Activities & Intents


This post introduces Activities in Android and how Intents are used for any communication between activities or any other application components.

Activities

Activities are the fundamental building blocks of Android applications. An activity is an interface between the screen and the application. Activities, services, any application components must be declared in the AndroidManifest.xml file. If it is not done, the application considers that component of the application does not exist and often runtime exceptions are thrown when the application is being run.

This declaring of an activity, however, does not have to be done manually in the manifest file. But any kind of attributes associated with the activity can be added to the activity explicitly in the AndroidManifest.xml. For instance, an activity can be declared as the launcher activity (the first screen of the application that shows up on our screens as it runs) can be done by adding the line <category android:name="android.intent.category.LAUNCHER"></category> in the corresponding activity element in the manifest.

In any case, if an activity has to be declared manually, it can be done by creating an <activity> element in the manifest file. Next, the name and label of the activity can be specified using android:name and android:label attributes respectively. More can be learned about its attributes at https://developer.android.com/guide/topics/manifest/activity-element.

Communicating with intents

Intents are the glue between activities. Any communication between activities happens via intent. Intent holds the necessary information needed to perform an action. An intent usually has two pieces of information – action and data. In other words, intents carry information that the system uses to determine which component to start plus the information that the recipient component uses to act to perform an action properly

Intents are of two types –

  • Explicit: Explicit intents explicitly mention the component which should be called. The Java class is used as an identifier in explicit intent call.
  • Implicit: In an implicit intent, the action to be performed is specified and data is optional. Based on the type of action and data, the component that fits the most is started by the system. In some cases, if multiple components fit, the user is prompted with possible options and thus the user can decide which component to use to perform the specified action.

Explicit intent example

Intent intent = new Intent(this, SecondActivity.class);
intent.putExtra(Intent.EXTRA_TEXT, "sample text");
startActivity(intent);

In the above example, the EXTRA_TEXT identifier is used as an identifier or any custom string can be used as well. The concept of key-value pairs is used when transmitting data using intents between two components. We use the same key to get the data out as the one we used to put data onto intent. getStringExtra() is used to fetch data from the intent. Intents are not just limited to sending String type data. Any basic Android data types can be transmitted between app components.

Implicit intent example

Intent intent = new Intent(Intent.ACTION_VIEW);
intent.setData(Uri.parse("https://www.google.com"));
startActivity(intent);

In the above example, an intent object is created by specifying ACTION_VIEW as what we want to do. A URL is specified in the data, since the intention is to view the website, the default browser is launched (different data could launch different apps). Intent resolution mechanism operates by matching the best intent from the installed application packages.

Complete running examples of starting a new activity from a current one using implicit intent call and switching between activities using explicit intent can be checked at https://github.com/kamalpreetgrewal/ActivityStarter and https://github.com/kamalpreetgrewal/ActivitySwitcher respectively.

More on activities like storing data persistently, returning results from an activity is coming up in the next post. Keep reading!

Fix MongoError on first connect

The default MongoDb port is 27017. An error occurs sometimes while I am running my Node & ExpressJS application, with the message:

message: 'failed to connect to server [localhost:27017] on first connect [MongoError: connect ECONNREFUSED 127.0.0.1:27017]' }

Several solutions are provided on the internet to fix this. But a lot of them did not work for me. The simplest method I find to fix this on MS Windows is:

1. Open your Command Prompt as administrator.
2. Type > net start mongodb

The mongodb service starts successfully.

Objects in JS

Objects in JS can be created by:

  1. Use of variables
  2. Use of functions
// table variable                      |              // use of function
var table = {                          |              var table = function() {
    "legs":4,                          |                  this.legs = 4;
    "color":"brown",                   |                  this.color = "brown";
    "material":"wood"                  |                  this.material = "wood";
};                                     |              };

An instance of these objects can be created as:

var myTable = new table();

Arguments can be passed as needed in the constructor with corresponding changes in definition of the constructor. All the variables defined as property of this are public. In order to make a variable private, var keyword is used. Private variables are created to control changes that happen to their values from outside. Functions to modify these values are then defined within the constructor. This is illustrated in Bike object below.

var car = function() {
    var speed = 20;            // private variable

    // public methods below
    this.accelerate = function(paceup) {
        speed += change;
    };

    this.decelerate = function() {
        speed -= 5;
    };

    this.getSpeed = function() {
        return speed;
    };
};

Hour 2 : JS

Continuing learned using boolean values, conditional statements, switch case and use of functions. Everything goes on like we might have learned in some other language. Encountered only a new operator category called the strict equality operator and strict inequality operator.

Strict equality operator uses === instead of the normal == for the equality operator. So is the case for strict inequality operator which uses !== and not !=. These strict comparison operators compare the types of values being compared unlike the normal comparison operator that coerces the data type and checks only the values. No more differences than standard practices of these operators and conditionals!

Hour 1 : JS

Having not used JS even to a novice level, I had planned to learn it and create something substantial. And today was the first day to executing the plan. I will brief out the things I learned in the sequence on a map.

undefined, null, boolean, string, number, symbol and object are the 7 data types. Variables are declared with var keyword and accessing the value of a variable that has not been initialised returns an error of undefined type or sometimes a NaN. Names of variables can have alphanumeric characters, $ or _ but not a space or cannot start with a number. JS is case-sensitive. Arithmetic operations work on universal principles, so nothing new there.

Since double quotes are used to encase string literals, they have to be escaped when one wants them to be a part of the literal. However, this can be avoided by using single quotes to enclose the literal and vice-versa. Square brackets with zero based indexing is used to access characters in a string. Strings in JS are immutable and hence have to be defined all over instead of manipulating only the characters one wishes to modify. For instance,

var myStr = "Jello World";
myStr = "Hello World"; // This has to be fixed all over instead of accessing the first character using myStr[0].

Multi-dimensional arrays can be created by nesting several elements as part of a single element. An example:

var myArr = [[1, 2], [3, 4]];
var a = myArr[1,1]; //This assigns value of 4 to variable <em>a</em>.

Few operations to update arrays are:

  1. push() – Appends data to end of the array.
  2. pop() – Pops an element from end of the array.
  3. shift() – Pops first element of the array.
  4. unshift() – Adds element in the beginning of the array.

function keyword is used to define a function which is called via its name and arguments, if it requires any. Variables defined inside the function have local scope and those outside have the global scope. The value of the global variable is overridden by the local variable, that is a rule, ofcourse when both have same names. Global variables can be defined without the keyword var which is not considered a good practice because it comes with consequences which I have to determine yet.

This is all I grabbed in the 1 hour today, à bientôt!

(Im)Mutable

Mutable and immutable are common English words. Something that can be changed is mutable and that cannot be is immutable. I came across these few years ago but it was an over-head-flow then. There are two classes in Java String and StringBuilder and both suffice almost the same purpose with some additional features in the latter. So I explored the difference in terms of their performance and genesis and came across these keywords.

The meanings of these words are similar in other programming languages. Here I will take the instance of Java only. So coming to the class String, it is immutable and the StringBuilder is mutable.

Considering the mutable objects, their content can be changed post construction. Performing any manipulation on the mutable object will over-write the content in the object initially created. Note no new object is created as most of us normally expect. Let us create a StringBuilder like:

StringBuilder sb = new StringBuilder();
sb.append("Kamalpreet");
sb.append("Kaur");
sb.append("Grewal");

In the memory heap, this will be only one object looking like:

The reference to the StringBuilder points to the same object. However such is not the case for immutable objects like String. Let us create like below:

String s = "Kamalpreet";
s.concat("Kaur");
s = s.concat("Grewal");

Due to immutable behavior of String class, the new results will be stored in a new object. Note the third line in the code above. If the assignment is not done, the reference will continue to point to the initial string with value Kamalpreet.

This all applies to all other mutable and immutable objects in different programming languages.

OAuth & OSM

The number of libraries, APIs available today to perform variegated tasks is profuse. Despite the abundance, sometimes alternatives are minimal and from these scarce resources, a solution that fits our specific needs are inconsequential. Lately working on OAuth for a project based on OpenStreetMap, I could not find a convenient guide to understand and implement. Persevering on making it functional, I authorized my Android application the access to OSM servers.

By the use of Signpost, things became a little simpler. Use of Twitter API, Facebook API with signpost is all over the web. A novice developer of Android like me cannot go on without a little fretting. Further in this post, I am sharing the screenshots of the authentication and authorization.

The screenshots are self-describing. In an upcoming post, I will write a tutorial on how to execute this task.

Change Main Activity in Android

Often we miss out adding splash screen of the application in the initial development phases, or for an instance you might want to change the main activity or the launcher activity at some later stage of development. In order to incorporate this change, a minor modification in AndroidManifest.xml file is to be done.

Say you have two activities A and B. Say initially A is the main activity and now you want to make activity B as the main activity. Open the AndroidManifest.xml file. Initially the entry for activity A in the manifest looks like:

<activity android:name=".A">
    <intent -filter>
        <action android:name="android.intent.action.MAIN"></action>
        <category android:name="android.intent.category.LAUNCHER"></category>
    </intent>
</activity>

Either delete the lines in the intent-filter tag or replace the activity name with B and add a new entry for activity A.

Overall the activity B’s entry should be like:

<activity android:name=".B">
    <intent -filter>
        <action android:name="android.intent.action.MAIN"></action>
        <category android:name="android.intent.category.LAUNCHER"></category>
    </intent>
</activity>

A single line entry for activity A will look like:

<activity android:name=".A"></activity>

It’s pretty simple and clear, all you have to do is replace the name or add a new entry.

Passing Data using SharedPreferences

Sometimes while passing data between two activities, we are just concerned with passing data and not launching the activity to which the data is passed. SharedPreferences make this data passing easier without necessitating launch of the recipient activity. The usage is discussed briefly below:

Say you have two activities A and B where A is the sender and B is the recipient. Use the following code in activity A.

SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(A.this);
SharedPreferences.Editor editor = prefs.edit();
editor.putString("string", "This is a string.");
editor.putInt("number", 50);
editor.commit();

In activity B, use the following code to receive the variables from sender activity:

SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(context);
String s = prefs.getString("string", "hello");
Integer i = prefs.getInt("number", 0);

Default values should be set in the getString(), getInt() or any of the get methods used in the recipient activity. The variables in which the received values are stored can be used for further processing in activity B.

For further details on SharedPreferences, the official documentation can be checked at this link.

Android Application Execution Error

Running my application in Android Emulator, I faced this error:

error:execution failed for task ':app:compiledebugjavawithjavac'.

My application got build up and ran in the emulator despite ignoring this issue. However, running it on my USB connected device was not a success.

Here’s how it is fixed. It demanded the environment variable JAVA_HOME to be set up. Open up your .bashrc file in your favorite editor. Add export JAVA_HOME="/path/to/your/jdk/installation" to the end of this file. I set the value to /usr/lib/jvm/java-1.8.0-openjdk-amd64. This set up the JAVA_HOME for me.

Next .gradle directory in the application directory needs to be deleted and then go to Build > Make Project in the Android Studio menu. This builds up the application’s fresh .gradle directory. And Run your application successfully.