Instantiable Classes: static vs instance

static fields

• Declaring a field static indicates that only one copy of the field variable exists.
• That one field value will be shared by all instances created of that class type.

Fields that should be declared static

• constant values that are the same for all instances of a class
• a counter that keeps track of the number of instances created so far
  (such a field could be used to provide the "next" serial number for each new instance)
• a system timer who's value changes, but whose value should be available to all objects in a class or program

// declare static fields inside the class and outside methods

// class (static) fields must be assigned initial values when declared

public static final char[] DIR = { 'N', 'E', 'S', 'W' };

private static int numberOfInstancesCreated = 0;

private static double timeStepsSinceStartOfProgram = 0;

instance fields

• Declaring a field without the static modifier indicates that each instance (object) of that class (type) will have its own copy of the field.
• Thus, each instance of the class will have its own value for that field.

Fields that should NOT be declared static

• constant values that will NOT have the same value for all instances of a class. Each instance can have its own serial number that never changes.  But, each instance has a different serial number. Could use the value of a static counter variable to know what serial number to assign to the newly created instance.
• constant or variable attributes of an instance, such as:
  • a name
  • number
  • date
  • color
  • size
  • etc...

// declare fields inside the class and outside methods

// instance fields will be assigned values in the constructor

public final int SERIAL_NUMBER; 

private int score;

private String name;

static methods (a.k.a. class methods)

• Declaring a method static is done if the method only needs to access or set static fields of the class.
• Methods that are declared static may be called without creating an instance of the class.
  • Math.pow( 2, 3 );   // calls static method of Math class

// declare static methods inside the class and outside other methods

// static methods may only access (use) static fields

public static int getNumberOfInstances() {

   return numberOfInstancesCreated;

} 

 

private static boolean isValidDirection( char d ) {

   boolean valid = false;

   for ( int i=0; i < DIR.length; i++ ) {

      if ( d == DIR[i] ) valid = true;

   }

   return valid;

}

 

private static double getTimeSteps() {

   return timeStepsSinceStartOfProgram;

}

instance methods (a.k.a. non-static methods)

• Declaring a method without the static modifier indicates that this method will need to access or set instance (non-static) fields of the class.
• Thus, an instance of the class will have to be created and the instance method will be called using the instance that was created.
  • Scanner stdin = new Scanner(System.in);  // create an instance
  • String line = stdin.nextLine(); // call instance method nextLine() using instance (object) named "stdin"

// declare non static methods inside the class and outside other methods

// instance (non static) methods may access (use) any fields

public long getMySerialNumber() {

   return SERIAL_NUMBER;

} 

 

private int getScore() {

   return score;

}

 

private String changeName( String newName ) {

   name = newName;

}

 

private String getName() {

   return name;

}

 

© 2014 Debra Deppeler, All rights reserved.