Advanced Java Interview Questions And Answers

1. Hashtable is synchronized, whereas HashMap is not. This makes HashMap better for non-threaded applications, as unsynchronized Objects typically perform better than synchronized ones.
2. Hashtable does not allow null keys or values. HashMap allows one null key and any number of null values.
3. One of HashMap’s subclasses is LinkedHashMap, so in the event that you’d want predictable iteration order (which is insertion order by default), you could easily swap out the HashMap for a LinkedHashMap. This wouldn’t be as easy if you were using Hashtable.

• An Error “indicates serious problems that a reasonable application should not try to catch.”
• An Exception “indicates conditions that a reasonable application might want to catch.”

• Use sockets (or channels). Basically one application will open the listening socket and start waiting until it receives some signal. The other application will connect there, and send signals when it had completed something. I’d say this is a preferred way used in 99.9% of applications.
• You can call winapi from Java (on windows).

• is a serializable object (that is, it implements java.io.Serializable, and does so correctly), that
• has “properties” whose getters and setters are just methods with certain names (like, say, getFoo() is the getter for the “Foo” property), and
• has a public 0-arg constructor (so it can be created at will and configured by setting its properties).

There is no syntactic difference between a JavaBean and another class – a class is a JavaBean if it follows the standards.

Dog myDog;

is not a Dog; it’s actually a pointer to a Dog.

So when you have

Dog myDog = new Dog("Rover");
foo(myDog);

you’re essentially passing the address of the created Dog object to the foo method.

1. == never throws NullPointerException

enum Color { BLACK, WHITE };

Color nothing = null;
if (nothing == Color.BLACK);      // runs fine
if (nothing.equals(Color.BLACK)); // throws NullPointerEx

2. == is subject to type compatibility check at compile time

enum Color { BLACK, WHITE };
enum Chiral { LEFT, RIGHT };

if (Color.BLACK.equals(Chiral.LEFT)); // compiles fine
if (Color.BLACK == Chiral.LEFT);      // DOESN'T COMPILE!!! Incompatible types!

• StringBuffer is synchronized, StringBuilder is not. When some thing is synchronized, then multiple threads can access, and modify it with out any problem or side effect. StringBuffer is synchronized, so you can use it with multiple threads with out any problem.
• StringBuilder is faster than StringBuffer because it’s not synchronized. Using synchronized methods in a single thread is overkill.

class StaticInit {
    public static int x;
    static {
        x = 32;
    }
    // other class members such as constructors and
    // methods go here...
}