static Nested classes

A Nested class with static modifier is called static nested class

    Syntax

    class OuterClass {

        static class StaticNestedClass {

        }

    }

Just like static methods, a static class, can't access instance properties and instance methods directly. To access them static nested class should create an object for the outer class.

How To Create an Object for static nested class

    Syntax

    OuterClass.StaticNestedClass nestedObject = new OuterClass.StaticNestedClass();

With an example, we can easily grasp the use of nested classes.

Consider the following requirement

Requirement: I want to maintain a database for an university, like How Many colleges are there, How many faculties, How many Students across the university.

See the below solution, developed using static nested classes.

class College{
 private String collegeName;
 private int maleFaculty, femaleFaculty, noOfFaculty;
 private int maleStudents, femaleStudents,noOFStudents;

 private static class Faculty{
  private static int totalNoOfFaculty;
  private static int totalMaleFaculty;
  private static int totalFemaleFaculty;
 }

 private static class Student{
  private static int totalNoOFStudents;
  private static int totalMaleStudents;
  private static int totalFemaleStudents;
 }

 private static class CollegeCount{
  private static int totalColleges;
 }

 void setFaculty(int totalMale, int totalFemale){
  maleFaculty = totalMale;
  femaleFaculty = totalFemale;
  noOfFaculty = totalMale+totalFemale;
    
  Faculty.totalNoOfFaculty = Faculty.totalNoOfFaculty + noOfFaculty;
  Faculty.totalMaleFaculty = Faculty.totalMaleFaculty + maleFaculty;
  Faculty.totalFemaleFaculty = Faculty.totalFemaleFaculty + femaleFaculty;
 }

 void setStudents(int totalMale, int totalFemale){
  maleStudents = totalMale;
  femaleStudents = totalFemale;
  noOFStudents = totalMale+totalFemale;

  Student.totalNoOFStudents = Student.totalNoOFStudents + noOFStudents;
  Student.totalMaleStudents = Student.totalMaleStudents + maleStudents;
  Student.totalFemaleStudents = Student.totalFemaleStudents + femaleStudents;
 } 

 College(String collegeName){
  this.collegeName = collegeName;
  CollegeCount.totalColleges++;
 }

 static void printConsolodateData(){
  System.out.println("Total Colleges " +CollegeCount.totalColleges);
  System.out.println("Total Faculties " + Faculty.totalNoOfFaculty);
  System.out.println("Total Male Faculties " + Faculty.totalMaleFaculty);
  System.out.println("Total Female Faculties " + Faculty.totalFemaleFaculty);
  System.out.println("Total Students " + Student.totalNoOFStudents);
  System.out.println("Total Male Students " + Student.totalMaleStudents);
  System.out.println("Total Female Students " + Student.totalFemaleStudents);
 }
}

As You see in the above program, class College contains 3 static nested classes Faculty, Student, CollegeCount.

Faculty class

Has 3 static variables to count total number of faculties, total number of male faculties and total number of femal faculties.

Student Class

Has 3 static variables t count total number of students, total number of male students, total number of female students.

CollegeCount class

Contains a variable to count total number of colleges.

class CollegeTest{
 public static void main(String args[]){
  College c1 = new College("SSIET");
  College c2 = new College("UCEK");
  
  c1.setFaculty(70, 30);
  c1.setStudents(1000, 700);
  c1.setFaculty(100, 70);
  c1.setStudents(2000, 1500);
  
  College.printConsolodateData();
 }
}

 

Output

Total Colleges 2
Total Faculties 270
Total Male Faculties 170
Total Female Faculties 100
Total Students 5200
Total Male Students 3000
Total Female Students 2200

Some Points to Remember

1. You can't access non static fields from static nested class directly

     Example

class Outer{
 String name = "Outer class";
 
 static class Nested{
  public void print(){
   System.out.println(name);
  }
 }
}

When you try to compile the above program, compiler throws below error      

Outer.java:5: error: non-static variable name cannot be referenced from a static context
System.out.println(name);
^
1 error

     

2. You can't access non static methods from static nested class directly

     Example

class Outer{
 void outerMethod(){
 }

 static class Nested{
  public void print(){
   outerMethod();
  }
 }
}

          

When you try to compile the above program, compiler throws below error

Outer.java:8: error: non-static method outerMethod() cannot be referenced from a static context
outerMethod();
^
1 error

3. To access non static fields, methods from static nested class, object for the outer class should be created.    

class Outer{
 void outerMethod(){
 }

 static class Nested{
  Outer obj = new Outer();
  public void print(){
   obj.outerMethod();
  }
 }
}

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