What is static in java?

Static means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without creating an instance of a class. Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can't override a static method with a nonstatic method. In other words, you can't change a static method into an instance method in a subclass.

What is an abstract class?

Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such. A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.

State the significance of public, private, protected, default modifiers

public : Public class is visible in other packages, field is visible everywhere (class must be public too).

private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature.

protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature.This access is provided even to subclasses that reside in a different package from the class that owns the protected feature.

default :What you get by default ie, without any access modifier (ie, public private or protected).It means that it is visible to all within a particular package.

What is an Iterator?

Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator.

What is the difference between a constructor and a method?

A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator. A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator.

Difference between Swing and Awt?

AWT are heavy-weight componenets. Swings are light-weight components. Hence swing works faster than AWT.

Usage of of Java packages.

Packages is a way of organize files when a project consists of multiple modules. It also helps resolve naming conflicts when different packages have classes with the same names. Packages access level also allows you to protect data from being used by the non-authorized classes.

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);