Overriding Methods

A subclass has the same instance method signature like the super class instance method is called overriding.

Example

class Animal{
 void print(){
  System.out.println("I am animal");
 }
}

class Tiger extends Animal{
 void print(){
  System.out.println("I am Tiger");
 }
}

As you observe, the classes Animal and Tiger has the same method signature print(). So print method in the class Tiger overrides the method in the class Animal.

Dynamic Method Dispatch

It is a mechanism, where a call to the overridden method is resolved at run time. It is also called as run time polymorphism.

Take the same examples Animal and Tiger.

class DynamicMethodDispatchEx{
 public static void main(String args[]){
  Animal anim1 = new Tiger();
  anim1.print();
 }
}

Output

I am Tiger

As you see the statement

     Animal anim1 = new Tiger();

An object of the Tiger class is assigned to the Animal reference variable. This is completely correct in Java. We can assign a subclass object to super class reference variable.

When the program calls the print(), then at run time JVM checks, the actual object pointed by the reference variable and calls the respective method of the actual object. So it calls the print method in the Tiger class.

Modifiers

The access specifier for an overriding method can allow more, but not less, access than the overridden method. For example, a default instance method in the superclass can be made public, protected, default, but not private, in the subclass.

Some points to Remember

1. Is run time polymorphism applicable to static methods also ?

No, static methods can't be overridden, they just hide the static method of the super class.

2. Is the instance variables overridden ?

No

class Animal{
 String str = "Animal";
 void print(){
  System.out.println("I am animal");
 }
}

class Tiger extends Animal{
 String str = "Tiger";
 void print(){
  System.out.println("I am Tiger");
 }
}

    

class DynamicMethodDispatchEx{
 public static void main(String args[]){
  Animal anim1 = new Tiger();
  anim1.print();
  System.out.println(anim1.str);
 }
}

   

Output

I am Tiger

Animal

If the instance variables are overridden, then the output should print the message “Tiger”. I.e, instance variables are just hide the instance variables of super class, these can't be overridden.

3. Can I apply static modifier to the subclass overriding method?

No, compiler throws an error.

 

 Example       

class Animal{
 String str = "Animal";
 
 void print(){
 System.out.println("I am animal");
 }
}

class Tiger extends Animal{
 String str = "Tiger";
 
 static void print(){
  System.out.println("I am Tiger");
 }
}

   

When tries to compile the Tiger class, below error thrown.

Tiger.java:3: error: print() in Tiger cannot override print() in Animal

static void print(){

^

overriding method is static

1 error

4. What is wrong with the below program ?    

class Animal{
 String str = "Animal";
 
 void print(){
  System.out.println("I am animal");
 }
}

class Tiger extends Animal{
 String str = "Tiger";
 
 private void print(){
  System.out.println("I am Tiger");
 }
}

When you tries to compile the above program, compiler throws the “Weaker access specifier error”

Tiger.java:3: error: print() in Tiger cannot override print() in Animal

private void print(){

^

attempting to assign weaker access privileges; was package

1 error

The access specifier for an overriding method can allow more, but not less, access than the overridden method. For example, a default instance method in the superclass can be made public, protected, default, but not private, in the subclass.

5. What is the difference between static binding and Dynamic binding ?

Static binding occurs at compile time, where as Dynamic binding at run time. Dynamic method dispatch is an example for Dynamic binding.

6. What is wrong with the below program ?

class Animal{
 String str = "Animal";
 
 void print(){
  System.out.println("I am animal");
 }
}

  

class Tiger extends Animal{
 String str = "Tiger";

 void print()throws Exception{
  System.out.println("I am Tiger");
 }
}

The overriding method cannot throw any exceptions that are not thrown by the overridden method.

When you tries to compile the above program compiler throws the below error.

Tiger.java:3: error: print() in Tiger cannot override print() in Animal

void print()throws Exception{

^

overridden method does not throw Exception

1 error

7. Is the constructors inherited ?

A subclass inherits all the members (fields, methods, and nested classes) from its superclass. Constructors are not members, so they are not inherited by subclasses.

8. When a subclass object assigned to super class reference variable, then the reference variable can call the methods which are defined in the super class, trying to call the methods which are not defined in super class cause the compile time error.

Example

class Animal{
 void print()throws Exception{
  System.out.println("I am animal");
 }
}

class Tiger extends Animal{
 void print(){
  System.out.println("I am Tiger");
 }

 void show(){
  System.out.println("I am show method");
 }
}

class DynamicMethodDispatchEx{
 public static void main(String args[]){
  Animal anim1 = new Tiger();
  anim1.show();
 }
}

    

When you are trying to compile the program, below error thrown, since the reference variable of class Animal, “anim1” trying to call the method “show: which is not defined in the class “Animal”.

DynamicMethodDispatchEx.java:5: error: cannot find symbol

anim1.show();

^

symbol: method show()

location: variable anim1 of type Animal

1 error

9. Can I specify weaker access specifier to the overriding method in the sub class ?

No

Multi path inheritance                                                 Covariant return type                                                 Home

How to define a method

A method is a programmed procedure that is defined as part of a class. A class (and thus an object) can have more than one method.

Syntax of method definition

    modifier returnType methodName(parameters) Exception List{

        Method Body

    }

Method definition composed of six components :

modifiers : like private, public, protected or default. We have already seen private, public and default. Will discuss more about these in the packages section.

ReturnType : The type of the value written by the method. If you don't want to return any value make the return type as void. Return type will be any primitive type or reference type.

MethodName : The name given to the method

parameters: Method is defined with parameters or with out parameters. A method can be defined with any number of parameters.

Exception List: A Method can throw any number of exceptions, will discuss about this on ExceptionHandling section

Method Body: The actual implementation of the method goes here

Example

    1. public int sum(int a, int b){

        return (a+b);

        }

        modifier : public

        returnType : int

        methodName : sum

        parameters : a,b

        Exception List : none

        Method body : calculates the sum of the two parameters and return the sum.

2. void sayHello(){

        System.out.println(“Hello”);

    }

    modifier : default

    returnType : void

    methodName : sayHello

    parameters :none

    Exception List : none

    Method body : simply prints the message “Hello” to the console.

If no access specifier (modifier) specified to a variable or method, it assumes default access specifier(modifier).

Naming a Method

By convention method names starts with a small character and starting character of subsequent words is capitalized.

    Example:

    walk()

    sleep()

    setColor(String s)

    getHeight()

Local Variables

Variables declared inside a method are called local variables. These are not accessible out side of the method. You must initialize local variables before using.

Some Points to remember

Trying to use local variables before initializing causes compile time error

Example

class LocalVariable{
 int sum(int a, int b){
  int c;
  System.out.println(c);
  return (a+b);
 }
} 

Since “c” is a local variable, when your program tries to access local variable, before initialization, compiler will throw the error.

 

For the above program, you will get below error
LocalVariable.java:5: error: variable c might not have been initialized
            System.out.println(c);
                                          ^
1 error

Instance methods                                                 More about methods                                                 Home

Instance variables

Instance variables are those which are associated with object. Person object personA has instance variables height, weight, color, country.

Instance variables are also called member variables or fields.

Fields declaration syntax

modifier dataType variableName;

Field declaration composed of three components

     1. modifier is any one of public, private, protected, default.

    2. dataType is any primitive or reference type

    3. name of the variable

Example:

private int var;

    we will discuss about the access modifiers private and public in this section.

Access Modifiers (public, private)

private : the field is accessible only within its own class.

Example consider the class Person. I given private access to instance variables height, weight, color and country. So any method which is defined inside the class Person can able to access these variables, but you can't access outside the class.

class Person{
 private float height, weight;
 private String color, country;

 float getHeight(){
  return height;
 }

 float getWeight(){
  return weight;
 }

 String getColor(){
  return color;
 }

 String getCountry(){
  return country;
 }
}

What happen if I tried to access the private variables outside of my class ?

Good Question, You will get compile time error.

Trying to access the private variables from other class PersonTest.

class PersonTest{
 public static void main(String args[]){
  Person personA = new Person();
  personA.height = 6;
 }
}

When you try to compile the above code, you will get the compiler error like below

 PersonTest.java:6: error: height has private access in Person
           personA.height = 6;
           ^
           1 error

    

Then how to solve this problem

Update the Person class by providing setter methods, to set the private properties.

class Person{
 private float height, weight;
 private String color, country;

 void setHeight(float h){
  height = h;
 }

 void setweight(float w){
  weight = w;
 }

 void setColor(String c){
  color = c;
 }

 void setCountry(String o){
  country = o;
 }

 float getHeight(){
  return height;
 }

 float getWeight(){
  return weight;
 }

 String getColor(){
  return color;
 }

 String getCountry(){
  return country;
 }
}

            

As you see in the above class Person, it is updated by adding 4 setter methods like setHeight, setWeight, setColor, setCountry to set the private properties height, weight, color, country respectively.

Update the PersonTest class also like below

class PersonTest{
 public static void main(String args[]){
  Person personA = new Person();
  Person personB = new Person();

  /* Setting properties for personA object */
  personA.setHeight(6);
  personA.setweight(75);
  personA.setColor("White");
  personA.setCountry("India");

  /* Setting properties for personB object */
  personB.setHeight(5.5f);
  personB.setweight(76);
  personB.setColor("Black");
  personB.setCountry("America");

  /* Calling the behaviors and printing the details of personA */
  System.out.println("PersonA Height is " + personA.getHeight());
  System.out.println("PersonA Weight is " + personA.getWeight());
  System.out.println("PersonA color is " + personA.getColor());
  System.out.println("PersonA country is " + personA.getCountry());
  System.out.println();

  /* Calling the behaviors and printing the details of personB */
  System.out.println("PersonB Height is " + personB.getHeight());
  System.out.println("PersonB Weight is " + personB.getWeight());
  System.out.println("PersonB color is " + personB.getColor());
  System.out.println("PersonB country is " + personB.getCountry());
 }
}

  

Output

PersonA Height is 6.0
PersonA Weight is 75.0
PersonA color is White
PersonA country is India

PersonB Height is 5.5
PersonB Weight is 76.0
PersonB color is Black
PersonB country is America

  

public modifier

    the field is accessible from all classes.

Some points to remember

1. If you don't initialize an instance variable, by default it is initialized to default value.

Data Type	Default Value
int	0
float	0
String	Null
boolean	FALSE
Reference type	null

Example

class DefaultValue{
 byte byteVar;
 short shortVar;
 int intVar;
 long longVar;
 char charVar;
 float floatVar;
 double doubleVar;
 boolean boolVar;
 String stringVar;

 public static void main(String args[]){
  DefaultValue obj = new DefaultValue();
  
  System.out.println("byte variable is set to " + obj.byteVar);
  System.out.println("short variable is set to " + obj.shortVar);
  System.out.println("int variable is set to " + obj.intVar);
  System.out.println("long variable is set to " + obj.longVar);
  System.out.println("char variable is set to " + obj.charVar);
  System.out.println("float variable is set to " + obj.floatVar);
  System.out.println("double variable is set to " + obj.doubleVar);
  System.out.println("bool variable is set to " + obj.boolVar);
  System.out.println("string variable is set to " + obj.stringVar);
 }
}

     

Output

byte variable is set to 0
short variable is set to 0
int variable is set to 0
long variable is set to 0
char variable is set to
float variable is set to 0.0
double variable is set to 0.0
bool variable is set to false
string variable is set to null

     

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