How to hide the TSB(nav bar - back, home, recent) no matter what version of Android you are using

You can do it with this:

   public void HideNavbar() {
        if(Build.VERSION.SDK_INT < 19) //API 18 or below
            View v = this.getWindow().getDecorView();
            v.setSystemUiVisibility(View.GONE);
        } else { //API 19 or above
            View decorView = getWindow().getDecorView(); 
            int uiOptions = View.SYSTEM_UI_FLAG_HIDE_NAVIGATION
                          | View.SYSTEM_UI_FLAG_IMMERSIVE_STICKY;
            decorView.setSystemUiVisibility(uiOptions);
        }
    }

How to start an Activity from within a Service

First, if you plan on doing this, please read this bit about interrupting the user: http://developer.android.com/guide/practices/seamlessness.html#interrupt

The trick is to set the intent's flag for FLAG_ACTIVITY_NEW_TASK. Also, as you probably already know, all life cycle methods like onCreate are executed on the UI thread which is required to start an Activity.

Here's an example:
  public void onCreate(){
    super.onCreate();
    Intent intent = new Intent("com.example.email.compose");
    intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK); //This is necessary!
    intent.putExtra("Contacts_ID", 0);
    startActivity(intent);
  }

How to add leading and trailing spaces in string.xml

Even if you use string formatting sometimes you still need white spaces at the beginning or the end. On these cases neither escaping with \ nor using   or the xml:space attribute helps.
You must use the UTF character \u0020 for white spaces. Just type this succession of characters and it will be interpreted as a space. The \u part tells android that the following number represents the character in the UTF table.

How to protect part of a public method from being executed by others

I recently had to face this problem while I was modifying the android framework to add a security feature and I didn't want 3rd party apps to be able to mess with it.

To solve this problem I used enforceCallingOrSelfPermission.

Anything you do in your code until you call that method will be runnable by anyone who can access your method but passed this point it will through a SecurityException If neither you nor the calling process has been granted a particular permission.

Here's what it looks like:
void myMethod() {
// my code every one has access to...
myContext.enforceCallingOrSelfPermission(myPermission, myMessage);
// my super code that noone else can execute
}

How to have a few layout elements wrap around

Here is a problem people might encounter when dealing with layouts on android.

Say you want to place an image at the middle of a sentence. you may add a couple TextView and an ImageView but if the horizontal space gets too small the two TextViews are going to start wrapping around independently wich is going to look weird. You'd want the image to be part of your TextView and the entire sentence to wrap around nicely.

To do so:
* replace the three objects by one Spannable TextView
* use a SpannableStringBuilder to append all your strings (don't forget to add an extra character (a space for example) that will be replaced by your image)
* and finally use setSpan(ImageSpan, begin, end, flag) on that builder where ImageSpan is an ImageSpan containing the drawable of your picture, begin is the index of the character you've inserted

So instead of having something that looks like this:

I really really really really like

Picasa because it's a very very very very nice free image editing software

You get something that looks more like that:

I really really really really like Picasa because it's a very very very very nice free image editing software

And here's what the code would look like:

add android:bufferType="spannable" to your TextView

SpannableStringBuilder builder = new SpannableStringBuilder();
builder.append(mContext.getText(R.string.part1));
int lengthOfPart1 = builder.length();
builder.append(" ");
builder.append(mContext.getText(R.string.part2));
Drawable d = mContext.getResources().getDrawable(R.drawable.picasaIcon);
d.setBounds(0, 0, d.getIntrinsicWidth(), d.getIntrinsicHeight()); // <---- Very important otherwise your image won't appear
ImageSpan myImage = new ImageSpan(d);
builder.setSpan(myImage, lengthOfPart1, lengthOfPart1 + 1, Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
myTextView.setText(builder);

How to create a custom progress bar

First you need to create a progress bar layout file. Let's call it my_custom_pb.xml

<layer-list xmlns:android="http://schemas.android.com/apk/res/android">

<item android:id="@android:id/background" android:drawable="@drawable/mybg"/>

<item android:id="@android:id/secondaryProgress">

<clip>

<shape>

<corners android:radius="5dip" />

<gradient

android:startColor="#80d3ff00"

android:centerColor="#80b6ff00"

android:centerY="0.75"

android:endColor="#a0cbff00"

android:angle="270"

/>

</shape>
</clip>
</item>

<item android:id="@android:id/progress">

<clip android:drawable="@drawable/myprogress"/>

</item>
</layer-list>

Notice you can either use a simple 9 patch drawable or make your own custom shape!
Then you would add that custom progress bar to the layout you want that bar to appear:

<ProgressBar

android:id="@+id/mypb"

android:layout_width="120px"

android:layout_height="10px"

android:progressDrawable="@drawable/my_custom_pb"

android:max="5"

android:progress="1" />

And finally you would access it from your code like you would with any regular progress bar:

myProgress = (ProgressBar)findViewById(R.id.mypb);

myProgress.setProgress((int)value);

--
Special Thx to Mikael for this one.

Using resources

When using resources the name used behind the @ sign is not the name of the resource file you will find that value in but the type of resource you are refering to. here xxxx is the type and yyyy is the name of the variable you're using. So your layout file would have something like this:

android:background="@xxxx/yyyy"

and your resource file (say mySuperRes.xml) would look something like that:

<xxxx name="yyyy">zzzz</xxxx>