Error:
Caused by: android.support.v4.app.Fragment$InstantiationException: Unable to instantiate fragment xxx: make sure class name exists, is public, and has an empty constructor that is public
原因：
当切换到其他应用时，会调用FragmentActivity的onSaveInstanceState方法，再次进入，Activity会重新创建，onCreate方法中会拿到savedInstanceState并re-instantiate。具体参考下源码：
public static Fragment instantiate(Context context, String fname, @Nullable Bundle args) {
try {
Class<?> clazz = sClassMap.get(fname);
if (clazz == null) {
// Class not found in the cache, see if it's real, and try to add it
clazz = context.getClassLoader().loadClass(fname);
if (!Fragment.class.isAssignableFrom(clazz)) {
throw new InstantiationException("Trying to instantiate a class " + fname
+ " that is not a Fragment", new ClassCastException());
}
sClassMap.put(fname, clazz);
}
Fragment f = (Fragment) clazz.getConstructor().newInstance();
if (args != null) {
args.setClassLoader(f.getClass().getClassLoader());
f.setArguments(args);
}
return f;
} catch (ClassNotFoundException e) {
throw new InstantiationException("Unable to instantiate fragment " + fname
+ ": make sure class name exists, is public, and has an"
+ " empty constructor that is public", e);
} catch (java.lang.InstantiationException e) {
throw new InstantiationException("Unable to instantiate fragment " + fname
+ ": make sure class name exists, is public, and has an"
+ " empty constructor that is public", e);
} catch (IllegalAccessException e) {
throw new InstantiationException("Unable to instantiate fragment " + fname
+ ": make sure class name exists, is public, and has an"
+ " empty constructor that is public", e);
} catch (NoSuchMethodException e) {
throw new InstantiationException("Unable to instantiate fragment " + fname
+ ": could not find Fragment constructor", e);
} catch (InvocationTargetException e) {
throw new InstantiationException("Unable to instantiate fragment " + fname
+ ": calling Fragment constructor caused an exception", e);
}
}
Fragment f = (Fragment) clazz.getConstructor().newInstance();
看到这行代码原因已经非常明显了，重新构建Fragment时，反射调用了Fragment的无参构造方法，找不到无参构造方法就创建失败了。
解决方法：
给只有有参构造方法的Fragment添加一个无参构造方法。