/*
 * Copyright 2002-2018 the original author or authors.
 *
 * 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
 *
 *      https://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.springframework.core.env;

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

import org.springframework.lang.Nullable;

Read-only Map<String, String> implementation that is backed by system properties or environment variables. 
Used by AbstractEnvironment when a SecurityManager prohibits access to System.getProperties() or System.getenv(). It is for this reason that the implementations of keySet(), entrySet(), and values() always return empty even though get(Object) may in fact return non-null if the current security manager allows access to individual keys. 
Author:
Arjen Poutsma, Chris Beams
Since:
3.0/**
 * Read-only {@code Map<String, String>} implementation that is backed by system
 * properties or environment variables.
 *
 * <p>Used by {@link AbstractEnvironment} when a {@link SecurityManager} prohibits
 * access to {@link System#getProperties()} or {@link System#getenv()}. It is for this
 * reason that the implementations of {@link #keySet()}, {@link #entrySet()}, and
 * {@link #values()} always return empty even though {@link #get(Object)} may in fact
 * return non-null if the current security manager allows access to individual keys.
 *
 * @author Arjen Poutsma
 * @author Chris Beams
 * @since 3.0
 */
abstract class ReadOnlySystemAttributesMap implements Map<String, String> {

	@Override
	public boolean containsKey(Object key) {
		return (get(key) != null);
	}

	
Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key. 
Params:
key – the name of the system attribute to retrieve
Throws:
IllegalArgumentException – if given key is non-String/**
	 * Returns the value to which the specified key is mapped, or {@code null} if this map
 	 * contains no mapping for the key.
	 * @param key the name of the system attribute to retrieve
	 * @throws IllegalArgumentException if given key is non-String
	 */
	@Override
	@Nullable
	public String get(Object key) {
		if (!(key instanceof String)) {
			throw new IllegalArgumentException(
					"Type of key [" + key.getClass().getName() + "] must be java.lang.String");
		}
		return getSystemAttribute((String) key);
	}

	@Override
	public boolean isEmpty() {
		return false;
	}

	
Template method that returns the underlying system attribute.
Implementations typically call System.getProperty(String) or System.getenv(String) here. /**
	 * Template method that returns the underlying system attribute.
	 * <p>Implementations typically call {@link System#getProperty(String)} or {@link System#getenv(String)} here.
	 */
	@Nullable
	protected abstract String getSystemAttribute(String attributeName);


	// Unsupported

	@Override
	public int size() {
		throw new UnsupportedOperationException();
	}

	@Override
	public String put(String key, String value) {
		throw new UnsupportedOperationException();
	}

	@Override
	public boolean containsValue(Object value) {
		throw new UnsupportedOperationException();
	}

	@Override
	public String remove(Object key) {
		throw new UnsupportedOperationException();
	}

	@Override
	public void clear() {
		throw new UnsupportedOperationException();
	}

	@Override
	public Set<String> keySet() {
		return Collections.emptySet();
	}

	@Override
	public void putAll(Map<? extends String, ? extends String> map) {
		throw new UnsupportedOperationException();
	}

	@Override
	public Collection<String> values() {
		return Collections.emptySet();
	}

	@Override
	public Set<Entry<String, String>> entrySet() {
		return Collections.emptySet();
	}

}