Differences

Android :

1. Differentiate Activities from Services.

Activities can be closed, or terminated anytime the user wishes. On the other hand, services are designed to run behind the scenes, and can act independently. Most services run continuously, regardless of whether there are certain or no activities being executed.

2. Difference between Stack and Heap in JAVA
Here are few differences between stack and heap memory in Java:

1) Main difference between heap and stack is that stack memory is used to store local variables and function call, while heap memory is used to store objects in Java. No matter, where object is created in code e.g. as member variable, local variable or class variable,  they are always created inside heap space in Java.

2) Each Thread in Java has there own stack which can be specified using -Xss JVM parameter, similarly you can also specify heap size of Java program using JVM option -Xms and -Xmx where -Xms is starting size of heap and -Xmx is maximum size of java heap. to learn more about JVM options see my post 10 JVM option Java programmer should know.

3) If there is no memory left in stack for storing function call or local variable, JVM will throw java.lang.StackOverFlowError, while if there is no more heap space for creating object, JVM will throw java.lang.OutOfMemoryError: Java Heap Space. Read more about how to deal with java.lang.OutOfMemoryError  in my post 2 ways to solve OutOfMemoryError in Java.

4) If you are using Recursion, on which method calls itself, You can quickly fill up stack memory. Another difference between stack and heap is that size of stack memory is lot lesser than size of  heap memory in Java.

5) Variables stored in stacks are only visible to the owner Thread, while objects created in heap are visible to all thread. In other words stack memory is kind of private memory of Java Threads, while heap memory is shared among all threads.

3. Difference between Thread and Process in java.

1) Both process and Thread are independent path of execution but one process can have multiple Threads.

2) Every process has its own memory space, executable code and a unique process identifier (PID) while every thread has its own stack in Java but it uses process main memory and share it with other threads.

3) Threads are also refereed as task or light weight process (LWP) in operating system

4) Threads from same process can communicate with each other by using Programming language construct like wait and notify in Java and much simpler than inter process communication.

5) Another difference between Process and Thread in Java is that it's How Thread and process are created. It's easy to create Thread as compared to Process which requires duplication of parent process.

6) All Threads which is part of same process share system resource like file descriptors , Heap Memory and other resource but each Thread has its own Exception handler and own stack in Java.

 4.Differences between String, StringBuffer and StringBuilder in java
1) String is immutable while StringBuffer and StringBuilder is mutable object.
2) StringBuffer is synchronized while StringBuilder is not which makes StringBuilder faster than StringBuffer.
3) Concatenation operator "+" is internal implemented using either StringBuffer or StringBuilder.
4) Use String if you require immutability, use Stringbuffer in java if you need mutable + thread-safety and use StringBuilder in Java if you require mutable + without thread-safety.

5.What is differences between BaseAdapter and ArrayAdapter.
BaseAdapter is a very generic adapter that allows you to do pretty much whatever you want. However, you have to do a bit more coding yourself to get it working.

ArrayAdapter is a more complete implementation that works well for data in arrays or ArrayLists. Similarly, there is a related CursorAdapter that you should use if your data is in a Cursor. Both of these extend BaseAdapter.

6. What is difference between Service and IntentService in android ?

Below are some key differences between Service and IntentService.

When to use?

• The Service can be used in tasks with no UI, but shouldn't be too long. If you need to perform long tasks, you must use threads within Service.
• The IntentService can be used in long tasks usually with no communication to Main Thread. If communication is required, can use Main Thread handler or broadcast intents. Another case of use is when callbacks are needed (Intent triggered tasks).

How to trigger?

• The Service is triggered by calling method startService().
• The IntentService is triggered using an Intent, it spawns a new worker thread and the method onHandleIntent() is called on this thread.

Triggered From

• The Service and IntentService may be triggered from any thread, activity or other application component.

Runs On

• The Service runs in background but it runs on the Main Thread of the application.
• The IntentService runs on a separate worker thread.

Limitations / Drawbacks

• The Service may block the Main Thread of the application.
• The IntentService cannot run tasks in parallel. Hence all the consecutive intents will go into the message queue for the worker thread and will execute sequentially.

When to stop?

• If you implement a Service, it is your responsibility to stop the service when its work is done, by calling stopSelf() or stopService(). (If you only want to provide binding, you don't need to implement this method).
• The IntentService stops the service after all start requests have been handled, so you never have to call stopSelf().

In short

A Service is a broader implementation for the developer to set up background operations, while an IntentService is useful for “fire and forget” operations, taking care of background Thread creation and cleanup.



7. What is difference between Padding and Margin in android ?

Padding is for inside/within components. Eg. TextView , Button, EditText etc.
Eg. space between the Text and Border


Margin is to be applied for the on-outside of the components.
Eg. space between left edge of the screen and border of your component


8. What is difference between Gravity and layout_gravity in android ?

• android:gravity sets the gravity of the content of the View its used on.(Inside)
• android:layout_gravity sets the gravity of the View or Layout in its parent. (Outsid)

9. What is difference between Service, Thread, IntentService and AsyncTask in android?

	Service	Thread	IntentService	AsyncTask
When to use ?	Task with no UI, but shouldn't be too long. Use threads within service for long tasks.	- Long task in general. - For tasks in parallel use Multiple threads (traditional mechanisms)	- Long task usually with no communication to main thread. (Update)- If communication is required, can use main thread handler or broadcast intents - When callbacks are needed (Intent triggered tasks).	- Small task having to communicate with main thread. - For tasks in parallel use multiple instances OR Executor
Trigger	Call to method onStartService()	Thread start() method	Intent	Call to method execute()
Triggered From (thread)	Any thread	Any Thread	Main Thread (Intent is received on main thread and then worker thread is spawed)	Main Thread
Runs On (thread)	Main Thread	Its own thread	Separate worker thread	Worker thread. However, Main thread methods may be invoked in between to publish progress.
Limitations / Drawbacks	May block main thread	- Manual thread management - Code may become difficult to read	- Cannot run tasks in parallel. - Multiple intents are queued on the same worker thread.	- one instance can only be executed once (hence cannot run in a loop)  - Must be created and executed from the Main thread