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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