You Gin

Hi guys, can somebody tell me why this happens, please read below.
I have my own class Data which extends Button. In my Activity's class I have a List<Data> which is appending with new Data object on each button click. I want to keep created objects of class Data on the screen after configuration changes, namely orientation change. So I've overrided onSaveInstanceState() and using it to restore my data in onCreate() method. In mentioned method I'm saving every Data object from the list of objects. In onCreate() method I'm trying to restore my data like this, the problem happens on line 18.

public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); button = (Button) findViewById(R.id.Button01); layout = (LinearLayout) findViewById(R.id.MyLayout); dataList = new ArrayList<Data>(); if(savedInstanceState != null) { int numberOfObjects = savedInstanceState.getInt("Number of objects"); for (int i = 1; i < numberOfObjects; i++) { Data tmp = (Data)savedInstanceState.getSerializable("Button name " + i); if (tmp != null) { dataList.add(tmp); } if (dataList.size() != 0) { // I'm using one of the lines at a time, uncommented here just for example: layout.addView((Data)dataList.get(i-1)); // IllegalStateException happens here with advice to invoke removeView() method. I tried to call it on the method beginning, but no success. layout.addView(new Data(getApplicationContext())); // This one is perfectly fine } } }

So in first case I'm getting IllegalStateException but it is working great for the second one. I don't understand the difference.

Just in case, here is my onSaveInstanceState() method:

@Override protected void onSaveInstanceState(Bundle outState) { super.onSaveInstanceState(outState); Log.d("Dev", "onSaveInstanceState()"); outState.putInt("Number of objects", dataList.size()); for (Data object : dataList) { outState.putSerializable("Button name " + object.getNumber() , object); } }

And my Data class, but nothing special:

package as.as.as; import java.io.Serializable; import android.content.Context; import android.widget.Button; public class Data extends Button implements Serializable { public static int counter; public int number; public Data(Context context) { super(context); counter++; this.number = counter; this.setText("Button number " + Integer.toString(number)); } public String getNumber() { return Integer.toString(number); } }

Thank you, I understand.

I got an advice recently to use:

List<MyClass> list = new ArrayList<MyClass>() instead of ArrayList<MyClass> list = new ArrayList<MyClass>()

in order to hide implementation details. I don't understand why is it so cool to hide something, why is it required?

Hi guys,

I'm about to finish with Head First Java book. I was advice to use it on this forum and that's why I want to ask you about next advice what book to be learned next from your standpoint.
Thank you very much.

Guys, what do you think, is it more perspective to learn Servlets and JSP rather than Android? I heard that servlets and JSP are losing their popularity for backend development and people more using .NET for websites and developing for Android is more in demand these days.

Thanks guys, that was useful - I read something I didn't know about.

Hi guys. I'm in doubts is it a good practice to set private methods in Constructor or not. Could you please have a look on my Class on the matter?

Thanks!

import java.io.File; import java.io.FileNotFoundException; import java.io.FileReader; public class ConfFile { private String confFile = "c:\\user.properties"; private File file; private FileReader fReader; ConfFile(){ generateFileStream(); } private void generateFileStream() { file = new File(confFile); try { fReader = new FileReader(file); } catch (FileNotFoundException e) { e.printStackTrace(); } } public void setConfFile(String confFile){ if (confFile.equals("")) throw new IllegalArgumentException("confFile can't be null"); this.confFile = confFile; generateFileStream(); } public FileReader getFileStream(){ return fReader; } }

Ok, ok... please prove my guess. I have a feeling that on if(str1 == "") new object of String type is instantiated and because the "" is already on a string pool those two are actually the same object and thus str == "" is true? Because if I would use:
if (str3 == new String("")){ System.out.println("3. Equal"); } else { System.out.println("3. Not Equal"); }

I'll get "Not Equal"

Thanks you for your explanation Ernest, but the question remains open. Why can' I use
if(str1 == ""){ System.out.println("Equal"); } else { System.out.println("Not Equal"); }

if it works just fine?

Hello Jeanne,

Can you tell me more about Strings comparing please?

I just can't get a difference here. Why should I use equal if == works perfectly fine?
public class Salmon { public static void main(String[] args) { String str1 = "Str1"; String str2 = ""; if(str1.equals("")){ System.out.println("Equal"); } else { System.out.println("Not Equal"); } if(str1 == ""){ System.out.println("Equal"); } else { System.out.println("Not Equal"); } if(str1.equals(str2)){ System.out.println("Equal"); } else { System.out.println("Not Equal"); } } }

Output:
Not Equal
Not Equal
Not Equal

Hello Jeanne,

Thank you very much for your useful comments, taking them all into account.

Regarding exception for incorrect data in a setter, yeah, I thought about it also and added exception throwing over there, thanks a lot!

Hi guys.

As it was highly recommended in one of the books I'm reading I've started to write tests for each of the class I'm creating. I'm not really sure what is a good practice to cover setters and getters or not. If yes I'm not sure how because to verify the setter method you need to call the getter for this variable and vice versa. So you will never know where the actual problem is.
Where I went is just similar tests for setters and getters. I don't really like it. Could you guys refer me to a right direction?
Thank you very much!

Here is my class and a corresponding test class.
import java.io.Serializable; public class Card implements Serializable { private String foreignWord; private String nativeWord; private int weight; // To be increased when card is answered correctly and decreased if not public int getWeight() { return weight; } public void setWeight(int weight) { this.weight = weight; } public String getForeignWord(){ return foreignWord; } public void setForeignWord(String foreignWord){ if (foreignWord != ""){ this.foreignWord = foreignWord; } else { System.out.println("Can't save a foreignWord because it is empty"); } } public String getNativeWord(){ return nativeWord; } public void setNativeWord(String nativeWord){ if (nativeWord != ""){ this.nativeWord = nativeWord; } else { System.out.println("Can't save a nativeWord because it is empty"); } } public void increaseWeight(){ weight += 1; } public void decreaseWeight(){ weight -= 1; } }

Test:
import static org.junit.Assert.*; import org.junit.Test; public class CardTest { @Test public void testSetForeignWord() { Card card = new Card(); card.setForeignWord("test"); assertTrue(card.getForeignWord() == "test"); } @Test public void testSetNativeWord() { Card card = new Card(); card.setNativeWord("test"); assertTrue(card.getNativeWord() == "test"); } @Test public void testGetForeignWord() { Card card = new Card(); card.setForeignWord("test"); assertTrue(card.getForeignWord() == "test"); } @Test public void testGetNativeWord() { Card card = new Card(); card.setNativeWord("test"); assertTrue(card.getNativeWord() == "test"); } @Test public void testGetWeight(){ Card card = new Card(); card.setWeight(2); assertTrue(card.getWeight() == 2); } @Test public void testSetWeight(){ Card card = new Card(); card.setWeight(2); assertTrue(card.getWeight() == 2); } @Test public void testIncreaseWeight(){ Card card = new Card(); card.increaseWeight(); assertTrue(card.getWeight() == 1); } @Test public void testDecreaseWeight(){ Card card = new Card(); card.decreaseWeight(); assertTrue(card.getWeight() == -1); } }

Thanks guys, now I understand this choice where to initialize your variables depends on you and current needs, but both cases are correct.

Hi guys,

I've got an impression that it is a good practice to make your instance variables initialization within class's constructor
class Salmon { private int i; Salmon(){ i = 3; } }

At the same time in most examples I see instance variables are initialized like:
class Salmon { private int i = 3; Salmon(){ } }

So, as far as I understand, by the time new is returning a reference to a newly created object the instance variable are initialized for both these cases.
Put it in a few words, am I right that there is no need to make initialization in Constructor for cases when I'm not defining initial value(s) for instance variable(s)?

Thank you very much.

Hi guys,

What I've noticed is that rt.jar is located in both JDK and JRE:
"c:\Program Files\Java\jdk1.6.0_21\jre\lib\rt.jar"
"c:\Program Files\Java\jre6\lib\rt.jar"

Several questions arise at the same time. Do you mind I'll list them all?

Here are they:

1. Which of them is using when I do import something in my code?
2. Why the size of these two is different?
3. Does it mean I can simply remove the jre6 folder and still be able to run java applications (as long as JRE is included in JDK)?