如何修复Java泛型强转的Unchecked cast编译警告

问题背景 #

实现链式存储结构的LinkedStack<E>栈类时，所有功能测试均已通过，代码可正常运行，但存在编译警告，需要通过修改equals方法的代码消除该警告。

警告信息 #

Unchecked cast: 'java.lang.Object' to 'LinkedStack'

原始问题代码 #

public final class LinkedStack<E> implements Stack<E> {

    private Node head = null;
    private int size = 0;

    @Override
    public int size() {
        return size;
    }

    @Override
    public boolean isEmpty() {
        return size == 0;
    }

    @Override
    public void clear() {
        head = null;
        size = 0;
    }

    @Override
    public boolean contains(E e) {
        for (int i = 0; i < size; i++) {
            E temp = pop();
            if (temp.equals(e)) {
                return true;
            }
        }
        return false;
    }

    @Override
    public E top() throws StackEmptyException {
        if (size == 0) {
            throw new StackEmptyException();
        }
        return head.element;
    }

    @Override
    public void push(E e) {
        head = new Node(e, head);
        size++;
    }

    @Override
    public E pop() throws StackEmptyException {

        if (size == 0) {
            throw new StackEmptyException();
        }
        E temp = head.element;
        head = head.next;
        size--;
        return temp;
    }


    public boolean equals(Object o) {
        if (o == null || getClass() != o.getClass()) return false;
        LinkedStack<E> that = (LinkedStack<E>) o;
        if (that.size() != this.size()) return false;
        Node j = that.head;
        int counter = 0;
        for (Node i = head; counter < size; i = i.next, j = j.next) {
            if (i.element == null && j.element == null) {
                counter++;
                continue;
            }
            if ((i.element != null) && (j.element != null)) {
                if (!i.element.equals(j.element)) {
                    return false;
                }
            } else {
                return false;
            }
            counter++;
        }
        return true;
    }

    @Override
    public int hashCode() {
        int resultat = 1;
        int counter = 0;
        for (Node i = head; counter < size; i = i.next) {
            if (i.element == null) {
                counter++;
                continue;
            }
            resultat = resultat * i.element.hashCode();
            counter++;
        }
        return resultat;
    }


    protected class Node {
        private final E element;
        private final Node next;

        protected Node(final E element, final Node next) {
            this.element = element;
            this.next = next;
        }
    }
}

警告产生原因 #

Java泛型在编译阶段会执行类型擦除，运行时不存在LinkedStack<E>的泛型类型信息，虚拟机无法验证Object到LinkedStack<E>的强转是否符合泛型类型约束，因此抛出未检查（unchecked）转换警告。

修复方案 #

该强转本身是类型安全的：代码已经在强转前通过getClass() != o.getClass()判断了传入对象的实际类型和当前类完全一致，后续元素比较逻辑仅依赖Object#equals方法，不依赖泛型参数的类型约束，不会产生运行时类型错误。
只需做两处修改即可消除警告：

• 为equals方法补上@Override注解
• 在equals方法上添加最小作用域的@SuppressWarnings("unchecked")注解，仅抑制该方法内的合法未检查转换警告，不影响其他代码的类型检查

修改后的equals方法代码 #

@Override
@SuppressWarnings("unchecked")
public boolean equals(Object o) {
    if (o == null || getClass() != o.getClass()) return false;
    LinkedStack<E> that = (LinkedStack<E>) o;
    if (that.size() != this.size()) return false;
    Node j = that.head;
    int counter = 0;
    for (Node i = head; counter < size; i = i.next, j = j.next) {
        if (i.element == null && j.element == null) {
            counter++;
            continue;
        }
        if ((i.element != null) && (j.element != null)) {
            if (!i.element.equals(j.element)) {
                return false;
            }
        } else {
            return false;
        }
        counter++;
    }
    return true;
}

内容的提问来源于stack exchange，提问作者matin_mhz