实例化内部类

Question

I working on a sample problem of over-ridding hashCode and equals method but getting an error: " No enclosing instance of type CustomHashCodeExample is accessible. Must qualify the allocation with an enclosing instance of type CustomHashCodeExample (egxnew A() where x is an instance of CustomHashCodeExample). " I wrote an inner class HashPerson and I am getting this error when I am trying to instantiate this inner class in another method called testHashCodeOverride().

public static void testHashCodeOverride(){   
    System.out.println("\nTest HashCode Override Method");
    System.out.println("==================================\n");

    HashPerson william = new HashPerson("willy");
    HashPerson bill = new HashPerson("willy");          
}

This code works fine, even though I dont see static inner class or instantiation of outer class, confused :(

public class HashCodeExample {

    public static void testHashCodeOverride() {

        HashPerson william = new HashPerson("Willy");
        HashPerson bill = new HashPerson("Willy");
        System.out.println("Hash code for william  = " + william.hashCode());
        System.out.println("Hash code for bill     = " + bill.hashCode());

        HashMap table = new HashMap();
        table.put(william, "Silly");

        if (table.containsKey(william)) {
            System.out.println(table.get(william));
        } else {
            System.out.println("Key " + william + " not found");
        }

        if (table.containsKey(bill)) {
            System.out.println(table.get(bill));
        } else {
            System.out.println("Key " + bill + " not found");
        }


    }

    class HashPerson {
        private static final int HASH_PRIME = 1000003;

        public HashPerson(String name) {
            this.name = name;
        }

        public String toString() {
            return name;
        }

        public boolean equals(Object rhs) {
            if (this == rhs)
                return true;

            // make sure they are the same class
            if (rhs == null || rhs.getClass() != getClass())
                return false;

            // ok, they are the same class. Cast rhs to HashPerson
            HashPerson other = (HashPerson) rhs;

            // our test for equality simply checks the name field
            if (!name.equals(other.name)) {
                return false;
            }

            // if we get this far, they are equal
            return true;
        }
        public int hashCode() {
            int result = 0;
            result = HASH_PRIME * result + name.hashCode();
            return result;
        }
        private String name;

    }
}

Answer 1

I think you want to declare the HashPerson class as static . Otherwise it can only be instantiated in the context of the containing class, either in a method of the containing class or using code like this:

ContainingClass container = new ContainingClass();
HashPerson william = container.new HashPerson("willy");

Actually, my rule-of-thumb is to make any nested class static, unless I have a special reason not to. This is also more efficient, because non-static nested classes (called inner classes ) always contain an implicit reference to the containing object.

Answer 2

You need to either make your inner class static, or refer to it through an instance of the outer class. Most likely you just want to make your inner class static.

Non-static members of a class (variables, methods, inner classes) are per instance of the class. Therefore, when accessing non-static members from a static context (such as a static method like testHashCodeOverride ), you need to specify an instance of the enclosing class.

Answer 3

As i can see there could be different possible ways to instantiate the Inner Classes

1. Static Inner Class : When Inner class is static, let say code looks like as describe.

    class OuterClass { static int outer_x = 10; int outer_y = 20; // static nested class static class StaticNestedClass { void display() { } } } OuterClass.StaticNestedClass nestedObject = new OuterClass.StaticNestedClass();

or Just this could be sufficient for static classes

 new StaticNestedClass();

2. Local inner Classes (Non Static) : Inner Classes which are not static, one good thing with local inner classes that they can access all private data members of enclosed class.

    OuterClass outerObject = new OuterClass(); OuterClass.InnerClass innerObject = outerObject.new InnerClass();

3. Anonymous Inner Class (implementing Interface) : This type of classes generally hidden, One can't directly see the Class{...} block in code , That is why known as Anonymous Inner Class. Here describes how to instantiate it in case inner class implementing an interface Runnable Interface.

    Runnable r = new Runnable() { //This is Anonymous Class public void run() { System.out.println("Child Thread"); } };

4. Anonymous Inner Class (Extending One Class) :We can have an anonymous inner class that extends a class, Here i am taking example of Thread Class,

    Thread t = new Thread(new Runnable() { //Anonymous Inner class public void run() { System.out.println("Child Thread"); } });

5. Anonymous Inner class that defines inside method/constructor argument : Inner Classes could be define within methods as well, here giving example how can we define and instantiate it within argument

   public static void main(String[] args) { //Here we are using Anonymous Inner class //that define inside argument, here constructor argument Thread t = new Thread(new Runnable() { public void run() { System.out.println("Child Thread"); } }); t.start();