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   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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  package java.security.cert;
  
  import java.util.Iterator;
  import java.util.Set;
  
  /**
   * An immutable valid policy tree node as defined by the PKIX certification
   * path validation algorithm.
   *
   * <p>One of the outputs of the PKIX certification path validation
   * algorithm is a valid policy tree, which includes the policies that
   * were determined to be valid, how this determination was reached,
   * and any policy qualifiers encountered. This tree is of depth
   * <i>n</i>, where <i>n</i> is the length of the certification
   * path that has been validated.
   *
   * <p>Most applications will not need to examine the valid policy tree.
   * They can achieve their policy processing goals by setting the
   * policy-related parameters in {@code PKIXParameters}. However,
   * the valid policy tree is available for more sophisticated applications,
   * especially those that process policy qualifiers.
   *
   * <p>{@link PKIXCertPathValidatorResult#getPolicyTree()
   * PKIXCertPathValidatorResult.getPolicyTree} returns the root node of the
   * valid policy tree. The tree can be traversed using the
   * {@link #getChildren getChildren} and {@link #getParent getParent} methods.
   * Data about a particular node can be retrieved using other methods of
   * {@code PolicyNode}.
   *
   * <p><b>Concurrent Access</b>
   * <p>All {@code PolicyNode} objects must be immutable and
   * thread-safe. Multiple threads may concurrently invoke the methods defined
   * in this class on a single {@code PolicyNode} object (or more than one)
   * with no ill effects. This stipulation applies to all public fields and
   * methods of this class and any added or overridden by subclasses.
   *
   * @since       1.4
   * @author      Sean Mullan
   */
  public interface PolicyNode {
  
      /**
       * Returns the parent of this node, or {@code null} if this is the
       * root node.
       *
       * @return the parent of this node, or {@code null} if this is the
       * root node
       */
      PolicyNode getParent();
  
      /**
       * Returns an iterator over the children of this node. Any attempts to
       * modify the children of this node through the
       * {@code Iterator}'s remove method must throw an
       * {@code UnsupportedOperationException}.
       *
       * @return an iterator over the children of this node
       */
      Iterator<? extends PolicyNode> getChildren();
  
      /**
       * Returns the depth of this node in the valid policy tree.
       *
       * @return the depth of this node (0 for the root node, 1 for its
       * children, and so on)
       */
      int getDepth();
  
      /**
       * Returns the valid policy represented by this node.
       *
       * @return the {@code String} OID of the valid policy
       * represented by this node. For the root node, this method always returns
       * the special anyPolicy OID: "2.5.29.32.0".
       */
      String getValidPolicy();
  
      /**
       * Returns the set of policy qualifiers associated with the
       * valid policy represented by this node.
       *
       * @return an immutable {@code Set} of
       * {@code PolicyQualifierInfo}s. For the root node, this
       * is always an empty {@code Set}.
       */
      Set<? extends PolicyQualifierInfo> getPolicyQualifiers();
  
      /**
       * Returns the set of expected policies that would satisfy this
       * node's valid policy in the next certificate to be processed.
       *
       * @return an immutable {@code Set} of expected policy
       * {@code String} OIDs. For the root node, this method
       * always returns a {@code Set} with one element, the
       * special anyPolicy OID: "2.5.29.32.0".
       */
      Set<String> getExpectedPolicies();
  
      /**
       * Returns the criticality indicator of the certificate policy extension
       * in the most recently processed certificate.
       *
       * @return {@code true} if extension marked critical,
       * {@code false} otherwise. For the root node, {@code false}
       * is always returned.
       */
      boolean isCritical();
  }
  