Is it possible to call one constructor from another if a class has multiple constructors?

Yes. Use this() syntax.

How constructors are different from other methods?

Constructors should have the same name as the class and they can’t return a value. And also they can be called only once while regular methods could be called many times.

Is it possible to write a Java class that could be used as applet as well as an application?

Yes. Add a main() method to the applet.

What is the difference between the sleep() and wait() methods?

The code sleep(1000); sets thread aside for exactly one second. Whereas the code wait(1000); causes a wait of up to one second.

How can you know if an explicit object casting is needed?

If you assign a superclass object to a variable of a subclass’s data type, you need to do explicit casting. For example:
Object a; Customer b; b = (Customer) a;
When you assign a subclass to a variable having a supeclass type, the casting is performed automatically.

Can garbage collection be forced?

NO. We can’t force garbage collection, but could request it by calling System.gc(). JVM does not guarantee that GC will be started immediately.

What is the use of keyword transient?

Transient keyword indicates that the value of this member variable does not have to be serialized with the object. When the class will be de-serialized, this variable will be initialized with a default value of its data type (i.e. zero for integers).

What is the neccesity of using a synchronized block vs. synchronized method?

Synchronized blocks set locks for shorter periods than synchronized methods.

What is the difference between an interface and an abstract class?

An abstract class is the one, which may contain code in method bodies, which is not allowed in an interface. With abstract classes, you have to inherit your class from it and Java does not allow multiple inheritance. On the other hand, you can implement multiple interfaces in your class.

How could Java classes direct program messages to the system console, but error messages, say to a file?

The class System has a variable that represents the standard output, and the variable err that represents the standard error device. By default, they both point at the system console. This is how the standard output could be re-directed:
Stream st = new Stream(new FileOutputStream(”output.txt”)); System.setErr(st); System.setOut(st);