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YAML 1.1 parser and emitter for Java
There is a newer version: 2.3
/**
 * Copyright (c) 2008-2013, http://www.snakeyaml.org
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.yaml.snakeyaml.nodes;

import org.yaml.snakeyaml.error.Mark;

/**
 * Base class for all nodes.
 * 
 * The nodes form the node-graph described in the YAML Specification.
 * 
 * 
 * While loading, the node graph is usually created by the
 * {@link org.yaml.snakeyaml.composer.Composer}, and later transformed into
 * application specific Java classes by the classes from the
 * {@link org.yaml.snakeyaml.constructor} package.
 * 
 */
public abstract class Node {
    private Tag tag;
    private Mark startMark;
    protected Mark endMark;
    private Class type;
    private boolean twoStepsConstruction;
    /**
     * true when the tag is assigned by the resolver
     */
    protected boolean resolved;
    protected Boolean useClassConstructor;

    public Node(Tag tag, Mark startMark, Mark endMark) {
        setTag(tag);
        this.startMark = startMark;
        this.endMark = endMark;
        this.type = Object.class;
        this.twoStepsConstruction = false;
        this.resolved = true;
        this.useClassConstructor = null;
    }

    /**
     * Tag of this node.
     * 
     * Every node has a tag assigned. The tag is either local or global.
     * 
     * @return Tag of this node.
     */
    public Tag getTag() {
        return this.tag;
    }

    public Mark getEndMark() {
        return endMark;
    }

    /**
     * For error reporting.
     * 
     * @see "class variable 'id' in PyYAML"
     * @return scalar, sequence, mapping
     */
    public abstract NodeId getNodeId();

    public Mark getStartMark() {
        return startMark;
    }

    public void setTag(Tag tag) {
        if (tag == null) {
            throw new NullPointerException("tag in a Node is required.");
        }
        this.tag = tag;
    }

    /**
     * Two Nodes are never equal.
     */
    @Override
    public final boolean equals(Object obj) {
        return super.equals(obj);
    }

    public Class getType() {
        return type;
    }

    public void setType(Class type) {
        if (!type.isAssignableFrom(this.type)) {
            this.type = type;
        }
    }

    public void setTwoStepsConstruction(boolean twoStepsConstruction) {
        this.twoStepsConstruction = twoStepsConstruction;
    }

    /**
     * Indicates if this node must be constructed in two steps.
     * 

     * Two-step construction is required whenever a node is a child (direct or
     * indirect) of it self. That is, if a recursive structure is build using
     * anchors and aliases.
     * 
     * 
     * Set by {@link org.yaml.snakeyaml.composer.Composer}, used during the
     * construction process.
     * 
     * 
     * Only relevant during loading.
     * 
     * 
     * @return true if the node is self referenced.
     */
    public boolean isTwoStepsConstruction() {
        return twoStepsConstruction;
    }

    @Override
    public final int hashCode() {
        return super.hashCode();
    }

    public boolean useClassConstructor() {
        if (useClassConstructor == null) {
            if (isResolved() && !Object.class.equals(type) && !tag.equals(Tag.NULL)) {
                return true;
            } else if (tag.isCompatible(getType())) {
                // the tag is compatible with the runtime class
                // the tag will be ignored
                return true;
            } else {
                return false;
            }
        }
        return useClassConstructor.booleanValue();
    }

    public void setUseClassConstructor(Boolean useClassConstructor) {
        this.useClassConstructor = useClassConstructor;
    }

    /**
     * Indicates if the tag was added by
     * {@link org.yaml.snakeyaml.resolver.Resolver}.
     * 
     * @return true if the tag of this node was resolved
     */
    public boolean isResolved() {
        return resolved;
    }
}