Java Questions

When creating a thread by extending the Thread class, it is not mandatory to override the run method (If we are not overriding the run method , it is useless), because Thread class have already given a default implementation for run method. But if we are implementing Runnable , it is mandatory to override the run method. The preferred way to create a thread is by implementing Runnable interface, because it give loose coupling.

ClassName refName = new ClassName();

or

ClassName refName;

refName = new ClassName();

When an exception is thrown , the catch block of the try statement are examined in the order in which they appear. The first catch block that is capable of handling the exception is executed. The remaining catch blocks are ignored

A thread can be created by subclassing Thread, or by implementing the Runnable interface.

Array is collection of same data type. Array size is fixed, It cannot be expanded.

But ArrayList is a growable collection of objects.ArrayList is a part of Collections Framework and can work with only objects.

- The Dictionary class is an abstract class.

- Classes like Hashtable which map keys to values inherit from this class. 

- An object can be looked for if a dictionary and a key is provided.

- Any non-null object can be used as a key and as a value.

Java uses sand box security model.

new ClassName();

This expression creates object in heap area and returns that obj address. Here JVM creates object but JVM does not returns object original address. JVM returns duplicate address, duplicate address is also called as hashcode.