  }
}
// Outputs 8 (5 + 3)

Method Overloading

With method overloading, multiple methods can have the same name with different parameters.

int myMethod(int x)
float myMethod(float x)
double myMethod(double x, double y)
Consider the following example:

static int plusMethodInt(int x, int y) {
  return x + y;
}
static double plusMethodDouble(double x, double y) {
  return x + y;
}
public static void main(String[] args) {
  int myNum1 = plusMethodInt(8, 5);
  double myNum2 = plusMethodDouble(4.3, 6.26);
  System.out.println("int: " + myNum1);
  System.out.println("double: " + myNum2);
}

Instead of defining two methods that should do the same thing, it is better to overload one.

static int plusMethod(int x, int y) {
  return x + y;
}
static double plusMethod(double x, double y) {
  return x + y;
}
public static void main(String[] args) {
  int myNum1 = plusMethod(8, 5);
  double myNum2 = plusMethod(4.3, 6.26);
  System.out.println("int: " + myNum1);
  System.out.println("double: " + myNum2);
}
Multiple methods can have the same name as long as the number and/or type of parameters are different.

Java Scope

In Java, variables are only accessible inside the region they are created. This is called scope.

Method Scope

Variables declared directly inside a method are available anywhere in the method following the line of code in which they were declared.

Example:

public class Main {
  public static void main(String[] args) {
    // Code here CANNOT use x
    int x = 100;
    // Code here can use x
    System.out.println(x);
  }
}

Block Scope

A block of code refers to all of the code between curly braces {}. Variables declared inside blocks of code are only accessible by the code between the curly braces, which follows the line in which the variable was declared.

Example:

public class Main {
  public static void main(String[] args) {
    // Code here CANNOT use x
    { // This is a block
      // Code here CANNOT use x
      int x = 100;
      // Code here CAN use x
      System.out.println(x);
   } // The block ends here
  // Code here CANNOT use x
  }
}
A block of code may exist on its own or it can belong to an if, while or for statement. In the case of for statements, variables declared in the statement itself are also available inside the block's scope.

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.

Example:

public class Person {
  private String name; // private = restricted access
  // Getter
  public String getName() {
    return name;
  }
  // Setter
  public void setName(String newName) {
    this.name = newName;
  }
}

public class Main {
  public static void main(String[] args) {
    Person myObj = new Person();
    myObj.name = "John";  // error
    System.out.println(myObj.name); // error 
  }
}

If the variable was declared as public, 

Then the output will be John.

However, as we try to access a private variable, we get an error.

Instead, we use the getName() and setName() methods to acccess and update the variable.

Example:

public class Main {
  public static void main(String[] args) {
    Person myObj = new Person();
    myObj.setName("John"); // Set the value of the name variable to "John"
    System.out.println(myObj.getName());
  }
}
// Outputs "John"

Why Encapsulation?

  • Better control of class attributes and methods
  • Class attributes can be made read-only (if you only use the get method), or write-only (if you only use the set method)
  • Flexible: the programmer can change one part of the code without affecting other parts
  • Increased security of data.

Comments

Post a Comment

Popular posts from this blog

Java Basics #4

  Java  Math The Java Math class has many methods that allows you to perform mathematical tasks on numbers. Math.max( x,y ) The  Math.max( x , y )  method can be used to find the highest value of  x  and  y. Example: Math.max(5, 10); Math.min( x,y ) The  Math.min( x , y )  method can be used to find the lowest value of  x  and  y. Example: Math.min(5, 10); Math.sqrt( x ) The  Math.sqrt( x )  method returns the square root of  x. Example: Math.sqrt(64); Math.abs( x ) The  Math.abs( x )  method returns the absolute (positive) value of  x. Example: Math.abs(-4.7); Random Numbers Math.random()  returns a random number between 0.0 (inclusive), and 1.0 (exclusive). Example: Math.random(); To get more control over the random number, e.g. you only want a random number between 0 and 100, you can use the following formula. Example: int randomNum = (int)(Math.random() * 101);  // 0 to 100 More Math ...
Read more

Java Basics #1

Simple Java program: public class Main {   public static void main(String[] args) {     System.out.println("Hello World");   } } Every line of code that runs in Java must be inside a  class . In our example, we named the class  Main . A class should always start with an uppercase first letter. The main Method The  main()  method is required and you will see it in every Java program. Any code inside the  main()  method will be executed. You don't have to understand the keywords before and after main. System.out.println() Inside the  main()  method, we can use the  println()  method to print a line of text to the screen. Java Comments Comments can be used to explain Java code, and to make it more readable. It can also be used to prevent execution when testing alternative code. Single-line comments start with two forward slashes ( // ). Java Variables Variables are containers for storing data values. In Java, there are d...
Read more