Why are you not able to declare a class as static in Java?

问题

回答1:

Only nested classes can be static. By doing so you can use the nested class without having an instance of the outer class.

class OuterClass{
    public static class StaticNestedClass{
    }

    public class InnerClass{
    }

    public InnerClass getAnInnerClass(){
        return new InnerClass();
    }

    //This method doesn't work
    public static InnerClass getAnInnerClassStatically(){
        return new InnerClass();
    }
}

class OtherClass{
    //Use of a static nested class:
    private OuterClass.StaticNestedClass staticNestedClass = new OuterClass.StaticNestedClass();

    //Doesn't work
    private OuterClass.InnerClass innerClass = new OuterClass.InnerClass();

    //Use of an inner class:
    private OuterClass outerclass= new OuterClass();
    private OuterClass.InnerClass innerClass2 = outerclass.getAnInnerClass();
    private OuterClass.InnerClass innerClass3 = outerclass.new InnerClass();
}

Sources :

Oracle tutorial on nested classes

On the same topic :

Java: Static vs non static inner class
Java inner class and static nested class

回答2:

Top level classes are static by default. Inner classes are non-static by default. You can change the default for inner classes by explicitly marking them static. Top level classes, by virtue of being top-level, cannot have non-static semantics because there can be no parent class to refer to. Therefore, there is no way to change the default for top-level classes.

回答3:

So, I'm coming late to the party, but here's my two cents - philosophically adding to Colin Hebert's answer.

At a high level your question deals with the difference between objects and types. While there are many cars (objects), there is only one Car class (type). Declaring something as static means that you are operating in the "type" space. There is only one. The top-level class keyword already defines a type in the "type" space. As a result "public static class Car" is redundant.

回答4:

Class with private constructor is static.

Declare your class like this:

public class eOAuth {

    private eOAuth(){}

    public final static int    ECodeOauthInvalidGrant = 0x1;
    public final static int    ECodeOauthUnknown       = 0x10;
    public static GetSomeStuff(){}

}

and you can used without initialization:

if (value == eOAuth.ECodeOauthInvalidGrant)
    eOAuth.GetSomeStuff();
...

回答5:

Sure they can, but only ~~inner~~ nested classes. There, it means that instances of the nested class do not require an enclosing instance of the outer class.

But for top-level classes, the language designers couldn't think of anything useful to do with the keyword, so it's not allowed.

回答6:

You can create a utility class (which cannot have instances created) by declaring an enum type with no instances. i.e. you are specificly declaring that there are no instances.

public enum MyUtilities {;
   public static void myMethod();
}

回答7:

public class Outer {
   public static class Inner {}
}

... it can be declared static - as long as it is a member class.

From the JLS:

Member classes may be static, in which case they have no access to the instance variables of the surrounding class; or they may be inner classes (§8.1.3).

and here:

The static keyword may modify the declaration of a member type C within the body of a non-inner class T. Its effect is to declare that C is not an inner class. Just as a static method of T has no current instance of T in its body, C also has no current instance of T, nor does it have any lexically enclosing instances.

A static keyword wouldn't make any sense for a top level class, just because a top level class has no enclosing type.

回答8:

As explained above, a Class cannot be static unless it's a member of another Class.

If you're looking to design a class "of which there cannot be multiple instances", you may want to look into the "Singleton" design pattern.

Beginner Singleton info here.

Caveat:

If you are thinking of using the singleton pattern, resist with all your might. It is one of the easiest DesignPatterns to understand, probably the most popular, and definitely the most abused. (source: JavaRanch as linked above)

回答9:

In addition to how Java defines static inner classes, there is another definition of static classes as per the C# world [1]. A static class is one that has only static methods (functions) and it is meant to support procedural programming. Such classes aren't really classes in that the user of the class is only interested in the helper functions and not in creating instances of the class. While static classes are supported in C#, no such direct support exists in Java. You can however use enums to mimic C# static classes in Java so that a user can never create instances of a given class (even using reflection) [2]:

public enum StaticClass2 {
    // Empty enum trick to avoid instance creation
    ; // this semi-colon is important

    public static boolean isEmpty(final String s) {
        return s == null || s.isEmpty();
    }
}

回答10:

Everything we code in java goes into a class. Whenever we run a class JVM instantiates an object. JVM can create a number of objects, by definition Static means you have the same set of copy to all objects.

So, if Java would have allowed the top class to be static whenever you run a program it creates an Object and keeps overriding on to the same Memory Location.

If You are just replacing the object every time you run it whats the point of creating it?

So that is the reason Java got rid of the static for top-Level Class.

There might be more concrete reasons but this made much logical sense to me.

回答11:

The only classes that can be static are inner classes. The following code works just fine:

public class whatever {
    static class innerclass {
    }
}

The point of static inner classes is that they don't have a reference to the outer class object.

回答12:

I think this is possible as easy as drink a glass of coffee!. Just take a look at this. We do not use static keyword explicitly while defining class.

public class StaticClass {

    static private int me = 3;
    public static void printHelloWorld() {
       System.out.println("Hello World");
    }



    public static void main(String[] args) {
        StaticClass.printHelloWorld();
        System.out.println(StaticClass.me);
    }
}

Is not that a definition of static class? We just use a function binded to just a class. Be careful that in this case we can use another class in that nested. Look at this:

class StaticClass1 {

    public static int yum = 4;

    static void  printHowAreYou() {
        System.out.println("How are you?");
    }
}

public class StaticClass {

    static int me = 3; 
    public static void printHelloWorld() {
       System.out.println("Hello World");
       StaticClass1.printHowAreYou();
       System.out.println(StaticClass1.yum);
    }



    public static void main(String[] args) {
        StaticClass.printHelloWorld();
        System.out.println(StaticClass.me);
    }
}

回答13:

One can look at PlatformUI in Eclipse for a class with static methods and private constructor with itself being final.

public final class <class name>
{
   //static constants
   //static memebers
}

回答14:

if the benefit of using a static-class was not to instantiate an object and using a method then just declare the class as public and this method as static.

来源：https://stackoverflow.com/questions/3584113/why-are-you-not-able-to-declare-a-class-as-static-in-java

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