获取全局唯一的整形常量，可以用于 requestCode 的定义，防止其在不同文件中定义时值的碰撞。

RequestCodeGenerator.java

public class RequestCode {
    private static final AtomicInteger seed = new AtomicInteger();
    public static int get() {
        return seed.incrementAndGet();
    }
}

Usage.java

private static final int REQUEST_CODE = RequestCode.get();
public void doSomething() {
    startActivityForResult(intentA, REQUEST_CODE);
}