Android性能优化之使用SparseArray代替HashMap_android 为什么view用sparsearray

Use new SparseArray<View> (...) instead for better performance

* Copyright (C) 2006 The Android Open Source Project
 *
 * 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 android.util;
 
import com.android.internal.util.ArrayUtils;
 
/**
 * SparseArrays map integers to Objects.  Unlike a normal array of Objects,
 * there can be gaps in the indices.  It is intended to be more efficient
 * than using a HashMap to map Integers to Objects.
 */
public class SparseArray<E> implements Cloneable {
    private static final Object DELETED = new Object();
    private boolean mGarbage = false;
 
    private int[] mKeys;
    private Object[] mValues;
    private int mSize;
 
    /**
     * Creates a new SparseArray containing no mappings.
     */
    public SparseArray() {
        this(10);
    }
 
    /**
     * Creates a new SparseArray containing no mappings that will not
     * require any additional memory allocation to store the specified
     * number of mappings.
     */
    public SparseArray(int initialCapacity) {
        initialCapacity = ArrayUtils.idealIntArraySize(initialCapacity);
        mKeys = new int[initialCapacity];
        mValues = new Object[initialCapacity];
        mSize = 0;
    }
    @Override
    @SuppressWarnings("unchecked")
    public SparseArray<E> clone() {
        SparseArray<E> clone = null;
        try {
            clone = (SparseArray<E>) super.clone();
            clone.mKeys = mKeys.clone();
            clone.mValues = mValues.clone();
        } catch (CloneNotSupportedException cnse) {
            /* ignore */
        }
        return clone;
    }
    /**
     * Gets the Object mapped from the specified key, or <code>null</code>
     * if no such mapping has been made.
     */
    public E get(int key) {
        return get(key, null);
    }
 
    /**
     * Gets the Object mapped from the specified key, or the specified Object
     * if no such mapping has been made.
     */
    @SuppressWarnings("unchecked")
    public E get(int key, E valueIfKeyNotFound) {
        int i = binarySearch(mKeys, 0, mSize, key);
        if (i < 0 || mValues[i] == DELETED) {
            return valueIfKeyNotFound;
        } else {
            return (E) mValues[i];
        }
    }
    /**
     * Removes the mapping from the specified key, if there was any.
     */
    public void delete(int key) {
        int i = binarySearch(mKeys, 0, mSize, key);
        if (i >= 0) {
            if (mValues[i] != DELETED) {
                mValues[i] = DELETED;
                mGarbage = true;
            }
        }
    }
    /**
     * Alias for {@link #delete(int)}.
     */
    public void remove(int key) {
        delete(key);
    }
    /**
     * Removes the mapping at the specified index.
     */
    public void removeAt(int index) {
        if (mValues[index] != DELETED) {
            mValues[index] = DELETED;
            mGarbage = true;
        }
    }
    ...

public SparseArray() {
    this(10);
}
 
public SparseArray(int initialCapacity) {
    initialCapacity = ArrayUtils.idealIntArraySize(initialCapacity);
 
    mKeys = new int[initialCapacity];
    mValues = new Object[initialCapacity];
    mSize = 0;
}

public void put(int key, E value) {}
public void append(int key, E value){}

public void put(int key, E value) {
    int i = binarySearch(mKeys, 0, mSize, key);
 
    if (i >= 0) {
        mValues[i] = value;
    } else {
        i = ~i;
 
        if (i < mSize && mValues[i] == DELETED) {
            mKeys[i] = key;
            mValues[i] = value;
            return;
        }
 
        if (mGarbage && mSize >= mKeys.length) {
            gc();
 
            // Search again because indices may have changed.
            i = ~binarySearch(mKeys, 0, mSize, key);
        }
 
        if (mSize >= mKeys.length) {
            int n = ArrayUtils.idealIntArraySize(mSize + 1);
 
            int[] nkeys = new int[n];
            Object[] nvalues = new Object[n];
 
            // Log.e("SparseArray", "grow " + mKeys.length + " to " + n);
            System.arraycopy(mKeys, 0, nkeys, 0, mKeys.length);
            System.arraycopy(mValues, 0, nvalues, 0, mValues.length);
 
            mKeys = nkeys;
            mValues = nvalues;
        }
 
        if (mSize - i != 0) {
            // Log.e("SparseArray", "move " + (mSize - i));
            System.arraycopy(mKeys, i, mKeys, i + 1, mSize - i);
            System.arraycopy(mValues, i, mValues, i + 1, mSize - i);
        }
 
        mKeys[i] = key;
        mValues[i] = value;
    
    }
}

public void put(int key, E value)
public void setValueAt(int index, E value)

public E get(int key)
public E get(int key, E valueIfKeyNotFound)

public int keyAt(int index)

public E valueAt(int index)

public int indexOfValue(E value)

private static int binarySearch(int[] a, int start, int len, int key) {
    int high = start + len, low = start - 1, guess;
 
    while (high - low > 1) {
        guess = (high + low) / 2;
 
        if (a[guess] < key)
            low = guess;
        else
            high = guess;
    }
 
    if (high == start + len)
        return ~(start + len);
    else if (a[high] == key)
        return high;
    else
        return ~high;
}
 
相应的也有SparseBooleanArray，用来取代HashMap<Integer, Boolean>，SparseIntArray用来取代HashMap<Integer, Integer>，大家有兴趣的可以研究。

SparseArray是android里为<Interger,Object>这样的Hashmap而专门写的类,目的是提高效率，其核心是折半查找函数（binarySearch）。在Android中，当我们需要定义
    HashMap<Integer, E> hashMap = new HashMap<Integer, E>();
时，我们可以使用如下的方式来取得更好的性能.
    SparseArray<E> sparseArray = new SparseArray<E>();

public class ActivityGroupManager {
    static final String TAG = ActivityGroupManager.class.getName();
 
    private SparseArray<View> array;
    private ViewGroup container;
 
    public ActivityGroupManager() {
    	array = new SparseArray<View>();
    }
 
    public void setContainer(ViewGroup container) {
    	this.container = container;
    }
 
    public void showContainer(int num, View view) {
    	if (array.get(num) == null) {
    		array.put(num, view);
    		container.addView(view);
    	}
    	for (int i = 0; i < array.size(); i++) {
    		View v = array.valueAt(i);
    		v.setVisibility(View.GONE);
    	}
    	view.setVisibility(View.VISIBLE);
    }
}