Static and Dynamic Binding in Java

Every method call and field access in your code has to be linked to the actual code or memory it refers to. That linking is called binding. If it's settled while compiling, it's static binding (early binding). If the final decision waits for the running program, it's dynamic binding (late binding). This lesson pulls together what the last few lessons showed into one rule you can apply to any line of code.

The Rule

Seeing Both in One Program

public class Binding {
    static class Vehicle {
        String type = "vehicle";
        static String category() { return "Vehicle category"; }
        String describe() { return "A vehicle"; }
    }
 
    static class Bike extends Vehicle {
        String type = "bike";
        static String category() { return "Bike category"; }
        @Override String describe() { return "A bike"; }
    }
 
    public static void main(String[] args) {
        Vehicle v = new Bike();
        System.out.println(v.describe());
        System.out.println(v.type);
        System.out.println(v.category());
    }
}

A bike
vehicle
Vehicle category

How it works

1. v.describe(): an overridable instance method, so dynamic binding. The object is a Bike: "A bike".
2. v.type: a field, so static binding to the declared type Vehicle: "vehicle".
3. v.category(): a static method, so static binding to Vehicle: "Vehicle category". The object isn't even consulted. Writing Vehicle.category() makes that honest.

Common Mistakes

Saying "all method calls are dynamic in Java". Static, private and final methods, constructors and super calls aren't.

Calling static methods through instances. It hides the fact that the object is ignored; use the class name.

Redeclaring fields in subclasses and expecting them to override. Fields are statically bound.

Under the Hood

The bytecode instruction tells you which kind of binding each call uses:

         0: new           #7                  // class Binding$Bike
        12: invokevirtual #16                 // Method Binding$Vehicle.describe:()Ljava/lang/String;
        22: getfield      #28                 // Field Binding$Vehicle.type:Ljava/lang/String;
        33: invokestatic  #32                 // Method Binding$Vehicle.category:()Ljava/lang/String;

Notice getfield ... Binding$Vehicle.type: the field is fixed to Vehicle, which is why the Bike object's own type is never read.

Interview Corner

Practice

1. Easy: For each line, say static or dynamic: Math.max(2, 3); list.size(); this.helper() where helper is private; account.balance; super.toString().
2. Harder: Compile Binding.java, run javap -c Binding, then change v.category() to Bike.category() and see which instruction and class change.

Key Takeaways