2017/02/05

Why written nested classes in Android

About

When I am working on my android project, I am quite curious about why templates from Android Studio prefers Nested classes design. After digging deeper in this issue, I believe this is worth for me to write a blog to record them down.

What is a `Nested classes`?

For detail please read this tutorial and this answer. Basically, Nested classes are classes defined inside of another class. For example,

class OuterClass {
    ...
    class InnerClass {
        ...
    }
}

InnerClass is a kind of Nest classes.

There are 4 types of inner classes in general:

static class: declared as a static member of another class
inner class: declared as an instance member of another class
local inner class: declared inside an instance method of another class
anonymous inner class: like a local inner class, but written as an expression which returns a one-off object

Why using `nested classes`?

According to the tutorial, pros for using nested classes:

Able to access container’s private attributes, methods for non static inner class
Saves some typing in certain cases for anonymous inner class like click handlers
Better code organizations for related classes
Enhance performances in certain cases like reusing identical handlers

According to this answer and this answer, cons for using nested classes:

Waste memories as there are implicit references from inner class to outer class
High potential causing memory leaks when using non static inner class

Idea of how memory is leaked

Objects are unable to be garbage collected if there are references to them
Inner class always holds a implicit reference to outer class
If there is a reference to the inner class object outside of the outer class,
- Inner class object is unable to be collected
- Outer class object is unable to be collected too as it is pointed by the inner class object
- This is a serious leak in android activities (outer class) as they have associated many resources like views and context etc

Words before end…

I strongly suggested you to go through the answers I pasted in this blog especially this one. The one I highlighted answering why and how memory leaks in nested class very well.

References:

2017/01/27

How to connect multiple NOX instances in Android studio

About

NOX is a great android emulator. So I am wondering how to replace the default emulator from Android studio with NOX.

How to create multiple nox instances

After installing the NOX, there should be an icon about Multi-drive.

So, double click it and you can add more than one NOX instances just like below

For demonstration (and that’s all I need), there are 2 NOX instances.

How to connect NOX instances

For each of your NOX instances, following tutorial here to enable the developer mode.

Keep instances running and head to the console to get the IP:PORT information of your instances by command nox_adb.exe devices just like below

If you don’t know what adb is, here is a link from google to know more about it.

Link up android studio and NOX instances

After knowing IP:PORT of your NOX instances, you can setup a link for android studio and NOX.

At first, andriod studio cannot detect them after clicking the start button in the red square:

Back to the console, and type command nox_adb.exe connect IP:PORT

Then, you should be able to see them from android studio

Happy coding :P

2017/01/19

How to do array initialization on python efficiently

Question

Suppose we would like to create an array with 100 elements.

2 types of elements will be tested

None
integers from 0 to 99

Methods

3 ways to initialize an array

By append


def byAppend():
    a = []
    for i in xrange(100):
        a.append(None)

def byAppendInit():
    a = []
    for i in xrange(100):
        a.append(i)

By list comprehension


def byListComprehension():
    [None for i in xrange(100)]

def byComprehensionInit():
    [i for i in xrange(100)]

By multiplication


def byMultiplication():
    [None] * 100

def byMultiplicationInit():
    a = [None] * 100
    for i in xrange(100):
        a[i] = i

Results


import timeit
print 'By append'
print(timeit.timeit('byAppend()', setup="from __main__ import byAppend"))
print 'By append init'
print(timeit.timeit('byAppendInit()', setup="from __main__ import byAppendInit"))
print 'By list comprehension'
print(timeit.timeit('byListComprehension()', setup="from __main__ import byListComprehension"))
print 'By list comprehensionInit'
print(timeit.timeit('byComprehensionInit()', setup="from __main__ import byComprehensionInit"))
print 'By byMultiplication'
print(timeit.timeit('byMultiplication()', setup="from __main__ import byMultiplication"))
print 'By byMultiplication init'
print(timeit.timeit('byMultiplicationInit()', setup="from __main__ import byMultiplicationInit"))

Output from consoles

C:\Python27\python.exe C:/Users/Mond/PycharmProjects/hackercup/round1/test.py
By append
10.2452175728
By append init
9.83314044088

By list comprehension
4.45988583823
By list comprehensionInit
4.37547215318

By byMultiplication
0.840903578289
By byMultiplication init
5.83873665989

Conclusion

If you need to create a “fixed sized” array only, multiplication is the fastest.
If you need to initialize array with some constants, list comprehension performs better.
Never ever using the way for appending.

Gotcha

For 2d array initialization, you must be careful not to do it via multiplication.

Below are codes for initializing a 2D array with 3 rows and try to add a string hello to the first array


a = [[]] * 3
a[0].append('hello')
print a

What a will be output is

[['hello'], ['hello'], ['hello']]

instead of

[['hello'], [], []]

Therefore, we can see initializing 2D array via multiplication is not a good way as they are all the same array even though arrays seems to be multiplied (initialized).

The proper way is using list comprehension

# The underscore `_` means I do not care what value it is
a = [[] for _ in xrange(3)]

a[0].append('hello')
print a

Then, a will be output as

[['hello'], [], []]

2017/01/09

Clean up cache inactive ports

As there is a shortage on my mac book, I need to free storage from mac ports

sudo port -f clean --all all
sudo port -f uninstall inactive

References
http://superuser.com/questions/165652/how-can-i-clean-up-my-macports-installation

How to update npm modules

So, when was the last time you have been updated your npm modules?

After I have been updated for few of machines, I believe it is quite useful and necessary for me to record steps for doing so to save my life. Here we go.

1. Get a list of outdated packages

npm provides a useful command for us to do so. Below is an example for listing global modules which are outdated. Of cause, you can run that in your local project once you got the idea.

$ npm -g outdated
Package                 Current  Wanted  Latest  Location
eslint                   3.12.2  3.13.0  3.13.0  /opt/local/lib > eslint
eslint-config-airbnb     13.0.0  14.0.0  14.0.0  /opt/local/lib > eslint-config-airbnb
eslint-plugin-jsx-a11y    2.2.3   3.0.2   3.0.2  /opt/local/lib > eslint-plugin-jsx-a11y
eslint-plugin-react       6.8.0   6.9.0   6.9.0  /opt/local/lib > eslint-plugin-react
npm                     2.15.11   4.0.5   4.0.5  /opt/local/lib > npm

Here is the documentation about npm outdated. Briefly speaking, you should take care versions in the middle column.

NOTE:

Current column shows versions you own now.
Wanted column shows latest versions you can get after considering your package dependencies
Latest column shows the latest available version from corresponding projects

2. Update the npm modules

As my npm is managed by macport, I will let macport to upgrade my npm.

$ sudo port upgrade outdated

Install rest of the modules by specifying name and version together.

$ sudo npm -g install eslint@3.13.0 eslint-config-airbnb@14.0.0 eslint-plugin-jsx-a11y@3.0.2 eslint-plugin-react@6.9.0

NOTE:

My global modules requires a root privilege

Sometime, there are warnings about mismatched dependencies.

npm WARN eslint-config-airbnb@14.0.0 requires a peer of eslint@^3.13.0 but none was installed.
npm WARN eslint-config-airbnb@14.0.0 requires a peer of eslint-plugin-import@^2.2.0 but none was installed.
npm WARN eslint-config-airbnb-base@11.0.1 requires a peer of eslint@^3.13.0 but none was installed.
npm WARN eslint-config-airbnb-base@11.0.1 requires a peer of eslint-plugin-import@^2.2.0 but none was installed.
npm WARN eslint-plugin-jsx-a11y@3.0.2 requires a peer of eslint@^2.10.2 || 3.x but none was installed.
npm WARN eslint-plugin-react@6.9.0 requires a peer of eslint@^2.0.0 || ^3.0.0 but none was installed.

Don’t be afraid. Make sure they are really a problem first.

$ npm -g list

If you can read those warnings again, they are really problems. Semver explains syntax in the version, for example ^, x etc. I really suggest you to go through that article before solving the warnings.

Basically, they are talking about version mismatched. Take a serious look and install the required packages and problems will be solved.

3. End

You can double check the modules by calling npm outdated again and see whether there are missing modules.

In my opinion, the troublesome parts are resolving versions between packages. This is a smooth and easy process after resolving them.

4. More about npm’s dependencies

Actually, I have posted a question about missing dependencies in StackOverflow here. There are links for you to further study npm’s dependencies. (Hopefully, :D)

5. Search with `semver`

The best way to know whether the semver includes certain version is testing.

For example, I would like to know how many versions can be search for eslint with ^3.17.x

mondwan@mondwan-All-Series:~/Documents/git/abc$ npm -g view eslint@^3.17.x | grep ^eslint
eslint@3.17.0  
eslint@3.17.1

Change logs

2017-03-14: Add a command for searching package’s version
2017-03-11: Add a reference from StackOverflow

2016/10/28

Createa a bootable USB stick for ubuntu on ubuntu

Things below are just an alternative. There is an official way to do this.

Get a USB stick

Make sure you have an usb stick with enough storage. Mine is 8 GB.

Format the USB Stick

There are plenty options to do this. My solution is using the built in one disks

Click the USB device
Click the square button
Select EXT4 as the type of parition
Done

Download Ubuntu ISO

https://www.ubuntu.com/download/desktop

Bootable USB

Install and Use UnetBootin

sudo apt-get install unetbootin

Launch UnetBootin

Select the Ubuntu ISO
Select your USB drive
OK
Done

Now, you have a Bootable USB for ubuntu :D

2016/02/26

Lua string.match vs string.gmatch

Overview

Just now, I spent a while on coding with string.gmatch and string.match. In my opinion, they are not quite intuitive for their usage. So, I tried to placed my example here for future references.

Code example

gmatch aims for global search while match ends at 1st match.

> line = "Jan  1 08:17:02 ABC hotplug: [OK] ifup: 'lan' => 'br-lan'"
> pat = '([^ ]+)'

# End at Jan
> return line:match('([^ ]+)')
Jan

# Equivalent to split by ' '
> for w in line:gmatch(pat) do print(w) end
Jan
1
08:17:02
ABC
hotplug:
[OK]
ifup:
'lan'
=>
'br-lan'

gmatch does not good at dealing with ^

> line = "Jan  1 08:17:02 ABC hotplug: [OK] ifup: 'lan' => 'br-lan'"
> pat = '^([A-Z][a-z][a-z])'
# Able to get Jan
> =line:match(pat)
Jan
# Unable to get anything
> for w in line:gmatch(pat) do print(w) end
>

Reference

2016/02/09

Notes on studying LTN12

Background

Suppose you don’t know what LTN is, LTN stands for Lua technical note.

Below are notes dropped from LTN12 which is the 12th documents talking about filters, sink , source, chain and pump.

Source codes from the author is available here

Interpretations

XXX: My interpretations may or not may correct. Let me know if there are any misunderstands.

Combinations of terms I highlighted models question producer and consumer very well
What is a filter ?

Functions accept successive chunks of input, and produce successive chunks of output.

Mathematically speaking,

Suppose f1, f2, ..., fn are filters s.t. F = f1 + f2 + ... + fn
Suppose x1, x2, ... xn are input s.t. X = x1 + x2 + ... + xn

F(x) == f1(x1) + f2(x2) + ... + fn(xn)

What is a chain?

A function combines two *similar* functions. By *similar*, I mean same function signatures and return data type.

Mathematically speaking,

Suppose f1, f2, ..., fn are functions s.t. C = f1 + f2 + ... + fn
Suppose x1, x2, ... xn are input s.t. X = x1 + x2 + ... + xn

C(x) == f1(x1) + f2(x2) + ... + fn(xn)

What are sources and sinks?

Filters form internal nodes inside an arbitrary *data flow*. *Data* in *data flow* means things shared between producer and consumer while *flow* means the direction of that things which is from *Producer* to *Consumer*

Given picture above, we can interpret *Sources* and *Sinks* as initial and final nodes of the *data flow*.

What is pump?

The driving force between internal nodes (filters)

Example application?

-- Goal: convert any input text ends with \r\n

-- source.file() returns a kind of filter
-- normalize() returns a kind of filter
-- source.chain() shows how to combine *similar* filters
-- input is a kind of *source*
input = source.chain(source.file(io.stdin), normalize("\r\n"))

-- sink.file() returns a kind of filter
-- output is a kind of *sink*
output = sink.file(io.stdout)

-- Driving force between input and output
-- Or, start the jobs
pump(input, output)

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2017/02/05

About

What is a Nested classes?

Why using nested classes?

Idea of how memory is leaked

Words before end…

References:

2017/01/27

About

How to create multiple nox instances

How to connect NOX instances

2017/01/19

Question

Methods

Results

Conclusion

Gotcha

2017/01/09

1. Get a list of outdated packages

2. Update the npm modules

3. End

4. More about npm’s dependencies

5. Search with semver

Change logs

2016/10/28

Get a USB stick

Format the USB Stick

Download Ubuntu ISO

Bootable USB

2016/02/26

Overview

Code example

Reference

2016/02/09

What is a `Nested classes`?

Why using `nested classes`?

5. Search with `semver`