Java Language

Classes and Objects

Introduction#

Objects have states and behaviors. Example: A dog has states - color, name, breed as well as behaviors – wagging the tail, barking, eating. An object is an instance of a class.

Class − A class can be defined as a template/blueprint that describes the behavior/state that the object of its type support.

Syntax#

  • class Example {} //class keyword, name, body

Simplest Possible Class

class TrivialClass {}

A class consists at a minimum of the class keyword, a name, and a body, which might be empty.

You instantiate a class with the new operator.

TrivialClass tc = new TrivialClass();

Object Member vs Static Member

With this class:

class ObjectMemberVsStaticMember {

    static int staticCounter = 0;
    int memberCounter = 0;

    void increment() {
        staticCounter ++;
        memberCounter++;
    }
}

the following code snippet:

final ObjectMemberVsStaticMember o1 = new ObjectMemberVsStaticMember();
final ObjectMemberVsStaticMember o2 = new ObjectMemberVsStaticMember();

o1.increment();

o2.increment();
o2.increment();

System.out.println("o1 static counter " + o1.staticCounter);
System.out.println("o1 member counter " + o1.memberCounter);
System.out.println();

System.out.println("o2 static counter " + o2.staticCounter);
System.out.println("o2 member counter " + o2.memberCounter);
System.out.println();

System.out.println("ObjectMemberVsStaticMember.staticCounter = " + ObjectMemberVsStaticMember.staticCounter);

// the following line does not compile. You need an object
// to access its members
//System.out.println("ObjectMemberVsStaticMember.staticCounter = " + ObjectMemberVsStaticMember.memberCounter);

produces this output:

o1 static counter 3
o1 member counter 1

o2 static counter 3
o2 member counter 2

ObjectMemberVsStaticMember.staticCounter = 3

Note: You should not call static members on objects, but on classes. While it does not make a difference for the JVM, human readers will appreciate it.

static members are part of the class and exists only once per class. Non-static members exist on instances, there is an independent copy for each instance. This also means that you need access to an object of that class to access its members.

Overloading Methods

Sometimes the same functionality has to be written for different kinds of inputs. At that time, one can use the same method name with a different set of parameters. Each different set of parameters is known as a method signature. As seen per the example, a single method can have multiple signatures.

public class Displayer {

    public void displayName(String firstName) {
        System.out.println("Name is: " + firstName);
    }

    public void displayName(String firstName, String lastName) {
        System.out.println("Name is: " + firstName + " " + lastName);
    }

    public static void main(String[] args) {
        Displayer displayer = new Displayer();
        displayer.displayName("Ram");          //prints "Name is: Ram"
        displayer.displayName("Jon", "Skeet"); //prints "Name is: Jon Skeet"
    }
}

The advantage is that the same functionality is called with two different numbers of inputs. While invoking the method according to the input we are passing, (In this case either one string value or two string values) the corresponding method is executed.

Methods can be overloaded:

  1. Based on the number of parameters passed.

    Example: method(String s) and method(String s1, String s2).

  2. Based on the order of parameters.

    Example: method(int i, float f) and method(float f, int i)).

Note: Methods cannot be overloaded by changing just the return type (int method() is considered the same as String method() and will throw a RuntimeException if attempted). If you change the return type you must also change the parameters in order to overload.

Basic Object Construction and Use

Objects come in their own class, so a simple example would be a car (detailed explanations below):

public class Car {
    
    //Variables describing the characteristics of an individual car, varies per  object
   private int milesPerGallon;
   private String name;
   private String color;
   public int numGallonsInTank; 
    
    public Car(){
        milesPerGallon = 0;
        name = "";
        color = "";
        numGallonsInTank = 0;
    }
    
    //this is where an individual object is created
    public Car(int mpg, int, gallonsInTank, String carName, String carColor){
        milesPerGallon = mpg;
        name = carName;
        color = carColor;
        numGallonsInTank = gallonsInTank;
    }

    //methods to make the object more usable

    //Cars need to drive
    public void drive(int distanceInMiles){
        //get miles left in car
        int miles = numGallonsInTank * milesPerGallon;
        
        //check that car has enough gas to drive distanceInMiles
        if (miles <= distanceInMiles){
            numGallonsInTank = numGallonsInTank - (distanceInMiles / milesPerGallon)
            System.out.println("Drove " + numGallonsInTank + " miles!");
        } else {
            System.out.println("Could not drive!");
        }
    }

    public void paintCar(String newColor){
        color = newColor;
    }
        //set new Miles Per Gallon
    public void setMPG(int newMPG){
        milesPerGallon = newMPG;
    }

       //set new number of Gallon In Tank
    public void setGallonsInTank(int numGallons){
        numGallonsInTank = numGallons;
    }
    
    public void nameCar(String newName){
        name = newName;
    }

    //Get the Car color
    public String getColor(){
        return color;
    }

    //Get the Car name
    public String getName(){
        return name;
    }

    //Get the number of Gallons
    public String getGallons(){
        return numGallonsInTank;
    }
    
}

Objects are instances of their class. So, the way you would create an object would be by calling the Car class in one of two ways in your main class (main method in Java or onCreate in Android).

Option 1

`Car newCar = new Car(30, 10, "Ferrari", "Red");

Option 1 is where you essentially tell the program everything about the Car upon creation of the object. Changing any property of the car would require calling one of the methods such as the repaintCar method. Example:

 newCar.repaintCar("Blue");

Note: Make sure you pass the correct data type to the method. In the example above, you may also pass a variable to the repaintCar method as long as the data type is correct`.

That was an example of changing properties of an object, receiving properties of an object would require using a method from the Car class that has a return value (meaning a method that is not void). Example:

String myCarName = newCar.getName();  //returns string "Ferrari"

Option 1 is the best option when you have all the object’s data at the time of creation.

Option 2

`Car newCar = new Car();

Option 2 gets the same effect but required more work to create an object correctly. I want to recall this Constructor in the Car class:

public void Car(){
        milesPerGallon = 0;
        name = "";
        color = "";
        numGallonsInTank = 0;
    }

Notice that you do not have to actually pass any parameters into the object to create it. This is very useful for when you do not have all the aspects of the object but you need to use the parts that you do have. This sets generic data into each of the instance variables of the object so that, if you call for a piece of data that does not exist, no errors are thrown.

Note: Do not forget that you have to set the parts of the object later that you did not initialize it with. For example,

Car myCar = new Car();
String color = Car.getColor(); //returns empty string

This is a common mistake amongst objects that are not initialized with all their data. Errors were avoided because there is a Constructor that allows an empty Car object to be created with stand-in variables (public Car(){} ), but no part of the myCar was actually customized. Correct example of creating Car Object:

Car myCar = new Car();
myCar.nameCar("Ferrari");
myCar.paintCar("Purple");
myCar.setGallonsInTank(10);
myCar.setMPG(30);

And, as a reminder, get an object’s properties by calling a method in your main class. Example:

String myCarName = myCar.getName(); //returns string "Ferrari"

Constructors

Constructors are special methods named after the class and without a return type, and are used to construct objects. Constructors, like methods, can take input parameters. Constructors are used to initialize objects. Abstract classes can have constructors also.

public class Hello{
    // constructor
    public Hello(String wordToPrint){
        printHello(wordToPrint);
    }
    public void printHello(String word){
        System.out.println(word);
    }
}
// instantiates the object during creating and prints out the content
// of wordToPrint

It is important to understand that constructors are different from methods in several ways:

  1. Constructors can only take the modifiers public, private, and protected, and cannot be declared abstract, final, static, or synchronized.

  2. Constructors do not have a return type.

  3. Constructors MUST be named the same as the class name. In the Hello example, the Hello object’s constructor name is the same as the class name.

  4. The this keyword has an additional usage inside constructors. this.method(...) calls a method on the current instance, while this(...) refers to another constructor in the current class with different signatures.

Constructors also can be called through inheritance using the keyword super.

public class SuperManClass{

    public SuperManClass(){
        // some implementation
    }
    
    // ... methods
}


public class BatmanClass extends SupermanClass{
    public BatmanClass(){
        super();
    }
    //... methods...
}

See Java Language Specification #8.8 and #15.9

Initializing static final fields using a static initializer

To initialize a static final fields that require using more than a single expression, a static initializer can be used to assign the value. The following example initializes a unmodifiable set of Strings:

public class MyClass {

    public static final Set<String> WORDS;
    
    static {
        Set<String> set = new HashSet<>();
        set.add("Hello");
        set.add("World");
        set.add("foo");
        set.add("bar");
        set.add("42");
        WORDS = Collections.unmodifiableSet(set);
    }
}

Explaining what is method overloading and overriding.

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