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Google API Client Library for Java. Supports Java 5 (or higher) desktop (SE) and web (EE), Android, and Google App Engine.
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/*
 * Copyright (c) 2010 Google Inc.
 * 
 * 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 com.google.api.client.util;

import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.HashSet;
import java.util.IdentityHashMap;
import java.util.Map;
import java.util.TreeMap;
import java.util.TreeSet;
import java.util.WeakHashMap;

/**
 * Parses class information to determine data key name/value pairs associated
 * with the class.
 * 
 * @since 1.0
 * @author Yaniv Inbar
 */
public final class ClassInfo {
  private static final ThreadLocal, ClassInfo>> CACHE =
      new ThreadLocal, ClassInfo>>() {
        @Override
        protected WeakHashMap, ClassInfo> initialValue() {
          return new WeakHashMap, ClassInfo>();
        }
      };

  /** Class. */
  public final Class clazz;

  /** Map from data key name to its field information or {@code null} for none. */
  private final IdentityHashMap keyNameToFieldInfoMap;

  /**
   * Returns the class information for the given class.
   * 
   * @param clazz class or {@code null} for {@code null} result
   * @return class information or {@code null} for {@code null} input
   */
  public static ClassInfo of(Class clazz) {
    if (clazz == null) {
      return null;
    }
    WeakHashMap, ClassInfo> cache = CACHE.get();
    ClassInfo classInfo = cache.get(clazz);
    if (classInfo == null) {
      classInfo = new ClassInfo(clazz);
      cache.put(clazz, classInfo);
    }
    return classInfo;
  }

  /**
   * Returns the information for the given data key name.
   * 
   * @param keyName data key name or {@code null} for {@code null} result
   * @return field information or {@code null} for none or for {@code null}
   *         input
   */
  public FieldInfo getFieldInfo(String keyName) {
    if (keyName == null) {
      return null;
    }
    IdentityHashMap keyNameToFieldInfoMap =
        this.keyNameToFieldInfoMap;
    if (keyNameToFieldInfoMap == null) {
      return null;
    }
    return keyNameToFieldInfoMap.get(keyName.intern());
  }

  /**
   * Returns the field for the given data key name.
   * 
   * @param keyName data key name or {@code null} for {@code null} result
   * @return field or {@code null} for none or for {@code null} input
   */
  public Field getField(String keyName) {
    FieldInfo fieldInfo = getFieldInfo(keyName);
    return fieldInfo == null ? null : fieldInfo.field;
  }

  /**
   * Returns the number of data key name/value pairs associated with this data
   * class.
   */
  public int getKeyCount() {
    IdentityHashMap keyNameToFieldInfoMap =
        this.keyNameToFieldInfoMap;
    if (keyNameToFieldInfoMap == null) {
      return 0;
    }
    return keyNameToFieldInfoMap.size();
  }

  /** Returns the data key names associated with this data class. */
  public Collection getKeyNames() {
    IdentityHashMap keyNameToFieldInfoMap =
        this.keyNameToFieldInfoMap;
    if (keyNameToFieldInfoMap == null) {
      return Collections.emptySet();
    }
    return Collections.unmodifiableSet(keyNameToFieldInfoMap.keySet());
  }

  /** Creates a new instance of the given class using reflection. */
  public static  T newInstance(Class clazz) {
    T newInstance;
    try {
      newInstance = clazz.newInstance();
    } catch (IllegalAccessException e) {
      throw handleExceptionForNewInstance(e, clazz);
    } catch (InstantiationException e) {
      throw handleExceptionForNewInstance(e, clazz);
    }
    return newInstance;
  }

  private static IllegalArgumentException handleExceptionForNewInstance(
      Exception e, Class clazz) {
    StringBuilder buf =
        new StringBuilder("unable to create new instance of class ")
            .append(clazz.getName());
    if (Modifier.isAbstract(clazz.getModifiers())) {
      buf.append(" (and) because it is abstract");
    }
    if (clazz.getEnclosingClass() != null
        && !Modifier.isStatic(clazz.getModifiers())) {
      buf.append(" (and) because it is not static");
    }
    if (!Modifier.isPublic(clazz.getModifiers())) {
      buf.append(" (and) because it is not public");
    } else {
      try {
        clazz.getConstructor();
      } catch (NoSuchMethodException e1) {
        buf.append(" (and) because it has no public default constructor");
      }
    }
    throw new IllegalArgumentException(buf.toString(), e);
  }

  /**
   * Returns a new instance of the given collection class.
   * 
   * If a concrete collection class in the The class of the returned collection
   * instance depends on the input collection class as follows (first that
   * matches):
   * 

   * {@code null} or {@link ArrayList} is an instance of the collection
   * class: returns an {@link ArrayList}
   * Concrete subclass of {@link Collection}: returns an instance of that
   * collection class
   * {@link HashSet} is an instance of the collection class: returns a
   * {@link HashSet}
   * {@link TreeSet} is an instance of the collection class: returns a
   * {@link TreeSet}
   * 
   * 
   * @param collectionClass collection class or {@code null} for
   *        {@link ArrayList}.
   * @return new collection instance
   */
  public static Collection