Head First Java Chapter 04 summary

Bullet Points

Instance Variable Vs Local Variable.

Equality of primitive or reference

Encapsulation.

The whole idea behind encapsulation is to hide the implementation details from users. If a data member is private it means it can only be accessed within the same class. No outside class can access private data member (variable) of other class.

However if we setup public getter and setter methods to update (for example void setSSN(int ssn))and read (for example int getSSN()) the private data fields then the outside class can access those private data fields via public methods.

This way data can only be accessed by public methods thus making the private fields and their implementation hidden for outside classes. That’s why encapsulation is known as data hiding. Lets see an example to understand this concept better.

Example of Encapsulation in Java

How to implement encapsulation in java:
1) Make the instance variables private so that they cannot be accessed directly from outside the class. You can only set and get values of these variables through the methods of the class.
2) Have getter and setter methods in the class to set and get the values of the fields.

class EncapsulationDemo{
private int ssn;
private String empName;
private int empAge;
//Getter and Setter methods
public int getEmpSSN(){
return ssn;
}
public String getEmpName(){
return empName;
}
public int getEmpAge(){
return empAge;
}
public void setEmpAge(int newValue){
empAge = newValue;
}
public void setEmpName(String newValue){
empName = newValue;
}
public void setEmpSSN(int newValue){
ssn = newValue;
}
}
public class EncapsTest{
public static void main(String args[]){
EncapsulationDemo obj = new EncapsulationDemo();
obj.setEmpName("Mario");
obj.setEmpAge(32);
obj.setEmpSSN(112233);
System.out.println("Employee Name: " + obj.getEmpName());
System.out.println("Employee SSN: " + obj.getEmpSSN());
System.out.println("Employee Age: " + obj.getEmpAge());
}
}

Output:

Employee Name: Mario
Employee SSN: 112233
Employee Age: 32

In above example all the three data members (or data fields) are private which cannot be accessed directly. These fields can be accessed via public methods only. Fields empName, ssn and empAge are made hidden data fields using encapsulation technique of OOPs.

Advantages of encapsulation

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Software Engineer at Virtusa

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