android/util中的一些常用类

1.SparseArrays

sparseArrays是映射Integer To Objects，可参照HashMap的作用.它的目的是更省内存,内存效率更高，因为它避免了keys的自动装箱，而且它的结构也不用依赖每一个entry-set。它内部是用二分查找去查询，所以不适合key很多的情况，大概在几百个元素的情况下，比hashmap的速度慢50%以下。

类似的有SparseBooleanArray,SparseIntArray,SparseLongArray,分别是映射Integer To Boolean,Int Long.

2.ArraySet ArrayMap

与上相同，数目不大情况下代替hashmap

3.AtomicFile

就是在写文件的同时本地保存了一份copy，如果写失败可以从备份文件恢复。

4.AttributeSet(interface),XmlPullAttributes(class),BridgeXmlPullAttributes(class)

AttributeSet只是一个解析xml的接口，子类Resources.Theme.obtainStyledAttributes()；

也可以

XmlPullParser parser = resources.getXml(myResouce);
 AttributeSet attributes = Xml.asAttributeSet(parser);

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5.Base64,Base64InputStream,Base64OutputStream

3个8位字节(3*8=24)转化为4个6位的字节(4*6=24),之后在6位的前面补两个0,形成8位一个字节的形式.适用于网络传输，加密解密.

6.ContainerHelpers

ArrayMap等中使用的二分查找就是这货的ContainerHelpers.binarySearch,没有其他作用了

7.DisplayMetrics

用于获取屏幕像素密度，low=120,mid=160,high=240...

DisplayMetrics metrics = new DisplayMetrics();
getWindowManager().getDefaultDisplay().getMetrics(metrics);

8.FastImmutableArraySet

重复利用iterator，如果要遍历数百次，效率是hashset的2.5倍。

9.FoatMath

基本不用了（效率低），类似Math的一些操作。

10.JsonReader,JsonWriter

json解析，用法

  public List<Message> readJsonStream(InputStream in) throws IOException {
      JsonReader reader = new JsonReader(new InputStreamReader(in, "UTF-8"));
     try {
        return readMessagesArray(reader);
      } finally {
        reader.close();
      }
    }
 
    public List<Message> readMessagesArray(JsonReader reader) throws IOException {
      List<Message> messages = new ArrayList<Message>();
 
     reader.beginArray();
      while (reader.hasNext()) {
        messages.add(readMessage(reader));
      }
      reader.endArray();
      return messages;
    }
 
    public Message readMessage(JsonReader reader) throws IOException {
      long id = -1;
     String text = null;
      User user = null;
      List<Double> geo = null;
 
      reader.beginObject();
      while (reader.hasNext()) {
        String name = reader.nextName();
        if (name.equals("id")) {
          id = reader.nextLong();
        } else if (name.equals("text")) {
          text = reader.nextString();
        } else if (name.equals("geo") && reader.peek() != JsonToken.NULL) {
          geo = readDoublesArray(reader);
        } else if (name.equals("user")) {
          user = readUser(reader);
        } else {
          reader.skipValue();
        }
      }
      reader.endObject();
      return new Message(id, text, user, geo);
    }
 
    public List<Double> readDoublesArray(JsonReader reader) throws IOException {
      List<Double> doubles = new ArrayList<Double>();
 
      reader.beginArray();
      while (reader.hasNext()) {
        doubles.add(reader.nextDouble());
      }
      reader.endArray();
      return doubles;
    }
 
    public User readUser(JsonReader reader) throws IOException {
      String username = null;
      int followersCount = -1;
 
      reader.beginObject();
      while (reader.hasNext()) {
        String name = reader.nextName();
        if (name.equals("name")) {
          username = reader.nextString();
        } else if (name.equals("followers_count")) {
          followersCount = reader.nextInt();
        } else {
          reader.skipValue();
        }
      }
      reader.endObject();
      return new User(username, followersCount);
    }

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*   public void writeJsonStream(OutputStream out, List<Message> messages) throws IOException {
 *     JsonWriter writer = new JsonWriter(new OutputStreamWriter(out, "UTF-8"));
 *     writer.setIndent("  ");
 *     writeMessagesArray(writer, messages);
 *     writer.close();
 *   }
 *
 *   public void writeMessagesArray(JsonWriter writer, List<Message> messages) throws IOException {
 *     writer.beginArray();
 *     for (Message message : messages) {
 *       writeMessage(writer, message);
 *     }
 *     writer.endArray();
 *   }
 *
 *   public void writeMessage(JsonWriter writer, Message message) throws IOException {
 *     writer.beginObject();
 *     writer.name("id").value(message.getId());
 *     writer.name("text").value(message.getText());
 *     if (message.getGeo() != null) {
 *       writer.name("geo");
 *       writeDoublesArray(writer, message.getGeo());
 *     } else {
 *       writer.name("geo").nullValue();
 *     }
 *     writer.name("user");
 *     writeUser(writer, message.getUser());
 *     writer.endObject();
 *   }
 *
 *   public void writeUser(JsonWriter writer, User user) throws IOException {
 *     writer.beginObject();
 *     writer.name("name").value(user.getName());
 *     writer.name("followers_count").value(user.getFollowersCount());
 *     writer.endObject();
 *   }
 *
 *   public void writeDoublesArray(JsonWriter writer, List<Double> doubles) throws IOException {
 *     writer.beginArray();
 *     for (Double value : doubles) {
 *       writer.value(value);
 *     }
 *     writer.endArray();
 *   }

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11.LruCache

用的是LinkedHashMap<K,V>,使用强引用，最近使用的元素移到队列的最前，所以不经常使用的元素被移出。由于弱引用回收时间不确定，LruCache更好的作为图片缓存.

 *   int cacheSize = 4 * 1024 * 1024; // 4MiB
 *   LruCache<String, Bitmap> bitmapCache = new LruCache<String, Bitmap>(cacheSize) {
 *       protected int sizeOf(String key, Bitmap value) {
 *           return value.getByteCount();
 *       }
 *   }}

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12.Patterns

写了常用的Ip，domain，email等的正则表达式

13.TimeUtils

获取时区

14.xml

xml解析