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A universal transaction manager that achieves database-agnostic transactions and distributed transactions that span multiple databases

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package com.scalar.db.api;

import com.google.common.base.MoreObjects;
import java.util.Collections;
import java.util.List;
import javax.annotation.Nonnull;
import javax.annotation.concurrent.Immutable;

/**
 * An optional parameter of {@link Update} for conditional update. {@link Update} with this
 * condition makes the operation take effect only if a specified entry exists.
 *
 * @author Hiroyuki Yamada
 */
@Immutable
public class UpdateIfExists implements MutationCondition {

  /** Constructs a {@code UpdateIfExists}. */
  UpdateIfExists() {}

  /**
   * Returns an empty list of {@link ConditionalExpression}s since any conditions are not given to
   * this object.
   *
   * @return an empty list of {@code ConditionalExpression}s
   */
  @Override
  @Nonnull
  public List getExpressions() {
    return Collections.emptyList();
  }

  @Override
  public void accept(MutationConditionVisitor visitor) {
    visitor.visit(this);
  }

  /**
   * Indicates whether some other object is "equal to" this object. The other object is considered
   * equal if:
   *
   * 
   *   it is also an {@code UpdateIfExists}
   * 
   *
   * @param o an object to be tested for equality
   * @return {@code true} if the other object is "equal to" this object otherwise {@code false}
   */
  @Override
  public boolean equals(Object o) {
    if (o == this) {
      return true;
    }
    return o instanceof UpdateIfExists;
  }

  @Override
  public int hashCode() {
    return UpdateIfExists.class.hashCode();
  }

  @Override
  public String toString() {
    return MoreObjects.toStringHelper(this).toString();
  }
}