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Google App Engine compliant variation of FreeMarker. FreeMarker is a "template engine"; a generic tool to generate text output based on templates.

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/*
 * Copyright 2014 Attila Szegedi, Daniel Dekany, Jonathan Revusky
 * 
 * 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 freemarker.template;

import java.util.Collection;
import java.util.Collections;
import java.util.Map;
import java.util.Set;

/**
 * Read-only empty map. {@link #remove(Object)}, {@link #clear()} and
 * {@link #putAll(Map)} with an empty {@link Map} as parameter are supported
 * operations (and do nothing) since FreeMarker 2.3.20. 
 * 
 * @deprecated Use {@link Collections#EMPTY_MAP} on J2SE 1.3 or later.   
 */
public class EmptyMap implements Map, Cloneable {
    public static final EmptyMap instance = new EmptyMap(); 
    
    public void clear() {
        // no op
    }
    
    public boolean containsKey(Object arg0) {
        return false;
    }
    
    public boolean containsValue(Object arg0) {
        return false;
    }
    
    public Set entrySet() {
        return Collections.EMPTY_SET;
    }
    
    public Object get(Object arg0) {
        return null;
    }
    
    public boolean isEmpty() {
        return true;
    }
    
    public Set keySet() {
        return Collections.EMPTY_SET;
    }
    
    public Object put(Object arg0, Object arg1) {
        throw new UnsupportedOperationException("This Map is read-only.");
    }
    
    public void putAll(Map arg0) {
        // Checking for arg0.isEmpty() wouldn't reflect precisely how putAll in
        // AbstractMap works. 
        if (arg0.entrySet().iterator().hasNext()) {
            throw new UnsupportedOperationException("This Map is read-only.");
        }
    }
    
    public Object remove(Object arg0) {
        return null;
    }
    
    public int size() {
        return 0;
    }
    
    public Collection values() {
        return Collections.EMPTY_LIST;
    }

}