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java interview questions

Top java frequently asked interview questions

Initialization of an ArrayList in one line

I want to create a list of options for testing purposes. At first, I did this:

ArrayList<String> places = new ArrayList<String>();
places.add("Buenos Aires");
places.add("Córdoba");
places.add("La Plata");

Then I refactored the code as follows:

ArrayList<String> places = new ArrayList<String>(
    Arrays.asList("Buenos Aires", "Córdoba", "La Plata"));

Is there a better way to do this?


Source: (StackOverflow)

Java += operator

Until today I thought that for example:

i += j;

is just a shortcut for:

i = i + j;

But what if we try this:

int i = 5;
long j = 8;

Then i = i + j; will not compile but i += j; will compile fine.

Does it mean that in fact i += j; is a shortcut for something like this i = (type of i) (i + j)?


Source: (StackOverflow)

How do I call one constructor from another in Java?

Is it possible to call a constructor from another (within the same class, not from a subclass)? If yes how? And what could be the best way to call another constructor (if there are several ways to do it)?


Source: (StackOverflow)

What is a serialVersionUID and why should I use it?

Eclipse issues warnings when a serialVersionUID is missing.

The serializable class Foo does not declare a static final serialVersionUID field of type long

What is serialVersionUID and why is it important? Please show an example where missing serialVersionUID will cause a problem.


Source: (StackOverflow)

Creating a memory leak with Java [closed]

I just had an interview, and I was asked to create a memory leak with Java. Needless to say I felt pretty dumb having no clue on how to even start creating one.

What would an example be?


Source: (StackOverflow)

Using java.net.URLConnection to fire and handle HTTP requests

Use of java.net.URLConnection is asked about pretty often here, and the Oracle tutorial is too concise about it.

That tutorial basically only shows how to fire a GET request and read the response. It doesn't explain anywhere how to use it to among others perform a POST request, set request headers, read response headers, deal with cookies, submit a HTML form, upload a file, etc.

So, how can I use java.net.URLConnection to fire and handle "advanced" HTTP requests?


Source: (StackOverflow)

How to test a class that has private methods, fields or inner classes?

How do I use JUnit to test a class that has internal private methods, fields or nested classes? It seems bad to change the access modifier for a method just to be able to run a test.


Source: (StackOverflow)

Converting String to Int in Java?

How can I convert a String to an int in Java?

My String contains only numbers and I want to return the number it represents.

For example, given the string "1234" the result should be the number 1234.


Source: (StackOverflow)

Is Java "pass-by-reference" or "pass-by-value"?

I always thought Java was pass-by-reference; however I've seen a couple of blog posts (for example, this blog) that claim it's not. I don't think I understand the distinction they're making.

What is the explanation?


Source: (StackOverflow)

Iterate through a HashMap [duplicate]

Possible Duplicate:
How do I iterate over each Entry in a Collection Map?

What's the best way to iterate over the items in a HashMap?


Source: (StackOverflow)

"implements Runnable" vs. "extends Thread"

From what time I've spent with threads in Java, I've found these two ways to write threads:

With implements Runnable:

public class MyRunnable implements Runnable {
    public void run() {
        //Code
    }
}
//Started with a "new Thread(new MyRunnable()).start()" call

Or, with extends Thread:

public class MyThread extends Thread {
    public MyThread() {
        super("MyThread");
    }
    public void run() {
        //Code
    }
}
//Started with a "new MyThread().start()" call

Is there any significant difference in these two blocks of code ?


Source: (StackOverflow)

Create ArrayList from array

I have an array that is initialized like:

Element[] array = {new Element(1), new Element(2), new Element(3)};

I would like to convert this array into an object of the ArrayList class.

ArrayList<Element> arraylist = ???;

Source: (StackOverflow)

Why does this code using random strings print "hello world"?

The following print statement would print "hello world". Could anyone explain this?

System.out.println(randomString(-229985452) + " " + randomString(-147909649));

And randomString() looks like this:

public static String randomString(int i)
{
    Random ran = new Random(i);
    StringBuilder sb = new StringBuilder();
    while (true)
    {
        int k = ran.nextInt(27);
        if (k == 0)
            break;

        sb.append((char)('`' + k));
    }

    return sb.toString();
}

Source: (StackOverflow)

Proper use cases for Android UserManager.isUserAGoat()?

I was looking at the new APIs introduced in Android 4.2. While looking at the UserManager class I came across the following method:

public boolean isUserAGoat()

Used to determine whether the user making this call is subject to teleportations.

Returns whether the user making this call is a goat.

How and when should this be used?


Source: (StackOverflow)

How can I create an executable JAR with dependencies using Maven?

I want to package my project in a single executable JAR for distribution.

How can I make Maven package all dependency JARs into my JAR?


Source: (StackOverflow)