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Hamcrest matcher library for Java
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package org.hamcrest;

/**
 * 
 * A matcher over acceptable values.
 * A matcher is able to describe itself to give feedback when it fails.
 * 
 * 
 * Matcher implementations should NOT directly implement this interface.
 * Instead, extend the {@link BaseMatcher} abstract class,
 * which will ensure that the Matcher API can grow to support
 * new features and remain compatible with all Matcher implementations.
 * 
 * 
 * N.B. Well designed matchers should be immutable.
 * 
 *
 * @see BaseMatcher
 */
public interface Matcher extends SelfDescribing {

    /**
     * Evaluates the matcher for argument item.
     *
     * This method matches against Object, instead of the generic type T. This is
     * because the caller of the Matcher does not know at runtime what the type is
     * (because of type erasure with Java generics). It is down to the implementations
     * to check the correct type.
     *
     * @param item the object against which the matcher is evaluated.
     * @return true if item matches, otherwise false.
     *
     * @see BaseMatcher
     */
    boolean matches(Object item);
    
    /**
     * Generate a description of why the matcher has not accepted the item.
     * The description will be part of a larger description of why a matching
     * failed, so it should be concise. 
     * This method assumes that matches(item) is false, but 
     * will not check this.
     *
     * @param item The item that the Matcher has rejected.
     * @param mismatchDescription
     *     The description to be built or appended to.
     */
    void describeMismatch(Object item, Description mismatchDescription);

    /**
     * This method simply acts a friendly reminder not to implement Matcher directly and
     * instead extend BaseMatcher. It's easy to ignore JavaDoc, but a bit harder to ignore
     * compile errors .
     *
     * @see Matcher for reasons why.
     * @see BaseMatcher
     * @deprecated to make
     */
    @Deprecated
    void _dont_implement_Matcher___instead_extend_BaseMatcher_();
}