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DWR is easy Ajax for Java. It makes it simple to call Java code directly from Javascript. It gets rid of almost all the boiler plate code between the web browser and your Java code.
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/*
 * Copyright 2005 Joe Walker
 *
 * 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.directwebremoting.json;

/**
 * JsonValue allows you to hold any of the JSON types and to get a string
 * version without needing to know what type you have.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public abstract class JsonValue
{
    /**
     * i.e. How do we send this from one system to another?
     * @return A string representing the portable version of this data
     */
    public abstract String toExternalRepresentation();

    /**
     * Often when dealing with a {@link JsonValue} you know which subtype it is,
     * this allows you to assume it is a string and get the Java value quickly.
     * @return A String representing the {@link JsonNumber} implementing this
     * @throws ClassCastException if this is not a JsonString
     */
    public String getString()
    {
        throw new ClassCastException(this.getClass().getSimpleName() + " is not a " + JsonString.class.getSimpleName());
    }

    /**
     * Often when dealing with a {@link JsonValue} you know which subtype it is,
     * this allows you to assume it is a number and get the Java value quickly.
     * @return An int representing the {@link JsonNumber} implementing this
     * @throws ClassCastException if this is not a JsonNumber
     */
    public int getInteger()
    {
        throw new ClassCastException(this.getClass().getSimpleName() + " is not a " + JsonNumber.class.getSimpleName());
    }

    /**
     * Often when dealing with a {@link JsonValue} you know which subtype it is,
     * this allows you to assume it is a number and get the Java value quickly.
     * @return A long representing the {@link JsonNumber} implementing this
     * @throws ClassCastException if this is not a JsonNumber
     */
    public long getLong()
    {
        throw new ClassCastException(this.getClass().getSimpleName() + " is not a " + JsonNumber.class.getSimpleName());
    }

    /**
     * Often when dealing with a {@link JsonValue} you know which subtype it is,
     * this allows you to assume it is a number and get the Java value quickly.
     * @return A double representing the {@link JsonNumber} implementing this
     * @throws ClassCastException if this is not a JsonNumber
     */
    public double getDouble()
    {
        throw new ClassCastException(this.getClass().getSimpleName() + " is not a " + JsonNumber.class.getSimpleName());
    }

    /**
     * Often when dealing with a {@link JsonValue} you know which subtype it is,
     * this allows you to assume it is a boolean and get the Java value quickly.
     * @return A boolean representing the {@link JsonNumber} implementing this
     * @throws ClassCastException if this is not a JsonNumber
     */
    public boolean getBoolean()
    {
        throw new ClassCastException(this.getClass().getSimpleName() + " is not a " + JsonNumber.class.getSimpleName());
    }

    /**
     * Often when dealing with a {@link JsonValue} you know which subtype it is,
     * this allows you to assume it is a JsonArray and get it quickly
     * @return A cast of this to {@link JsonArray}
     * @throws ClassCastException if this is not a JsonArray
     */
    public JsonArray getJsonArray()
    {
        throw new ClassCastException(this.getClass().getSimpleName() + " is not a " + JsonArray.class.getSimpleName());
    }

    /**
     * Often when dealing with a {@link JsonValue} you know which subtype it is,
     * this allows you to assume it is a JsonObject and get it quickly
     * @return A cast of this to {@link JsonObject}
     * @throws ClassCastException if this is not a JsonObject
     */
    public JsonObject getJsonObject()
    {
        throw new ClassCastException(this.getClass().getSimpleName() + " is not a " + JsonObject.class.getSimpleName());
    }
}