实例化内部类 [英] Instantiating inner class

问题描述

我正在处理一个覆盖 hashCode 和 equals 方法的示例问题，但收到错误:没有可访问类型 CustomHashCodeExample 的封闭实例.必须使用 CustomHashCodeExample 类型的封闭实例限定分配(egxnew A() 其中 x 是 CustomHashCodeExample 的实例."我写了一个内部类 HashPerson，当我尝试在另一个名为 testHashCodeOverride() 的方法中实例化这个内部类时出现这个错误.

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 (e.g. x.new 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("
Test HashCode Override Method");
    System.out.println("==================================
");

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

这段代码工作正常，即使我没有看到静态内部类或外部类的实例化，也很困惑:(

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;

    }
}

推荐答案

我认为您想将 HashPerson 类声明为 static.否则它只能在包含类的上下文中实例化，或者在包含类的方法中或者使用这样的代码:

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");

实际上，我的经验法则是将任何嵌套类设为静态，除非我有特殊原因不这样做.这也更有效，因为非静态嵌套类(称为内部类)总是包含对包含对象的隐式引用.

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.

这篇关于实例化内部类的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！