why do we need access modifiers in java code example

Example 1: access modifiers in java

In Java, access specifiers are the keywords which are used to define 
the access scope of the method, class, or a variable. 
  In Java, there are four access specifiers.
  
  * Public: The classes, methods, or variables which are defined as public, 
  can be accessed by any class or method.
  * Protected: Protected can be accessed by the class of the same package, 
or by the sub-class of this class, or within the same class.
  * Default: Default are accessible within the package only. 
    By default, all the classes, methods, and variables are of default scope.
  * Private: The private class, methods, or variables defined as private 
    can be accessed within the class only

Example 2: access modifiers in java

Example of private access modifier:

class Demo
{
   private void display()
   {
      System.out.println("Hello world java");
   }
}
public class PrivateAccessModifierExample
{
   public static void main(String[] args)
   {
      Demo obj = new Demo();
      System.out.println(obj.display());
   }
}