关于Methods和ArrayList的清晰度

Question

I would like to learn how can I create a ArrayList of methods.

public class a {
public static b (){}
public static c (){}
public static d (){}
public static e (){}

public static void main(String[] arg){
ArrayList<Method> lst = new ArrayList<Method>();
lst.add(1, a());
lst.add(2, b());
lst.add(3, c());
lst.add(4, d());
lst.add(5, e());
}

I'm wondering, why have it be like it, could someone be so kind to explain:

public static Collection<? extends Method> a(){}
...
lst.addAll(1, a());

Why using Collection? Why not use simply add(Object)?

Oh, one more thing. How can I execute later methods from ArrayList?

for (Method i : lst){
i;}

Thank you for your kind help.

Answer 1

If you want to use reflection, you should use getDeclaredMethod to get the method, as many other answers here have stated. However, more common (and perhaps cleaner) practice in Java code, to avoid use of reflection, is to make an interface that you can provide implementations of:

private static interface Foo {
  public void doFoo();
}

public static Foo a = new Foo() {
  public void doFoo() {
    // Do 'a' stuff here.
  }
}

public static Foo b = new Foo() {
  public void doFoo() {
    // Do 'b' stuff here.
  }
}

...

public static Foo e = new Foo() {
  public void doFoo() {
    // Do 'e' stuff here.
  }
}

public static void main(String[] arg){
  ArrayList<Foo> lst = new ArrayList<Foo>();
  lst.add(1, a);
  lst.add(2, b);
  lst.add(3, c);
  lst.add(4, d);
  lst.add(5, e);
}

Then to invoke the function, you can call doFoo() on each of the Foo objects in the list.

Answer 2

As of Java 7, you cannot treat a method as a variable as you're doing here. You need to use Reflection to access the Method object that represents the method. Use the getMethod method on the a class to retrieve the proper Method object, which you can add to your ArrayList .

The addAll method adds all elements in the collection argument to the collection on which you're calling addAll . If you were to call add(Collection) , then the Collection itself would just be the next new element.

To call a method that's represented by a Method , call the invoke method . Careful, both the getMethod and invoke methods throw their own Exception s that you'll have to catch.

Answer 3

You get Method objects by using Reflection on a class object.

And then you call them by supplying the object you wish them to act on, plus any parameters you wish to supply.

Probably a good idea to read the reflection tutorial : http://docs.oracle.com/javase/tutorial/reflect/

Answer 4

You can get a method declared in your class like this:

public class MyClass {
    public void a() {
        System.out.println("A!");
    }

    public static void main(String... args)
    {
        Method methodA = getClass().getDeclaredMethod("a");
    }
}

Answer 5

• You need to have the return type for all the methods (except for constructors) like public static void b (){} .
• lst.add(1, a()); will just call the method and try to add the return value of the method to the list at a given index.
• Also, you need to follow the java naming conventions (like class name should always starts with capital)

If at all you want to add the Method you need to use reflection list.add(a.class.getMethod("b"));