这本篇博客里面我想重点来分析一下ContentValues的源码以及它里面涉及到的继承接口Parcelabel,还有HashMap的源码。

相信使用过android里面数据库操作的朋友对于ContentValues一定不会感到陌生吧,它其实很像一个字典对象,可以用来存储键值对。比如代码如下:

ContentValues contentValues=new ContentValues();
contentValues.put("name","xiao");
contentValues.put("age",20);
contentValues.put("isStudent",true);

你会发现ContentValues里面可以用来put各种类型的数据,它是怎样拥有这种神奇的功能的呢?下面让我们来看看它的源码。首先,是ContentValues类的定义:

public final class ContentValues implements Parcelable {
}

我们可以看到它实现了Parcelabel接口,这个接口主要是用来实现数据安装、传输相关操作的。说到这里,让我们也来看看Parcelabel接口里面到底定义了哪些方法,源码如下:

public interface Parcelable {

    public static final int PARCELABLE_WRITE_RETURN_VALUE = 0x0001;

    public static final int CONTENTS_FILE_DESCRIPTOR = 0x0001;

    public int describeContents();

    public void writeToParcel(Parcel dest, int flags);

    public interface Creator<T> {

        public T createFromParcel(Parcel source);

        public T[] newArray(int size);
} public interface ClassLoaderCreator<T> extends Creator<T> { public T createFromParcel(Parcel source, ClassLoader loader);
}
}

我们可以看到里面有个writeToParcel方法是用来传输数据的,至于它是怎么用来包装数据的,就要看看具体实现Parcelabel接口类的实现了。

好了说回我们所要讨论的重点对象ContentValues,首先来看看ContentValues里面包括的构造函数,源码如下所示:

    private HashMap<String, Object> mValues;

    public ContentValues() {
// Choosing a default size of 8 based on analysis of typical
// consumption by applications.
mValues = new HashMap<String, Object>(8);
} /**
* Creates an empty set of values using the given initial size
*
* @param size the initial size of the set of values
*/
public ContentValues(int size) {
mValues = new HashMap<String, Object>(size, 1.0f);
} /**
* Creates a set of values copied from the given set
*
* @param from the values to copy
*/
public ContentValues(ContentValues from) {
mValues = new HashMap<String, Object>(from.mValues);
} /**
* Creates a set of values copied from the given HashMap. This is used
* by the Parcel unmarshalling code.
*
* @param values the values to start with
* {@hide}
*/
private ContentValues(HashMap<String, Object> values) {
mValues = values;
}

相信大家从注释里面就能够看看,ContentValues的构造主要是根据代码里面传入的具体参数来构造对应的HashMap对象,然后里面的各种put操作、get操作、remove操作都是针对HashMap进行的,其中put类型的方法源码如下:

    public void put(String key, String value) {
mValues.put(key, value);
} public void putAll(ContentValues other) {
mValues.putAll(other.mValues);
} public void put(String key, Byte value) {
mValues.put(key, value);
} public void put(String key, Short value) {
mValues.put(key, value);
} public void put(String key, Integer value) {
mValues.put(key, value);
} public void put(String key, Long value) {
mValues.put(key, value);
} public void put(String key, Float value) {
mValues.put(key, value);
} public void put(String key, Double value) {
mValues.put(key, value);
} public void put(String key, Boolean value) {
mValues.put(key, value);
} public void put(String key, byte[] value) {
mValues.put(key, value);
} public void putNull(String key) {
mValues.put(key, null);
}

通过上面的方法,我们就能够明白为什么ContentValues能够put各种类型的数值了吧,接下来让我们来看看get方法,源码如下:

 public Object get(String key) {
return mValues.get(key);
} public String getAsString(String key) {
Object value = mValues.get(key);
return value != null ? value.toString() : null;
} public Long getAsLong(String key) {
Object value = mValues.get(key);
try {
return value != null ? ((Number) value).longValue() : null;
} catch (ClassCastException e) {
if (value instanceof CharSequence) {
try {
return Long.valueOf(value.toString());
} catch (NumberFormatException e2) {
Log.e(TAG, "Cannot parse Long value for " + value + " at key " + key);
return null;
}
} else {
Log.e(TAG, "Cannot cast value for " + key + " to a Long: " + value, e);
return null;
}
}
} public Integer getAsInteger(String key) {
Object value = mValues.get(key);
try {
return value != null ? ((Number) value).intValue() : null;
} catch (ClassCastException e) {
if (value instanceof CharSequence) {
try {
return Integer.valueOf(value.toString());
} catch (NumberFormatException e2) {
Log.e(TAG, "Cannot parse Integer value for " + value + " at key " + key);
return null;
}
} else {
Log.e(TAG, "Cannot cast value for " + key + " to a Integer: " + value, e);
return null;
}
}
} public Short getAsShort(String key) {
Object value = mValues.get(key);
try {
return value != null ? ((Number) value).shortValue() : null;
} catch (ClassCastException e) {
if (value instanceof CharSequence) {
try {
return Short.valueOf(value.toString());
} catch (NumberFormatException e2) {
Log.e(TAG, "Cannot parse Short value for " + value + " at key " + key);
return null;
}
} else {
Log.e(TAG, "Cannot cast value for " + key + " to a Short: " + value, e);
return null;
}
}
} public Byte getAsByte(String key) {
Object value = mValues.get(key);
try {
return value != null ? ((Number) value).byteValue() : null;
} catch (ClassCastException e) {
if (value instanceof CharSequence) {
try {
return Byte.valueOf(value.toString());
} catch (NumberFormatException e2) {
Log.e(TAG, "Cannot parse Byte value for " + value + " at key " + key);
return null;
}
} else {
Log.e(TAG, "Cannot cast value for " + key + " to a Byte: " + value, e);
return null;
}
}
} public Double getAsDouble(String key) {
Object value = mValues.get(key);
try {
return value != null ? ((Number) value).doubleValue() : null;
} catch (ClassCastException e) {
if (value instanceof CharSequence) {
try {
return Double.valueOf(value.toString());
} catch (NumberFormatException e2) {
Log.e(TAG, "Cannot parse Double value for " + value + " at key " + key);
return null;
}
} else {
Log.e(TAG, "Cannot cast value for " + key + " to a Double: " + value, e);
return null;
}
}
} public Float getAsFloat(String key) {
Object value = mValues.get(key);
try {
return value != null ? ((Number) value).floatValue() : null;
} catch (ClassCastException e) {
if (value instanceof CharSequence) {
try {
return Float.valueOf(value.toString());
} catch (NumberFormatException e2) {
Log.e(TAG, "Cannot parse Float value for " + value + " at key " + key);
return null;
}
} else {
Log.e(TAG, "Cannot cast value for " + key + " to a Float: " + value, e);
return null;
}
}
} public Boolean getAsBoolean(String key) {
Object value = mValues.get(key);
try {
return (Boolean) value;
} catch (ClassCastException e) {
if (value instanceof CharSequence) {
return Boolean.valueOf(value.toString());
} else if (value instanceof Number) {
return ((Number) value).intValue() != 0;
} else {
Log.e(TAG, "Cannot cast value for " + key + " to a Boolean: " + value, e);
return null;
}
}
} public byte[] getAsByteArray(String key) {
Object value = mValues.get(key);
if (value instanceof byte[]) {
return (byte[]) value;
} else {
return null;
}
}

通过上面的代码我们也能很直观的看到,不同的get方法通过调用不同类型的((Number)value).intValue方法强转一次获取,如果拿不到的话就返回null。

既然ContentValues是基于HashMap去实现操作的,那么我们有必要来看看HashMap到底是怎么回事?首先是HashMap类定义,源码如下所示:

public class HashMap<K, V> extends AbstractMap<K, V> implements Cloneable, Serializable{
}

通过上面的代码,我们可以看到HashMap是基于泛型去构建的,同时实现了克隆和序列化接口。这就意味着在一定程度上面,我们可以实例化任何类型的HashMap,并且使它具有克隆、序列化的功能,请看如下代码:

HashMap<Integer,Object> hashOne=new HashMap<>();
HashMap<String,Object> hashTwo=new HashMap<>();
HashMap<Boolean,Object> hashThree=new HashMap<>();
HashMap<Float,Object> hashFour=new HashMap<>();

只不过我们通常在项目里面一般都习惯使用String类型的key。好了,让我们继续往下看,首先最应该说的就是HashMapEntry内部静态类了,源码如下:

static class HashMapEntry<K, V> implements Entry<K, V> {
final K key;
V value;
final int hash;
HashMapEntry<K, V> next; HashMapEntry(K key, V value, int hash, HashMapEntry<K, V> next) {
this.key = key;
this.value = value;
this.hash = hash;
this.next = next;
} public final K getKey() {
return key;
} public final V getValue() {
return value;
} public final V setValue(V value) {
V oldValue = this.value;
this.value = value;
return oldValue;
} @Override public final boolean equals(Object o) {
if (!(o instanceof Entry)) {
return false;
}
Entry<?, ?> e = (Entry<?, ?>) o;
return Objects.equal(e.getKey(), key)
&& Objects.equal(e.getValue(), value);
} @Override public final int hashCode() {
return (key == null ? 0 : key.hashCode()) ^
(value == null ? 0 : value.hashCode());
} @Override public final String toString() {
return key + "=" + value;
}
}

HashMapEntry类实现了Entry接口,而Entry接口又是Map接口里面的一个内部接口。通过实现Entry接口,从而使HashMap具有了getKey/getValue/setValue等相关功能。同时我们可以看到HashMap里面好多功能的实现都是针对HashMapEntry展开的。另外HashMap还有个比较重要的概念就是Set接口,让我们来看看里面final类型的私有内部类EntrySet,源码如下:

private final class EntrySet extends AbstractSet<Entry<K, V>> {
public Iterator<Entry<K, V>> iterator() {
return newEntryIterator();
}
public boolean contains(Object o) {
if (!(o instanceof Entry))
return false;
Entry<?, ?> e = (Entry<?, ?>) o;
return containsMapping(e.getKey(), e.getValue());
}
public boolean remove(Object o) {
if (!(o instanceof Entry))
return false;
Entry<?, ?> e = (Entry<?, ?>)o;
return removeMapping(e.getKey(), e.getValue());
}
public int size() {
return size;
}
public boolean isEmpty() {
return size == 0;
}
public void clear() {
HashMap.this.clear();
}
}

正如其名一样,Set接口里面主要是提供HashMap的设置相关操作。让我们来看看Set接口里面的源码,如下:

    public boolean add(E object);

    public boolean addAll(Collection<? extends E> collection);

    public void clear();

    public boolean contains(Object object);

    public boolean containsAll(Collection<?> collection);

    public boolean equals(Object object);

    public int hashCode();

    public boolean isEmpty();

    public Iterator<E> iterator();

    public boolean remove(Object object);

    public boolean removeAll(Collection<?> collection);

    public boolean retainAll(Collection<?> collection);

    public int size();

    public Object[] toArray();

    public <T> T[] toArray(T[] array);

好了,今天博客就到这里。技术有限,如有不对欢迎拍砖!

05-07 15:18