Why doesn't Java allow overriding of static methods?

Question

Why is it not possible to override static methods?

If possible, please use an example.

Answer 1

Overriding depends on having an instance of a class. The point of polymorphism is that you can subclass a class and the objects implementing those subclasses will have different behaviors for the same methods defined in the superclass (and overridden in the subclasses). A static method is not associated with any instance of a class so the concept is not applicable.

There were two considerations driving Java's design that impacted this. One was a concern with performance: there had been a lot of criticism of Smalltalk about it being too slow (garbage collection and polymorphic calls being part of that) and Java's creators were determined to avoid that. Another was the decision that the target audience for Java was C++ developers. Making static methods work the way they do had the benefit of familiarity for C++ programmers and was also very fast, because there's no need to wait until runtime to figure out which method to call.

Answer 2

Personally I think this is a flaw in the design of Java. Yes, yes, I understand that non-static methods are attached to an instance while static methods are attached to a class, etc etc. Still, consider the following code:

public class RegularEmployee {
    private BigDecimal salary;

    public void setSalary(BigDecimal salary) {
        this.salary = salary;
    }

    public static BigDecimal getBonusMultiplier() {
        return new BigDecimal(".02");
    }

    public BigDecimal calculateBonus() {
        return salary.multiply(getBonusMultiplier());
    }

    /* ... presumably lots of other code ... */
}

public class SpecialEmployee extends RegularEmployee {
    public static BigDecimal getBonusMultiplier() {
        return new BigDecimal(".03");
    }
}

This code will not work as you might expect. Namely, SpecialEmployee's get a 2% bonus just like regular employees. But if you remove the "static"s, then SpecialEmployee's get a 3% bonus.

(Admittedly, this example is poor coding style in that in real life you would likely want the bonus multiplier to be in a database somewhere rather than hard-coded. But that's just because I didn't want to bog down the example with a lot of code irrelevant to the point.)

It seems quite plausible to me that you might want to make getBonusMultiplier static. Perhaps you want to be able to display the bonus multiplier for all the categories of employees, without needing to have an instance of an employee in each category. What would be the point of searching for such example instances? What if we are creating a new category of employee and don't have any employees assigned to it yet? This is quite logically a static function.

But it doesn't work.

And yes, yes, I can think of any number of ways to rewrite the above code to make it work. My point is not that it creates an unsolvable problem, but that it creates a trap for the unwary programmer, because the language does not behave as I think a reasonable person would expect.

Perhaps if I tried to write a compiler for an OOP language, I would quickly see why implementing it so that static functions can be overriden would be difficult or impossible.

Or perhaps there is some good reason why Java behaves this way. Can anyone point out an advantage to this behavior, some category of problem that is made easier by this? I mean, don't just point me to the Java language spec and say "see, this is documented how it behaves". I know that. But is there a good reason why it SHOULD behave this way? (Besides the obvious "making it work right was too hard"...)

Update

@VicKirk: If you mean that this is "bad design" because it doesn't fit how Java handles statics, my reply is, "Well, duh, of course." As I said in my original post, it doesn't work. But if you mean that it is bad design in the sense that there would be something fundamentally wrong with a language where this worked, ie where statics could be overridden just like virtual functions, that this would somehow introduce an ambiguity or it would be impossible to implement efficiently or some such, I reply, "Why? What's wrong with the concept?"

I think the example I give is a very natural thing to want to do. I have a class that has a function that does not depend on any instance data, and which I might very reasonably want to call independent of an instance, as well as wanting to call from within an instance method. Why should this not work? I've run into this situation a fair number of times over the years. In practice I get around it by making the function virtual, and then creating a static method whose only purpose in life is to be a static method that passes the call on to the virtual method with a dummy instance. That seems like a very roundabout way to get there.

Answer 3

The short answer is: it is entirely possible, but Java doesn't do it.

package sp.trial;
public class Base {
  static void printValue() {
    System.out.println("  Called static Base method.");
  }
  void nonStatPrintValue() {
    System.out.println("  Called non-static Base method.");
  }
  void nonLocalIndirectStatMethod() {
    System.out.println("  Non-static calls overridden(?) static:");
    System.out.print("  ");
    this.printValue();
  }
}

Answer 4

Actually we were wrong.
Despite Java doesn't allow you to override static methods by default, if you look thoroughly through documentation of Class and Method classes in Java, you can still find a way to emulate static methods overriding by following workaround:

import java.lang.reflect.InvocationTargetException;
import java.math.BigDecimal;

class RegularEmployee {

    private BigDecimal salary = BigDecimal.ONE;

    public void setSalary(BigDecimal salary) {
        this.salary = salary;
    }
    public static BigDecimal getBonusMultiplier() {
        return new BigDecimal(".02");
    }
    public BigDecimal calculateBonus() {
        return salary.multiply(this.getBonusMultiplier());
    }
    public BigDecimal calculateOverridenBonus() {
        try {
            // System.out.println(this.getClass().getDeclaredMethod(
            // "getBonusMultiplier").toString());
            try {
                return salary.multiply((BigDecimal) this.getClass()
                    .getDeclaredMethod("getBonusMultiplier").invoke(this));
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            } catch (IllegalArgumentException e) {
                e.printStackTrace();
            } catch (InvocationTargetException e) {
                e.printStackTrace();
            }
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        } catch (SecurityException e) {
            e.printStackTrace();
        }
        return null;
    }
    // ... presumably lots of other code ...
}

final class SpecialEmployee extends RegularEmployee {

    public static BigDecimal getBonusMultiplier() {
        return new BigDecimal(".03");
    }
}

public class StaticTestCoolMain {

    static public void main(String[] args) {
        RegularEmployee Alan = new RegularEmployee();
        System.out.println(Alan.calculateBonus());
        System.out.println(Alan.calculateOverridenBonus());
        SpecialEmployee Bob = new SpecialEmployee();
        System.out.println(Bob.calculateBonus());
        System.out.println(Bob.calculateOverridenBonus());
    }
}

Answer 5

Static methods are treated as global by the JVM, there are not bound to an object instance at all.

It could conceptually be possible if you could call static methods from class objects (like in languages like Smalltalk) but it's not the case in Java.

EDIT

You can overload static method, that's ok. But you can not override a static method, because class are no first-class object. You can use reflection to get the class of an object at run-time, but the object that you get does not parallel the class hierarchy.

class MyClass { ... }
class MySubClass extends MyClass { ... }

MyClass obj1 = new MyClass();
MySubClass obj2 = new MySubClass();

ob2 instanceof MyClass --> true

Class clazz1 = obj1.getClass();
Class clazz2 = obj2.getClass();

clazz2 instanceof clazz1 --> false

You can reflect over the classes, but it stops there. You don't invoke a static method by using clazz1.staticMethod() , but using MyClass.staticMethod() . A static method is not bound to an object and there is hence no notion of this nor super in a static method. A static method is a global function; as a consequence there is also no notion of polymorphism and, therefore, method overriding makes no sense.

But this could be possible if MyClass was an object at run-time on which you invoke a method, as in Smalltalk (or maybe JRuby as one comment suggest, but I know nothing of JRuby).

Oh yeah... one more thing. You can invoke a static method through an object obj1.staticMethod() but that really syntactic sugar for MyClass.staticMethod() and should be avoided. It usually raises a warning in modern IDE. I don't know why they ever allowed this shortcut.

Answer 6

Method overriding is made possible by dynamic dispatching<\/a> , meaning that the declared type of an object doesn't determine its behavior, but rather its runtime type:

Animal lassie = new Dog();
lassie.speak(); // outputs "woof!"
Animal kermit = new Frog();
kermit.speak(); // outputs "ribbit!"

Answer 7

Yes. Practically Java allows overriding static method, and No theoretically if you Override a static method in Java then it will compile and run smoothly but it will lose Polymorphism which is the basic property of Java. You will Read Everywhere that it is not possible to try yourself compiling and running. you will get your answer. eg If you Have Class Animal and a static method eat() and you Override that static method in its Subclass lets called it Dog. Then when wherever you Assign a Dog object to an Animal Reference and call eat() according to Java Dog's eat() should have been called but in static Overriding Animals' eat() will Be Called.

class Animal {
    public static void eat() {
        System.out.println("Animal Eating");
    }
}

class Dog extends Animal{
    public static void eat() {
        System.out.println("Dog Eating");
    }
}

class Test {
    public static void main(String args[]) {
       Animal obj= new Dog();//Dog object in animal
       obj.eat(); //should call dog's eat but it didn't
    }
}


Output Animal Eating

According to Polymorphism Principle of Java, the Output Should be Dog Eating .
But the result was different because to support Polymorphism Java uses Late Binding that means methods are called only at the run-time but not in the case of static methods. In static methods compiler calls methods at the compile time rather than the run-time, so we get methods according to the reference and not according to the object a reference a containing that's why You can say Practically it supports static overring but theoretically, it doesn't.

Answer 8

In Java (and many OOP languages, but I cannot speak for all; and some do not have static at all) all methods have a fixed signature - the parameters and types. In a virtual method, the first parameter is implied: a reference to the object itself and when called from within the object, the compiler automatically adds this<\/code> .

However, by declaring the method static you have explicitly stated that the compiler must not include the implied object parameter at the beginning of that signature. Therefore, any other code that calls this must must not attempt to put a reference to an object on the stack<\/em> . If it did do that, then the method execution would not work since the parameters would be in the wrong place - shifted by one - on the stack.

virtual methods always have a reference to the context object (ie this<\/code> ) so then it is possible to reference anything within the heap that belong to that instance of the object. But with static methods, since there is no reference passed, that method cannot access any object variables and methods since the context is not known.

I see that in Ruby classes are really a special kind of object and one can create (even dynamically) new methods. Classes are full class objects in Ruby, they are not in Java. This is just something you will have to accept when working with Java (or C#). These are not dynamic languages, though C# is adding some forms of dynamic. In reality, Ruby does not have "static" methods as far as I could find - in that case these are methods on the singleton class object. You can then override this singleton with a new class and the methods in the previous class object will call those defined in the new class (correct?). So if you called a method in the context of the original class it still would only execute the original statics, but calling a method in the derived class, would call methods either from the parent or sub-class. Interesting and I can see some value in that. It takes a different thought pattern.

Why they did this? Well, probably to improve performance at the time based on the technology and understanding that was available. Computer languages are constantly evolving. Go back far enough and there is no such thing as OOP. In the future, there will be other new ideas.

Now that I see the differences and as I Java\/C# developer myself, I can understand why the answers you get from Java developers may be confusing if you are coming from a language like Ruby. Java static<\/code> methods are not the same as Ruby class<\/code> methods. Java developers will have a hard time understanding this, as will conversely those who work mostly with a language like Ruby\/Smalltalk. I can see how this would also be greatly confusing by the fact that Java also uses "class method" as another way to talk about static methods but this same term is used differently by Ruby. Java does not have Ruby style class methods (sorry); Ruby does not have Java style static methods which are really just old procedural style functions, as found in C.

I learned something new for me today about class methods (Ruby style).

Answer 9

Well... the answer is NO if you think from the perspective of how an overriden method should behave in Java. But, you don't get any compiler error if you try to override a static method. That means, if you try to override, Java doesn't stop you doing that; but you certainly don't get the same effect as you get for non-static methods. Overriding in Java simply means that the particular method would be called based on the run time type of the object and not on the compile time type of it (which is the case with overriden static methods). Okay... any guesses for the reason why do they behave strangely? Because they are class methods and hence access to them is always resolved during compile time only using the compile time type information. Accessing them using object references is just an extra liberty given by the designers of Java and we should certainly not think of stopping that practice only when they restrict it :-)

class SuperClass {
// ......
public static void staticMethod() {
    System.out.println("SuperClass: inside staticMethod");
}
// ......
}

public class SubClass extends SuperClass {
// ......
// overriding the static method
public static void staticMethod() {
    System.out.println("SubClass: inside staticMethod");
}

// ......
public static void main(String[] args) {
    // ......
    SuperClass superClassWithSuperCons = new SuperClass();
    SuperClass superClassWithSubCons = new SubClass();
    SubClass subClassWithSubCons = new SubClass();

    superClassWithSuperCons.staticMethod();
    superClassWithSubCons.staticMethod();
    subClassWithSubCons.staticMethod();
    // ...
}
}

Answer 10

overriding is reserved for instance members to support polymorphic behaviour. static class members do not belong to a particular instance. instead, static members belong to the class and as a result overriding is not supported because subclasses only inherit protected and public instance members and not static members. You may want to define an inerface and research factory and/or strategy design patterns to evaluate an alternate approach.

Answer 11

I like and double Jay's comment ( https:\/\/stackoverflow.com\/a\/2223803\/1517187<\/a> ).
I agree that this is the bad design of Java.
Many other languages support overriding static methods, as we see in previous comments. I feel Jay has also come to Java from Delphi like me.
Delphi (Object Pascal) was one of the languages implementing OOP before Java and one of the first languages used for commercial application development.
It is obvious that many people had experience with that language since it was in the past the only language to write commercial GUI products. And - yes, we could in Delphi override static methods. Actually, static methods in Delphi are called "class methods", while Delphi had the different concept of "Delphi static methods" which were methods with early binding. To override methods you had to use late binding, declare "virtual" directive. So it was very convenient and intuitive and I would expect this in Java.

"

Answer 12

By overriding we can create a polymorphic nature depending on the object type. Static method has no relation with object. So java can not support static method overriding.

Answer 13

In general it doesn't make sense to allow 'overriding' of static methods as there would be no good way to determine which one to call at runtime. Taking the Employee example, if we call RegularEmployee.getBonusMultiplier() - which method is supposed to be executed?

However, all this would do is to re-implement regular class methods, adding redundancy to the language without really adding any benefit.

Answer 14

By overriding, you achieve dynamic polymorphism. When you say overriding static methods, the words you are trying to use are contradictory.

Both are opposite in nature, and hence can't be used together.

JRE will map different instance methods of different classes based on what kind of object you are using.

So the term "overriding" static methods itself doesn't make any meaning.

Answer 15

What good will it do to override static methods. You cannot call static methods through an instance.

MyClass.static1()
MySubClass.static1()   // If you overrode, you have to call it through MySubClass anyway.

Answer 16

Overriding in Java simply means that the particular method would be called based on the runtime type of the object and not on the compile-time type of it (which is the case with overridden static methods). As static methods are class methods they are not instance methods so they have nothing to do with the fact which reference is pointing to which Object or instance, because due to the nature of static method it belongs to a specific class. You can redeclare it in the subclass but that subclass won't know anything about the parent class' static methods because, as I said, it is specific to only that class in which it has been declared. Accessing them using object references is just an extra liberty given by the designers of Java and we should certainly not think of stopping that practice only when they restrict it more details and example http:\/\/faisalbhagat.blogspot.com\/2014\/09\/method-overriding-and-method-hiding.html<\/a>

"

Answer 17

这个问题的答案很简单，标记为静态的方法或变量只属于类，因此静态方法不能在子类中继承，因为它们只属于超类。

Answer 18

Easy solution: Use singleton instance. It will allow overrides and inheritance.

In my system, I have SingletonsRegistry class, which returns instance for passed Class. If instance is not found, it is created.

Haxe language class:

package rflib.common.utils;
import haxe.ds.ObjectMap;



class SingletonsRegistry
{
  public static var instances:Map<Class<Dynamic>, Dynamic>;

  static function __init__()
  {
    StaticsInitializer.addCallback(SingletonsRegistry, function()
    {
      instances = null;
    });

  } 

  public static function getInstance(cls:Class<Dynamic>, ?args:Array<Dynamic>)
  {
    if (instances == null) {
      instances = untyped new ObjectMap<Dynamic, Dynamic>();      
    }

    if (!instances.exists(cls)) 
    {
      if (args == null) args = [];
      instances.set(cls, Type.createInstance(cls, args));
    }

    return instances.get(cls);
  }


  public static function validate(inst:Dynamic, cls:Class<Dynamic>)
  {
    if (instances == null) return;

    var inst2 = instances[cls];
    if (inst2 != null && inst != inst2) throw "Can\'t create multiple instances of " + Type.getClassName(cls) + " - it's singleton!";
  }

}

Answer 19

Here is a simple explanation. A static method is associated with a class while an instance method is associated with a particular object. Overrides allow calling the different implementation of the overridden methods associated with the particular object. So it is counter-intuitive to override static method which is not even associated with objects but the class itself in the first place. So static methods cannot be overridden based on what object is calling it, it will always be associated with the class where it was created.

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Answer 20

A Static method, variable, block or nested class belongs to the entire class<\/strong> rather than an object.

Here, as the method is static<\/strong> (ie, static method is used to represent the behaviour of a class only.) changing\/ overriding<\/strong> the behaviour of entire class will violate the phenomenon of one of the fundamental pillar of Object oriented programming ie, high cohesion<\/strong> . (remember a constructor is a special kind of method in Java.)

For example: A car class should produce only car objects and not bike, trucks, planes etc. But the Car class may have some features(behaviour) that belongs to itself only.

The language designers thought to allow developers to keep some behaviours of a class to itself only by making a method static in nature.

---

public class Vehicle {
static int VIN;

public static int getVehileNumber() {
    return VIN;
}}

class Car extends Vehicle {
static int carNumber;

public static int getVehileNumber() {
    return carNumber;
}}

Answer 21

Now seeing above answers everyone knows that we can't override static methods, but one should not misunderstood about the concept of accessing static methods from subclass<\/strong> .

           

            

               <\/blockquote><\/blockquote>
               
 
               

               
               
 
              
 
           

Answer 22

The following code shows that it is possible:

class OverridenStaticMeth {   

static void printValue() {   
System.out.println("Overriden Meth");   
}   

}   

public class OverrideStaticMeth extends OverridenStaticMeth {   

static void printValue() {   
System.out.println("Overriding Meth");   
}   

public static void main(String[] args) {   
OverridenStaticMeth osm = new OverrideStaticMeth();   
osm.printValue();   

System.out.println("now, from main");
printValue();

}   

}