static const PI = 3.14
private static PI:number = 3.14
private static final double PI = 3.14;
static keyword when applied to properties and functions/methodsThe static keyword can be applied to a function/method or a property of a class.
A static function/method does not use any instance specific values, instead it executes logic on the data passed in as function/method arguments. Static functions/methods are often 'utility' functions, for example finding the maximum of 2 numbers. We call a static function/method using the name of the class, for example:
Math.max(1, 2);
Static properties belong to the class as a whole rather than one specific instance. We often use static properites to hold constant values, for example, the value of PI.
static const PI = 3.14
private static PI:number = 3.14
private static final double PI = 3.14;
Imagine we have an air traffic control system to keep track of all the planes using 2 runways. We have a rule that only a certain number of planes can be on either of the 2 runways at a time, otherwise our air traffic controllers will get overloaded!
We'll create 2 instance of a Runway class - one for the landing planes and one for departing planes. We'll add a static property to hold a constant representing the maximum number of planes allowed on the runway plus a static property holding a list of all the planes currently on either of the runways (note this code is not thread safe, it is shown for example purposes only). We'll then add a static function/method to add a plane to the runway.
class Runway {
static MAX_PLANES_ALLOWED_ON_ALL_RUNWAYS = 100;
static planes = [];
name;
constructor(name) {
this.name = name
}
add(plane) {
if (Runway.planes.length>Runway.MAX_PLANES_ALLOWED_ON_ALL_RUNWAYS) {
throw new Error ("runways at full capacity!")
}
Runway.planes.push(plane)
}
}
TODO
public class Runway {
private static final int MAX_PLANES_ALLOWED_ON_ALL_RUNWAYS = 100;
private String name;
private static List<Plane> planes = new ArrayList<Plane>();
public Runway(String name) {
this.name = name;
}
public void add(Plane plane) throws Exception {
if (this.planes.size()>Runway.MAX_PLANES_ALLOWED_ON_ALL_RUNWAYS) {
throw new Exception ("runways at full capacity!");
}
this.planes.add(plane);
}
}
Research OO languages which use static classes - what does the static keyword mean when applied to a class?