cse 476 mobile application development

9/24/2020

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CSE 476 Dennis PhillipsMobile Application Development11

Java Stuff

Things you need to know about Java

Or just might find handy…


Are you sure what this does?

int a1 = 1;int b1 = 1;

String a = "This is string " + a1;String b = "This is string " + b1;textView1.setText(a);textView2.setText(b);

if(a == b) {textView.setText("Equal");

} else {textView.setText("Not equal");

} Output:This is string 1This is string 1???

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String equality tests

if(a.equals(b)) {textView.setText("Equal");

}

a == b tests if the objects are the same, not if the contents are the same


Inheritance

Inheritance creates new classes based on classes that have already been defined. The capabilities of the base class are inherited by the derived class.

public class HatterView extends View {

@Overrideprotected void onDraw(Canvas canvas) {

super.onDraw(canvas);}

}

Here HatterViewextends View. HatterView is aView.

It’s a View and then some… Diagram

Representation

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Inheritance

We also say:

HatterView is derived from ViewView is a superclass of HatterViewView is a base class of HatterView


Polymorphism

The ability of an object of one type to appear as and be used as another type.

Lines lines = new Lines();Drawable item = lines;

The variable lines is a reference to an object of type Lines.The variable item is a reference to the same object, but only knows it as a Drawable (it just knows that much about the object)

Polymorphism applies when classes are derived from other classes

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The Key Idea of Polymorphism

An object can be known by its class or any class it is derived from.

Example:

Screen – A screen in a computer game. Can be 2D or 3D. Scene – A scene in a computer game. 3D only. Derived from Screen.CityScene – A scene set in a city. Derived from Scene.

We create an object for a city scene. We can pass that object to functions that expect a reference to a Scene or Screen.

Think of it this way: A CityScene is a Scence. A Scene is a Screen; A CityScence is a Screen;


Polymorphism in Java

Suppose we have this code:

The variable item is a reference to an object that is actually of type Lines(). That is the underlying type. We can get it back using a down cast:

Lines lines = new Lines();Drawable item = lines;

Lines linesObj = (Lines)item;

But, if item is not a reference to a Lines object, you will get a runtime exception

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android.widget.Button


Checking if something is of the expected type

Lines linesObj = null;if(item instanceof Lines) {

linesObj = (Lines)item;}

The IDE will actually warn you sometimes if it thinks a down cast is dangerous. It’s expecting you to do a check like this.

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Using instanceof public class Animal {}

public class Fish extends Animal {public void swim() {

// swim code…}

}

public class Bird extends Animal {public void fly() {

// fly code…}

}

for(Animal animal : animals) {if(animal instanceof Fish) {

Fish fish = (fish) animal;fish.swim();

} else if(animal instanceof Bird) {

Bird bird = (bird) animal;bird.fly();

}}

You have a collection of Animals. You can check each to determine its type and then call the appropriate function.


Virtual Functions public class Animal {public void move() {}

}

public class Fish extends Animal {@Overridepublic void move() {

// swim code…}

}

public class Bird extends Animal {@Overridepublic void move() {

// fly code…}

}

for(Animal animal : animals) {animal.move();

}

If a base class version of a function appears in a derived class, the version in the actual underlying type will be called.

Notice how much simpler the code is in this case? Virtual functions allow us to avoid instanceof and down casts.

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Example

private ArrayList array = new ArrayList();

for(Object obj: array) {if(obj instanceof Square) {

Square square = (Square)obj;square.draw();

} else if(obj instanceof Cube) {Cube cube = (Cube)obj;cube.displayMe();

}}

public class Square {public void draw() {

//…}

}

public class Cube {public void displayMe() {

//..}

}


instanceof and down casts considered harmful?

Well designed object-oriented programs do not need instanceof or down casts. They can utilize virtual functions and other methods to avoid these.

Many experts consider these an indication of a bad design. However, they are pretty common in Java programming and the Android API.

hatterView = (HatterView)findViewById(R.id.hatterView);colorButton = (Button)findViewById(R.id.buttonColor);featherCheck = (CheckBox)findViewById(R.id.checkFeather);spinner = (Spinner)findViewById(R.id.spinnerHat);

Try to design so as to minimize their use

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Why down casts become almost essential

Android creates the views for you in many cases, you only get the polymorphic View reference.


Collaboration

public class Client {

public Client() {worker = new Worker();

worker.doSomeWork();}

}

public class Worker {

public Worker() {

}

public int doSomeWork() {// Do somethingreturn 0;

}

}I have a worker class. I ask it to do some work. In this case, how does Client know the work is done?

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Synchronous Tasks




}


public Worker() {

}

public int doSomeWork() {// Do somethingreturn 0;

}

}Worker completes all if its work before it returns. This is a synchronous task You use synchronous tasks all

of the time…


Asynchronous Tasks




}


public Worker() {

}

public void doSomeWork() {// Start doing something//...

}

private void workIsDone() {// Tell the client we are done...//...

}}

I have a worker class. I ask it to do some work. doSomeWorkimmediately returns. Worker continues to do the work and later needs to tell Client it is done. This is called an asynchronous task.

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Asynchronous what?

How often does this happen?

Every program you have ever written in this class.

Common cases:➢ I create a GUI component and later it needs to tell me I

clicked on it.➢ I create a thread and it needs to tell me it is done

computing.➢ I create a background service and it needs to tell me a

message has arrived.

The other class may be computing in a thread or it may be computing when the GUI calls it.


One way to do this…


private Worker worker;

public Client() {worker = new Worker(this);


public void workerIsDone() {

}}


private Client client;

public Worker(Client client) {this.client = client;

}


}

private void workIsDone() {// Tell the client we are done...client.workerIsDone();

}

}Worker knows about client, client knows about worker

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UML

Client has a reference to the worker, so it can call functions on the worker. Worker has a reference to the client, so it can call functions on the client.

Classic bidirectional association


But…

Worker class has turned out to be handy. Client2 wants to also use it.

public class Client2 {


public Client2() {worker = new Worker(this);



}}


private Client client;


}


}


}

}

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One solution…


private Client client = null;private Client2 client2 = null;


}

public Worker(Client2 client) {this.client2 = client;

}


}

private void workIsDone() {// Tell the client we are done...if(client != null) {

client.workerIsDone();}

if(client2 != null) {client2.workerIsDone();

}}

}


Really bad solution


private Client client = null;private Client2 client2 = null;


}

public Worker(Client2 client) {this.client2 = client;

}


}

private void workIsDone() {// Tell the client we are done...if(client != null) {

client.workerIsDone();}

if(client2 != null) {client2.workerIsDone();

}}

}

Client2 was very impressed. Here comes Client3, Client4, etc.

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Inheritance to the rescue!


WorkerIsDone class

public class WorkerIsDone {


}}


private WorkerIsDone client;

public Worker(WorkerIsDone client) {this.client = client;

}


}


}

}

public class Client extends WorkerIsDone {private Worker worker;

public Client() {worker = new Worker(this); worker.doSomeWork();

}


}}

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Along comes Client2

public class WorkerIsDone {


}}

public class Client2 extends WorkerIsDone {private Worker worker;

public Client2() {worker = new Worker(this); worker.doSomeWork();

}


}}


private WorkerIsDone client;

public Worker(WorkerIsDone client) {this.client = client;

}


}


}

}


Happy, happy, joy, joy until…

Java (and C# and Objective-C, BTW) does not allow multiple inheritance

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Interfaces

public interface WorkerIsDone {void workerIsDone();

}

public class Client implements WorkerIsDone {





}}

An interface specifies functions that a derived class must implement!

Java allows only one base class, but as many interfaces as you like.

Only function definitionsAll assumed publicNo function bodyNo member variables


Nested Interfacepublic class Worker {

public interface IsDone {void workerIsDone();

}

private IsDone client;

public Worker(IsDone client) {this.client = client;

}

…

public class Client implements Worker.IsDone {





}}

If the interface goes with the class, nest it

You can define an interface inside a class

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Case Study – A target view

View calls a function in the activity when you touch the target.


TargetViewonTouchEvent()

@Overridepublic boolean onTouchEvent(MotionEvent event) {

switch(event.getActionMasked()) {case MotionEvent.ACTION_DOWN:

float x = event.getX();float y = event.getY();

// How far are we from the center?float cx = getWidth() / 2;float cy = getHeight() / 2;float dist = (float)Math.sqrt((x - cx) * (x - cx) + (y - cy) * (y - cy)) / cx;

float score = 10 - dist * 10;

// Send the score to the calling activity???????????????????return true;

}

return super.onTouchEvent(event);}

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If only will ever work with one activity…

public class TargetActivity extends Activity {

//…

public void onTargetHit(float score) {//...

}

}

float score = 10 - dist * 10;

// Send the score to the calling activity((TargetActivity)getContext()).onTargetHit(score);return true;

This works because Activity is derived from Context and the activity is known by the context in the View. Polymorphism…


But, what if I want this to work with different activities?

Add this interface to TargetView

Derive TargetActivityfrom the interface

public interface Score {void onTargetHit(float score);

}

public class TargetActivityextends Activity implements TargetView.Score {

And call it this way// Send the score to the calling activity

((Score)getContext()).onTargetHit(score);

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We have been using this…

private class ShuffleListener implements DialogInterface.OnClickListener {

From Step 3


Nested Classes

private class Puzzle {


@Overridepublic void onClick(DialogInterface dialog, int which) {

shuffle();view.invalidate();

}

}

}

Classes declared inside another class.Saves creating an extra .java file.Associates the class with what it is nested in.

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Nested Classes – Extra Java Feature




}

}

Nested classes declared like this have an automatic association with the enclosing class, so they can call functions and access member variables.

Declared in Puzzle

But, the allocation must be in a function in this class


Use of a nested class

AlertDialog.Builder builder = new AlertDialog.Builder(view.getContext());

ShuffleListener listener = new ShuffleListener();builder.setNegativeButton(R.string.shuffle, listener);




}

}

Nested Class

Where it is used

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Anonymous classesDeclared and used in only one place and has no name

AlertDialog.Builder builder = new AlertDialog.Builder(view.getContext());

builder.setNegativeButton(R.string.shuffle, new DialogInterface.OnClickListener() {@Overridepublic void onClick(DialogInterface dialog, int which) {


}});

private class ShuffleListener implements DialogInterface.OnClickListener {@Overridepublic void onClick(DialogInterface dialog, int which) {


}}

Original Nested Class

Anonymous Nested Class

Worksheet


Problem 1

public class GameActivity extends Activity implements HourGlassView.HourGlass {

@Overrideprotected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);setContentView(R.layout.activity_game);

HourGlassView hourGlassView = (HourGlassView)findViewById(R.id.hourGlass);

hourGlassView.setClient(this);}

@Overridepublic void turned() {}

@Overridepublic void expired() {}

}

public class HourGlassView extends View {

public interface HourGlass {void turned();void expired();

}//…

}

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Problem 2public class GameActivity extends Activity {



HourGlassView hourGlassView = (HourGlassView)findViewById(R.id.hourGlass);hourGlassView.setClient(new HourGlassCallback());

}

private class HourGlassCallback implements HourGlassView.HourGlass {@Overridepublic void turned() {}@Overridepublic void expired() {}

}}


Problem 3public class GameActivity extends Activity {



HourGlassView hourGlassView = (HourGlassView)findViewById(R.id.hourGlass);hourGlassView.setClient(new HourGlassView.HourGlass() {

@Overridepublic void turned() {}

@Overridepublic void expired() {}

});}

}

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Named to Anonymous

hourGlassView.setClient(new HourGlassCallback());

private class HourGlassCallback implements HourGlassView.HourGlass {public void turned() {}

public void expired() {}

}

hourGlassView.setClient(new HourGlassView.HourGlass() {public void turned() {}

public void expired() {}

});


Static Nested Classes

private static class Parameters {

public String imagePath = null;public int hat = HAT_BLACK;

}

Static nested classes work like C++ nested classes. They do not have the association with the enclosing object. You can’t call member functions or access variables outside the nested class itself

They cost less than regular nested classes if you don’t need that access. You can instantiate them anywhere they are visible. And, they make some other things possible…

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Static Members

public static final int HAT_BLACK = 0;

private static Parameters params = new Parameters();

Static members are associated with the class, not an object of the class

params acts like a global variable. It is created when the program starts and it stays around. Turns out to be useful in Android in many cases…

What if I exit this activity and return later on?

The value of static variables will persist as long

as the class is loaded, strange behavior …


Static vs. normal member functions

public class Vector3 {

public final float [] v = new float[3];

public static float length(Vector3 vector) {float [] v = vector.v;return (float)Math.sqrt(v[0] * v[0] + v[1] * v[1] + v[2] * v[2]);

}

public float length() {return (float)Math.sqrt(v[0] * v[0] + v[1] * v[1] + v[2] * v[2]);

}

public static float dot(Vector3 a, Vector3 b) {return (float)Math.sqrt(a.v[0] * b.v[0] + a.v[1] * b.v[1] + a.v[2] * b.v[2]);

}}

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Serialization

Object Serialization

Deserialization Object

Byte Stream

Byte Stream

Serialization converts a Java object into a stream of bytes. Deserialization reverses the process


What does it take?

private static class Parameters implements Serializable {private static final long serialVersionUID = -2846716033219475583L;public String imagePath = null;public int hat = HAT_BLACK;

}

But, every class Parameters is derived from must be serializable.

Every member of Parameters must be serializable (with one exception).

You are supposed to provide a variable serialVersionUID with an integer value. Some IDE’s will generate one for you.

“If a serializable class does not explicitly declare a serialVersionUID, then the serialization runtime will calculate a default serialVersionUID value for that class”. Java docs

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Using this…public void putToBundle(String key, Bundle bundle) {

bundle.putSerializable(key, params);}

public void getFromBundle(String key, Bundle bundle) {params = (Parameters)bundle.getSerializable(key);

// Ensure the options are all setsetColor(params.color);setImagePath(params.imagePath);setHat(params.hat);

}


transient

Declare a member variable transient if you don’t want it serialized…

private static class Parameters implements Serializable {public String imagePath = null;public int hat = HAT_BLACK;public transient Bitmap imageBmp = null;

}

There are often members that represent state and members that are temporary, derived, loaded, or not serializable at all.

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Serialization is supposed to be really slow

I’m seeing numerous reports online that serialization is very slow and should never be used.


Serialization is supposed to be really slow

I’m seeing numerous reports online that serialization is very slow and should never be used.

Note that for a benchmark of 10,000 values, very slow was 0.3 seconds… Often you are moving 10 values…

But, use only for small amounts of data…

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Moving data around in Android

Save into bundles and intentsSerialize into bundles and intentsStatic variablesParcelable – You implement serialization yourself

Parcelable is the recommended solution for larger objects, but some report it slower than serializable


Using static member variables to share data

Suppose you have a large data structure that is passed between activities.

Serialization/deserialization, no matter how you do it, can be costly and result in redundant copies in memory.

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Using static member variables to share data

public class SomeActivity extends Activity {

private static ApplicationData data = new ApplicationData();

public static ApplicationData getApplicationData() {return data;

}

//…

You can also allocate the object in the constructor, so it does not exist until you need it. Use sparingly: you are consuming memory as long as your application exists.

In some other activity:

ApplicationData data = SomeActivity.getApplicationData();

If you need to do this much, look at extending the Application class. It has functions that will indicate if memory is getting low.


Another approach…

public class SomeActivity extends Activity {

private static ApplicationData data = null;

public static ApplicationData getApplicationData() {if(data == null) {

data = new ApplicationData();}return data;

}

//…

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Singleton Patternpublic class ServerComm {

// Singleton pattern objectprivate static final ServerComm server = new ServerComm();

// Private constructor prevents other classes from// creating a new object of this typeprivate ServerComm() {}

// Get a reference to the singleton server objectpublic static ServerComm get() {

return server;} Exactly one of this object will always exist.

You can get a reference to it from anywhere this way:

ServerComm serverComm = ServerComm.get();

This is referred to as “eager initialization” since the object is created when the program starts…


Singleton PatternLazy initialization

public class ServerComm {

// Singleton objectprivate static ServerComm server = null;

// Private constructor prevents other classes from// creating a new object of this typeprivate ServerComm() {}

// Get a reference to the singleton server objectpublic static ServerComm get() {

if(server == null) {server = new ServerComm();

}return server;

}

Maybe you don’t know that you’ll use this object, or you want to make the program start faster?

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Exceptions

Some operations can fail and cause Exceptions:

EditText num = (EditText)findViewById(R.id.temperature);float x = Float.parseFloat(num.getText().toString());

What if I enter the wrong value?

General rule of programming: Assume all input is evil!


NumberFormatException

02-02 16:24:48.986: E/AndroidRuntime(10354): java.lang.RuntimeException: Unable to start activity: java.lang.NumberFormatException: Invalid float: “nahnah"

Exceptions are the standard way to capture errors in Java. Most failures are indicated by exceptions.

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try/catchEditText editText = (EditText) dlg

.findViewById(R.id.dlgFloatOne);float x;try {

x = Float.parseFloat(editText.getText().toString());

} catch (NumberFormatException ex) {

// Value is badeditText.requestFocus();editText.selectAll();return;

}

What you put in the try block will be tried. If it throws an exception, control moves immediately to the catch block


Multiple catch blocks

try {// Parse the URLURL url = new URL(COMMUNITY_URL);

// Create a new HTTP connection// and get a streamHttpURLConnection connection = (HttpURLConnection)url.openConnection();int responseCode = connection.getResponseCode();if(responseCode == HttpURLConnection.HTTP_OK) {

stream = connection.getInputStream();}

} catch(MalformedURLException ex) {return;

} catch(IOException ex) {return;

}

If more than one exception is possible, create a catch for each.

Some exceptions MUST be handled. You have to have a try around them.

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Mutable and immutable

A class is said to be mutable if the state can be changed after creation.

Otherwise, it is immutable.

String is immutable. You cannot change the value of a string.

So, what is this code doing:

String s1 = “I’m okay”;s1 = s1 + “, you’re okay”;


Example

String s1 = “I’m okay”; “I’m okay”

“I’m okay, you’re okay”

String s2 = s1;

s1 = s1 + “, you’re okay”;

s1

s2

“I’m okay”s1

“I’m okay”s2

s1

Whenever the string changes, a new object is created with the new string in it

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cse 476 mobile application development

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