Sunday 20 October 2013

How to take screenshots in Android 4.0 in above.

If you have a shiny new phone with Ice Cream Sandwich(4.0) or above, screenshots are built right into your phone! Just press the Volume Down and Power buttons at the same time, hold them for a second, and your phone will take a screenshot. It'll show up in your Gallery app for you to share with whomever you wish!


It is slightly different for Samsung devices. You need to press and hold

Power Button + Home Button



Monday 14 October 2013

Why overridden methods cannot have more strict access modifier?


 You must have come across a lot of examples in which access modifier of overridden method is less strict. For example is access modifier of a method is private than the overriding method in the subclass can have access modifier defualt(no modifier), protected or private.


One commonly used example is the protected Object clone() method from java.lang.Object class. It is generally overridden as public Object clone().

Most obvious question to follow is why such a behavior?


The answer lies in the question itself. Try to answer a counter question what happens if more strict access modifier is allowed for the overridden method?

Lets take a simple example to identify the problem. Lets say we have two classes Animal and Dog as follows -  

Class Animal :

package animalsData;

public class Animal {
    public void run() {
        System.out.println("Running");
    }
}


Class Dog : 

package animalsData;

public class Dog extends Animal {   
    private void run()
    {
        System.out.println("Dog is running");
    }
}


If  you are using an IDE(Eclipse,Netbeans etc) you will directly see the error. If you are using command line or a simple text editor(gedit, notepad etc) try compiling above code. You will get the following error -



Lets analyze the potential problem in above scenario. Try executing following piece of code

    public static void main(String args[]) {
        Animal animal = new Dog();
        animal.run();
    }


At compile time all java compiler does is see the reference type of the object(Animal in our case) and check whether the method called is available(declared or defined) class of reference type(In our case run() method is present in the Animal class which is the reference type). So there would be no errors in compilation but what happens when we run our code. Runtime class of the object is Dog and the corresponding run() method of Dog class will be invoked(Refer how function overriding is a runtime phenomenon ). But as per the access modifiers private methods must be called from the class only(not by it's instances). As this scenario violates basic principles laid down by java language it is not allowed.

On the other hand access modifier of the overriding method can be relaxed because it makes sense. In our example above if run() method in Animal class was private and that in Dog class was public we could easily execute our main code.[Keeping into consideration access modifier restrictions]

Sunday 13 October 2013

Troubleshooting steps when Eclipse ADT does not recognize your Android device.

Device also does not show up in adb. Try adb devices . It will not show up unless you have enabled USB debugging and installed the right driver.

If you have not even enabled Developer option you may want to do it now

How to enable developers option in your android device?

Before going to the troubleshooting steps make sure following basic checks are passed.

  1. Make sure you have installed Google USB driver. If not you do it from Android SDK manager.

  2. Next make sure you have enabled USB Debugging on your android device. You will find this option in System Settings -> Developer Options -> USB Debugging.
  3. Make sure your device it compatible with the minimum API level you have set on your android project.

Debugging Steps

You need to install proper driver of your device. Only then your Eclipse can detect your device to launch you application on it.

 1. Go to Computer(right click) -> Manage ->  Device Manager.
 2. If your device is categorized in "other devices" or "portable" devices" something is not right. Double click on the device listed in Device manager. Inside it check your driver details.
 3. If there are no driver details shown or the button is not clickable then the driver is not installed. If it is you need to update it.
 4. Windows should automatically do this for you. If not you can click on the icon when it is searching for the device driver and select "install most suitable driver automatically". [You will have to enable automatic update for this]
 5. Either case mentioned in point 3 the end result should be that after installing your driver(mind any suitable driver if your device driver is not available). Your device should be listed under the category "Android USB Devices"
 6. Now you can be sure that ADT would detect your device.

PS : I have mentioned any suitable driver because in my case I had Micromax A69(ICS) and it installed HTC driver. After this Android ADT started detecting my device.

 I am using Windows 7. Attaching a screen shot just for the reference - 


PS : You may also have to manually download and install driver if not found automatically. So when you click update select from local directory.
 

Samsung/ SAFE devices


For example when I tried this with samsung Galaxy S5 driver was not automatically installed. So I downloaded it manually from their site and installed it.



Use Browse my computer option.For samsung driver from above link you will get exe file that will install driver for you.

Moto G/E




Recent I bought Mote E and had the same problem. Again you need to manually download the suitable driver and install.

Remove the img extension and install.

Relate Links


    Friday 11 October 2013

    Overriding Methods in Java - How it works?

    Background


    Method overloading and method overriding are two important concepts in Java. It is very crucial to know the way they are work and the way they are used.

    The main motive of this post is to understand Method overriding and it's usage. Lets quickly understand what both terms mean and move on to the overriding part for details.


    Method Overloading

    Method or Function overloading means two or more functions with same name but different number and type of arguments(Signature). The purpose of this is to provide common functionality but in different possible scenarios. Example -

    public void setEmployeeInfo(String name);
    public void setEmployeeInfo(String name, int age);
    public void  setEmployeeInfo(String name, int age, String id);
    


    Name of the function remains the same but the number and type of arguments changes.

    Note : return type has noting to do with overloading. In fact return type of a method is not a part of it's signature. Only the function name and the arguments form a part of method signature. Also as we know that two methods with same signature are not allowed following scenario is not allowed -

    public String getEmployee(String name);
    public Employee getEmployee(String name);
    



    Note :  Also keep in mind method overloading is a compile time phenomenon. Which if the overloaded method is to be executed is decided at compile time based the method signature.

    Method overloading has some resolution rules that decides which method to pick up at compile time. Compilation will fail if no methods are matched. For example consider following methods -

    • public void getData(String dataName)
    • public void getData(Object dataName)
    Now what happens when you call getData("employeeName"). The one with String argument will be chosen because it is more specific. Anyway that was just a simple example. Things can be more complicated. Refer to following post to see the rules java applies for this resolution -

    Method Overriding

    Method overriding comes into picture only when there is inheritance involved. If there is a method in a super class then a sub class can override it. Let us first see an example and then dive into various aspects of it.
    Lets say we have a Animal class in a package called worldEnteties. It has  a move method as follows.

    package worldEnteties;
    
    
    
    public class Animal {
        
        public void move() {
            System.out.println("Animal is moving");
        }
    }
    

    Also we have a Dog class which extends this Animal class. In this Dog class we override the move() method to do some different movement(Dog specific).

    package worldEnteties;

    public class Dog extends Animal {
       
        @Override
        public void move(){
            System.out.println("Dog is moving");        
        }
    }
    

    Now when you create a normal Animal object and call move() on it, it will simply execute the function in Animal class.

    Code : 

        public static void main(String args[]) {       
            Animal animal = new Animal();
            animal.move();
        }
    

    Output :
    Animal is moving

    Now when we create a Dog object and call move() on it, it will execute the corresponding move() function.

    Code : 

        public static void main(String args[]) {      
            Dog dog = new Dog();
            dog.move();
        }
    
    Output :
    Dog is moving

     Now the question may arise -
    Q ) What if I wast to invoke move() in Animal when move() in Dog is invoked? After all Dog is an Animal. 

    Ans ) The answer is you can call super.move() inside the overridden function to call corresponding function in the superclass. Yes it happens by default in constructors but in normal functions you have to explicitly invoke super.functionName() call. A sample code would be as follows - 

    Modify the overridden move() method in Dog class as follows

     package worldEnteties;
    
    public class Dog extends Animal {
        
        @Override
        public void move(){
            super.move();
            System.out.println("Dog is moving");        
        }
    }
    

    Now execute the following code and examine the results -

    Code :

        public static void main(String args[]) {        
            Dog dog = new Dog();
            dog.move();
        }
    
    Output :
    Animal is moving
    Dog is moving
     

    Method Overriding is a runtime phenomenon!!!!

      So far so good!! Now lets involve polymorphism in it and lets see what makes method overriding a runtime phenomenon.[Always remember polymorphism as superclass reference to subclass object] Consider the following code(No super call just plain simple inheritance and explained in the 1st case above) - 
    code :
        public static void main(String args[]) {        
            Animal dog = new Dog();
            dog.move();
        } 
    

    Output:
    Dog is moving

    How it works?

    At compile time all that java compiler knows is the reference type Animal in our case. All compiler does is check whether move() method is present(or at-least declared) in the Animal class(If not compilation error will occur). As in our case it is present and code compiles fine. Now When we run the code, it is at this time the JVM will know the runtime class of the object [You can also print it using dog.getClass()] which is class Dog in our case. Now JVM will execute the function in Dog and we get our output.

    Code Snap :

     

    Another important point to note is that the access modifier of the overriding function can be liberal! Meaning if access modifier of super class is private than modifier of overriding method is subclass can be default(no modifier), protected or public. It cannot be the other way around(overriding method cannot have more strict access modifier).

    Note : Access to static/instance fields and static methods depend on class of the polymorphic reference variable and not the actual object to which reference point to. Also variables are never overridden. They are shadowed.

    So if you have

    public class A {
        public static String test = "testA";
        public static void test() {
            System.out.println(test);
        }
    }
    


    and

    public class B extends A {
        public static String test = "testB";
        public static void test() {
            System.out.println(test);
        }
    }
    


    and you run

    public class Test {
        public static void main(String args[]) {
            A a  = new B();
            a.test();
        }
    }
    


    it will print testA

    Note : In method overridden return type can be same or subclass of the super class method. Same goes for Exception thrown. Exception thrown by the overridden method can be same or subclass of Exception thrown by method in the super class. However the access modifies for the overridden method should be more strict. (See Rules below)

    Rules for method overriding


    For overriding, the overridden method has a few rules:
    1. The access modifier must be the same or more accessible.
    2. The return type must be the same or a more restrictive type, also known as covariant return types.
    3. If any checked exceptions are thrown, only the same exceptions or subclasses of those
      exceptions are allowed to be thrown.
    4. The methods must not be static. (If they are, the method is hidden and not
      overridden.)

    Related Links


    t> UA-39527780-1 back to top