java基础之ConcurrentHashMap

1、基础不牢固，请问接口里面怎么有方法？ 这个和一般的方法有啥区别？

public interface ConcurrentMap<K, V> extends Map<K, V> {    /**     * {@inheritDoc}     *     * @implNote This implementation assumes that the ConcurrentMap cannot     * contain null values and {@code get()} returning null unambiguously means     * the key is absent. Implementations which support null values     * <strong>must</strong> override this default implementation.     *     * @throws ClassCastException {@inheritDoc}     * @throws NullPointerException {@inheritDoc}     * @since 1.8     */    @Override    default V getOrDefault(Object key, V defaultValue) {        V v;        return ((v = get(key)) != null) ? v : defaultValue;    }

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