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*/@Overridedefault V getOrDefault(Object key, V defaultValue) {V v;return ((v = get(key)) != null) ? v : defaultValue;}

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