为什么数组上的方法必须是静态的（Java）？

Question

I don't understand why methods on arrays, in Java, have to be static. Static methods can only be used on static variables right? So this would mean arrays are static variables, variables shared by a class? But what class would this be?

Can someone help me understand this?

Edit: to be more specific, I am creating methods to act on arrays but if I just write "public int[] expandArr(int[]a, int v)" and I try to use this method in the main method, I get an error saying I can't make a static reference to a non-static method. When I write "public static int[] expandArr(int[]a, int v)" it works then.

I understand you cannot change the size of an array, the method I wrote makes a new array with increased size and copies the first one into it.

Thank you.

Answer 1

You say you tried to write this:

public int[] expandArr(int[]a, int v)

The thing is, you had to write it in some class, since you can't just have free-floating methods in your program. Therefore, it must operate on a instance of the class. For example:

public class MyProgram {

    public int[] expandArr(int[]a, int v) { ... }

    public static void main(String[] args) { ... }

}

expandArr requires an instance of MyProgram , since you didn't declare it to be static . And that has nothing to do with arrays. It would be the same if you wrote

public class MyProgram {

    public String expandString(String s, int v) { ... }

    public static void main(String[] args) { 
         String s = expandString(args[0], 1);  // ILLEGAL
         String s2 = args[0].expandString(1);  // ILLEGAL
    }
}

Although the first parameter of expandString is a String , this actually operates on a MyProgram , and you cannot use expandString without an instance of MyProgram to operate on. Making it static means that you can (the first use of expandString in my example would become legal.)

In general, you can't add methods to a class without modifying the source of that class. If you want to write a new method that does something with objects of a certain class C , and you can't modify class C (perhaps because it's in the Java library or someone else's library), then you'll need to put the method in some other class C2 , and most of the time you will need to make the method static since it doesn't involve objects of class C2 .

Answer 2

You can't call a non-static method from a static method unless you first instantiate the an object of the class.

eg

In class Whatever...

public boolean ok() {
    return true;
}

public static void main(String[] args) {
    Whatever w = new Whatever();
    System.out.println(w.ok());
}

Answer 3

You cant call a non static method from a static context. A static method belongs to the class, non static or instance methods are copied to each instance of the class (they each have their own). If I have 10 instances of class A, and class A has a static method, which all of them share, then I try to invoke a non static method in class A from class A's static method, which instance of class A gets its method invoked? The behavior is undefined.

The question really has nothing to do with arrays.

This question is related: Can't call non static method

Answer 4

I think what you're referring to is the fact that you can't extend an array class and hence can't add a method to it:

// No way to do this
int[] array;
array.myMethod();

This means your only option is to make a static helper method somewhere:

// So you have to do this
int[] array;
Utils.myMethod(array);

class Utils {
    static void myMethod(int[] array) {
        ...
    }
}

An example of this is the Arrays class, which has lots of static methods for operating on arrays. Conceptually it would be clearer if these methods could be added to the array classes, but you can't do that in Java (you can in other languages like Javascript).

Answer 5

That is why we use other classes like the ArrayList to make them grow dynamically in size. At the core of an arraylist you will simply find an array that is renewed in size whenever the class figures out it necessary. If I'm understanding your question right.