Java Interview Questions and Answers
What is the purpose of the wait(),
notify(), and notifyAll() methods?
The wait(),notify(), and notifyAll() methods are used to provide an
efficient way for threads to communicate each other.
What are the high-level thread states?
The high-level thread states are ready, running, waiting, and dead.
What is the difference between yielding and sleeping?
When a task invokes its yield() method, it returns to the ready state.
When a task invokes its sleep() method, it returns to the waiting state.
What happens when a thread cannot acquire a lock on an object?
If a thread attempts to execute a synchronized method or synchronized
statement and is unable to acquire an object's lock, it enters the
waiting state until the lock becomes available.
What is the difference between Process and
A process can contain multiple threads. In most multithreading operating
systems, a process gets its own memory address space; a thread doesn't. Threads
typically share the heap belonging to their parent process. For instance, a JVM
runs in a single process in the host O/S. Threads in the JVM share the heap
belonging to that process; that's why several threads may access the same
object. Typically, even though they share a common heap, threads have their own
stack space. This is how one thread's invocation of a method is kept separate
from another's. This is all a gross oversimplification, but it's accurate enough
at a high level. Lots of details differ between operating systems. Process vs.
Thread A program vs. similar to a sequential program an run on its own vs.
Cannot run on its own Unit of allocation vs. Unit of execution Have its own
memory space vs. Share with others Each process has one or more threads vs. Each
thread belongs to one process Expensive, need to context switch vs. Cheap, can
use process memory and may not need to context switch More secure. One process
cannot corrupt another process vs. Less secure. A thread can write the memory
used by another thread
Can an inner class declared inside of a method
access local variables of this method?
It's possible if these variables are final.
What can go wrong if you replace &emp;&emp;
with &emp; in the following code: String a=null; if (a!=null && a.length()>10)
A single ampersand here would lead to a NullPointerException.
What is the Vector class?
The Vector class provides the capability to implement a
growable array of objects
What modifiers may be used with an inner class
that is a member of an outer class?
A (non-local) inner class may be declared as
public, protected, private, static, final, or abstract.
If a method is declared as protected, where may
the method be accessed?
A protected method may only be accessed by classes or interfaces of the
same package or by subclasses of the class in which it is declared.
What is an Iterator interface?
The Iterator interface is used to step through the elements of a
How many bits are used to represent Unicode,
ASCII, UTF-16, and UTF-8 characters?
Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII
character set uses only 7 bits, it is usually represented as 8 bits. UTF-8
represents characters using 8, 16, and 18 bit patterns. UTF-16 uses 16-bit and
larger bit patterns.
What's the main difference between a Vector and
Java Vector class is internally synchronized and ArrayList is not.
What are wrapped classes?
Wrapped classes are classes that allow primitive types to be accessed as
Does garbage collection guarantee that a
program will not run out of memory?
No, it doesn't. It is possible for programs to use up memory resources
faster than they are garbage collected. It is also possible for programs to
create objects that are not subject to garbage collection.
What is the difference between preemptive
scheduling and time slicing?
Under preemptive scheduling, the highest priority task executes until it
enters the waiting or dead states or a higher priority task comes into
existence. Under time slicing, a task executes for a predefined slice of time
and then reenters the pool of ready tasks. The scheduler then determines which
task should execute next, based on priority and other factors.
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