java.match.BigInteger 可以解决范围溢出的问题,但是开辟内存空间更大
android的FloatMath 更高效率的解决双精度的运算
当键是整数时,sparseArray比hashMap更加高效 理由;HashMap使用的是泛型,这时需要Integer 而SparseArray使用的是基本类型Int,不会创建很多Integer对象，因此SpareArray更省内存 在这里强调一点，以为java很拿手的同学,android一般都很烂,自信没问题,但是忽略的内存
实例:
new Thread(new Runnable() {
@Override
public void run() {
int loopTimes = 1000;
long addStartTime1 = SystemClock.elapsedRealtime();
for (int i = 0; i < loopTimes; i++) {
testMap.put(i, i);
}
long addEndTime1 = SystemClock.elapsedRealtime();
LogUtils.d("------->hashMap add " + loopTimes + " items take time:" + (addEndTime1 - addStartTime1));
long addStartTime2 = SystemClock.elapsedRealtime();
for (int i = 0; i < loopTimes; i++) {
testArray.put(i, i);
}
long addEndTime2 = SystemClock.elapsedRealtime();
LogUtils.d("------->sparseIntArray add " + loopTimes + " items take time:" + (addEndTime2 - addStartTime2));
}
}).start();
运行5次的结果:
09-06 02:05:16.152 3845-4010/com.xuan.study_project D/LogUtils: ------->hashMap add 1000 items take time:16
09-06 02:05:16.154 3845-4010/com.xuan.study_project D/LogUtils: ------->sparseIntArray add 1000 items take time:2
09-06 02:05:30.976 3845-4208/com.xuan.study_project D/LogUtils: ------->hashMap add 1000 items take time:1
09-06 02:05:30.977 3845-4208/com.xuan.study_project D/LogUtils: ------->sparseIntArray add 1000 items take time:0
09-06 02:05:32.248 3845-4226/com.xuan.study_project D/LogUtils: ------->hashMap add 1000 items take time:0
09-06 02:05:32.251 3845-4226/com.xuan.study_project D/LogUtils: ------->sparseIntArray add 1000 items take time:2
09-06 02:05:33.478 3845-4244/com.xuan.study_project D/LogUtils: ------->hashMap add 1000 items take time:2
09-06 02:05:33.479 3845-4244/com.xuan.study_project D/LogUtils: ------->sparseIntArray add 1000 items take time:0
09-06 02:05:40.726 3845-4341/com.xuan.study_project D/LogUtils: ------->hashMap add 1000 items take time:2
09-06 02:05:40.726 3845-4341/com.xuan.study_project D/LogUtils: ------->sparseIntArray add 1000 items take time:1
结论:SparseIntArray的性能比HashMap好,java程序员所熟悉的hashMap 大多数情况在android平台不适合
在android中定义了很多这样的稀疏数组：SparseArray(键为整数,值为对象) SparseBoolArray和SparseIntArray,非常快速和省内存
LruCache
API等级判断
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) { itme_name_3.setNestedScrollingEnabled(false); item_more_3.setNestedScrollingEnabled(false);}
Exception
Exception 通常不期望能够恢复的,如果并非所有得分异常都是Exception的子类，但所有异常都是Throwable的子类
strictMode 检查
strictMode 是android系统提供的不良行为检测工具
用法 在baseApplication中添加如下代码:
StrictMode.setThreadPolicy( new StrictMode.ThreadPolicy.Builder() .detectCustomSlowCalls() .detectDiskReads() .detectDiskWrites() .detectNetwork() .penaltyLog() .penaltyFlashScreen() .build() ); try { //其实和性能无关,但如果使用strictMode 最好也定义一下VM策略 StrictMode.setVmPolicy( new StrictMode.VmPolicy.Builder() .detectLeakedSqlLiteObjects() .detectLeakedClosableObjects() .setClassInstanceLimit(Class.forName("com.apress.proandroid.SomeClass"), 100) .penaltyLog() .build()); } catch (ClassNotFoundException e) { e.printStackTrace(); }
运行程序:
String.format与StringBuilder 性能对比
在得到一个拼凑字符串的方式有很多,如++,format StringBuilder,接下来演示其速度
public static class User { public int id; public String name; public int age; public User(String name, int age, int id) { this.name = name; this.age = age; this.id = id; } }new Thread(new Runnable() { @Override public void run() { int loopTimes = 1000; //先模拟1000条数据 List<User> users = new ArrayList<User>(); for (int i = 0; i < loopTimes; i++) { users.add(new User("name_" + i, new Random().nextInt(100), i)); } long startTime1 = SystemClock.elapsedRealtime(); for (int i = 0; i < users.size(); i++) { User user = users.get(i); String userItem = String.format("id:%d name:%s age:%d", user.id, user.name, user.age); } long endTime1 = SystemClock.elapsedRealtime(); LogUtils.d("------->format: " + loopTimes + " items take time:" + (endTime1 - startTime1)); long startTime2 = SystemClock.elapsedRealtime(); for (int i = 0; i < users.size(); i++) { User user = users.get(i); StringBuilder stringBuilder = new StringBuilder(); stringBuilder.append("id"); stringBuilder.append(user.id); stringBuilder.append("name"); stringBuilder.append(user.name); stringBuilder.append("age"); stringBuilder.append(user.age); String userItem = stringBuilder.toString(); } long endTime2 = SystemClock.elapsedRealtime(); LogUtils.d("------->StringBuilder: " + loopTimes + " items take time:" + (endTime2 - startTime2)); } }).start();运行结果:
09-06 04:51:34.420 20101-20216/com.xuan.study_project D/LogUtils: ------->format: 1000 items take time:48
09-06 04:51:34.435 20101-20216/com.xuan.study_project D/LogUtils: ------->StringBuilder: 1000 items take time:15
09-06 04:51:35.356 20101-20228/com.xuan.study_project D/LogUtils: ------->format: 1000 items take time:43
09-06 04:51:35.358 20101-20228/com.xuan.study_project D/LogUtils: ------->StringBuilder: 1000 items take time:2
09-06 04:51:36.057 20101-20240/com.xuan.study_project D/LogUtils: ------->format: 1000 items take time:40
09-06 04:51:36.059 20101-20240/com.xuan.study_project D/LogUtils: ------->StringBuilder: 1000 items take time:1
09-06 04:51:36.678 20101-20252/com.xuan.study_project D/LogUtils: ------->format: 1000 items take time:44
09-06 04:51:36.680 20101-20252/com.xuan.study_project D/LogUtils: ------->StringBuilder: 1000 items take time:2
09-06 04:51:37.345 20101-20265/com.xuan.study_project D/LogUtils: ------->format: 1000 items take time:84
09-06 04:51:37.346 20101-20265/com.xuan.study_project D/LogUtils: ------->StringBuilder: 1000 items take time:1
可以看到stringBuilder的性能远比format 主要场景有 sqlite 如
insert into user values(%d,$s,%d)
建议用stringBuilder