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java interview questions

Top java frequently asked interview questions

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 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)

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)

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)

Why is subtracting these two times (in 1927) giving a strange result?

If I run the following program, which parses two date strings referencing times one second apart and compares them:

public static void main(String[] args) throws ParseException {
    SimpleDateFormat sf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");  
    String str3 = "1927-12-31 23:54:07";  
    String str4 = "1927-12-31 23:54:08";  
    Date sDt3 = sf.parse(str3);  
    Date sDt4 = sf.parse(str4);  
    long ld3 = sDt3.getTime() /1000;  
    long ld4 = sDt4.getTime() /1000;
    System.out.println(ld4-ld3);
}

The output is:

353

Why is ld4-ld3 not 1 (as I would expect from the one-second difference in the times), but 353?

If I change the dates to times one second later:

String str3 = "1927-12-31 23:54:08";  
String str4 = "1927-12-31 23:54:09";  

Then ld4-ld3 will be 1.


Java version:

java version "1.6.0_22"
Java(TM) SE Runtime Environment (build 1.6.0_22-b04)
Dynamic Code Evolution Client VM (build 0.2-b02-internal, 19.0-b04-internal, mixed mode)

Timezone(TimeZone.getDefault()):

sun.util.calendar.ZoneInfo[id="Asia/Shanghai",
offset=28800000,dstSavings=0,
useDaylight=false,
transitions=19,
lastRule=null]

Locale(Locale.getDefault()): zh_CN

Source: (StackOverflow)

Read/convert an InputStream to a String

If you have java.io.InputStream object, how should you process that object and produce a String?


Suppose I have an InputStream that contains text data, and I want to convert this to a String (for example, so I can write the contents of the stream to a log file).

What is the easiest way to take the InputStream and convert it to a String?

public String convertStreamToString(InputStream is) { 
    // ???
}

Source: (StackOverflow)

Why is char[] preferred over String for passwords in Java?

In Swing, the password field has a getPassword() (returns char[]) method instead of the usual getText() (returns String) method. Similarly, I have come across a suggestion not to use String to handle passwords.

Why does String pose a threat to security when it comes to passwords? It feels inconvenient to use char[].


Source: (StackOverflow)

When to use LinkedList over ArrayList?

I've always been one to simply use:

List<String> names = new ArrayList<String>();

I use the interface as the type name for portability, so that when I ask questions such as these I can rework my code.

When should LinkedList be used over ArrayList and vice-versa?


Source: (StackOverflow)

Difference between public, default, protected, and private?

Are there clear rules on when to use each of these when making classes and interfaces and dealing with inheritance?


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)