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java+英文面试题.pdf
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Question: Name the containers which uses Border Layout as their default layout?
Answer: Containers which uses Border Layout as their default are: window, Frame and
Dialog classes.
Question: What do you understand by Synchronization?
Answer: Synchronization is a process of controlling the access of shared resources by the
multiple threads in such a manner that only one thread can access one resource at a time. In
non synchronized multithreaded application, it is possible for one thread to modify a shared
object while another thread is in the process of using or updating the object's value.
Synchronization prevents such type of data corruption.
E.g. Synchronizing a function:
public synchronized void Method1 () {
// Appropriate method-related code.
}
E.g. Synchronizing a block of code inside a function:
public myFunction (){
synchronized (this) {
// Synchronized code here.
}
}
Question: What is Collection API?
Answer: The Collection API is a set of classes and interfaces that support operation on
collections of objects. These classes and interfaces are more flexible, more powerful, and more
regular than the vectors, arrays, and hashtables if effectively replaces.
Example of classes:
HashSet
,
HashMap
,
ArrayList
,
LinkedList
,
TreeSet
and
TreeMap
.
Example of interfaces:
Collection
,
Set
,
List
and
Map.
Question: Is Iterator a Class or Interface? What is its use?
Answer: Iterator is an interface which is used to step through the elements of a Collection.
Question: What is similarities/difference between an Abstract class and Interface?
Answer: Differences are as follows:
Interfaces provide a form of multiple inheritance. A class can extend only one other class.
Interfaces are limited to public methods and constants with no implementation. Abstract
classes can have a partial implementation, protected parts, static methods, etc.
A Class may implement several interfaces. But in case of abstract class, a class may
extend only one abstract class.
Interfaces are slow as it requires extra indirection to to find corresponding method in in
the actual class. Abstract classes are fast.
Similarities:
Neither Abstract classes or Interface can be instantiated.
Question: How to define an Abstract class?
Answer: A class containing abstract method is called Abstract class. An Abstract class can't be
instantiated.
Example of Abstract class:
abstract class testAbstractClass {
protected String myString;
public String getMyString() {
return myString;
}
public abstract string anyAbstractFunction();
}
Question: How to define an Interface?
Answer: In Java Interface defines the methods but does not implement them. Interface can
include constants. A class that implements the interfaces is bound to implement all the
methods defined in Interface.
Emaple of Interface:
public interface sampleInterface {
public void functionOne();
public long CONSTANT_ONE = 1000;
}
Question: Explain the user defined Exceptions?
Answer: User defined Exceptions are the separate Exception classes defined by the user for
specific purposed. An user defined can created by simply sub-classing it to the Exception class.
This allows custom exceptions to be generated (using throw) and caught in the same way as
normal exceptions.
Example:
class myCustomException extends Exception {
// The class simply has to exist to be an exception
}
Question: Explain the new Features of JDBC 2.0 Core API?
Answer: The JDBC 2.0 API includes the complete JDBC API, which includes both core and
Optional Package API, and provides inductrial-strength database computing capabilities.
New Features in JDBC 2.0 Core API:
Scrollable result sets- using new methods in the ResultSet interface allows
programmatically move the to particular row or to a position relative to its current
position
JDBC 2.0 Core API provides the Batch Updates functionality to the java applications.
Java applications can now use the ResultSet.updateXXX methods.
New data types - interfaces mapping the SQL3 data types
Custom mapping of user-defined types (UTDs)
Miscellaneous features, including performance hints, the use of character streams, full
precision for java.math.BigDecimal values, additional security, and support for time
zones in date, time, and timestamp values.
Question: Explain garbage collection?
Answer: Garbage collection is one of the most important feature of Java. Garbage collection
is also called automatic memory management as JVM automatically removes the unused
variables/objects (value is null) from the memory. User program cann't directly free the object
from memory, instead it is the job of the garbage collector to automatically free the objects
that are no longer referenced by a program. Every class inherits finalize() method from
java.lang.Object, the finalize() method is called by garbage collector when it determines no
more references to the object exists. In Java, it is good idea to explicitly assign null into a
variable when no more in use. I Java on calling System.gc() and Runtime.gc(), JVM tries to
recycle the unused objects, but there is no guarantee when all the objects will garbage
collected.
Question: How you can force the garbage collection?
Answer: Garbage collection automatic process and can't be forced.
Question: What is OOPS?
Answer: OOP is the common abbreviation for Object-Oriented Programming.
Question: Describe the principles of OOPS.
Answer: There are three main principals of oops which are called Polymorphism, Inheritance
and Encapsulation.
Question: Explain the Encapsulation principle.
Answer: Encapsulation is a process of binding or wrapping the data and the codes that
operates on the data into a single entity. This keeps the data safe from outside interface and
misuse. One way to think about encapsulation is as a protective wrapper that prevents code
and data from being arbitrarily accessed by other code defined outside the wrapper.
Question: Explain the Inheritance principle.
Answer: Inheritance is the process by which one object acquires the properties of another
object.
Question: Explain the Polymorphism principle.
Answer: The meaning of Polymorphism is something like one name many forms.
Polymorphism enables one entity to be used as as general category for different types of
actions. The specific action is determined by the exact nature of the situation. The concept of
polymorphism can be explained as "one interface, multiple methods".
Question: Explain the different forms of Polymorphism.
Answer: From a practical programming viewpoint, polymorphism exists in three distinct
forms in Java:
从一个实际编程的观点来看 , 多态存在于三种截然不同的形式 ,
Method overloading
Method overriding through inheritance
Method overriding through the Java interface
Question: What are Access Specifiers available in Java?
Answer: Access specifiers are keywords that determines the type of access to the member of a
class. These are:
Public
Protected
Private
Defaults
Question: Describe the wrapper classes in Java.
Answer: Wrapper class is wrapper around a primitive data type. An instance of a wrapper
class contains, or wraps, a primitive value of the corresponding type.
Following table lists the primitive types and the corresponding wrapper classes:
Primitive Wrapper
boolean java.lang.Boolean
byte java.lang.Byte
char java.lang.Character
double java.lang.Double
float java.lang.Float
int java.lang.Integer
long java.lang.Long
short java.lang.Short
void java.lang.Void
Question: Read the following program:
public class test {
public static void main(String [] args) {
int x = 3;
int y = 1;
if (x = y)
System.out.println("Not equal");
else
System.out.println("Equal");
}
}
What is the result?
A. The output is 揈 qual?br> B. The output in 揘 ot Equal?br> C. An error at " if (x =
y)" causes compilation to fall.
D. The program executes but no output is show on console.
Answer: C
Question: what is the class variables ?
什么是类变量?
Answer: When we create a number of objects of the same class, then each object will share a
common copy of variables. That means that there is only one copy per class, no matter how
many objects are created from it. Class variables or static variables are declared with the static
keyword in a class, but mind it that it should be declared outside outside a class. These
variables are stored in static memory. Class variables are mostly used for constants, variable
that never change its initial value. Static variables are always called by the class name. This
variable is created when the program starts i.e. it is created before the instance is created of
class by using new operator and gets destroyed when the programs stops. The scope of the
class variable is same a instance variable. The class variable can be defined anywhere at class
level with the keyword static. It initial value is same as instance variable. When the class
variable is defined as int then it's initial value is by default zero, when declared boolean its
default value is false and null for object references. Class variables are associated with the
class, rather than with any object.
Question: What is the difference between the instanceof and getclass, these two are same or
not ?
Answer: instanceof is a operator, not a function while getClass is a method of
java.lang.Object class. Consider a condition where we use
if(o.getClass().getName().equals("java.lang.Math")){ }
This method only checks if the classname we have passed is equal to java.lang.Math. The
class java.lang.Math is loaded by the bootstrap ClassLoader. This class is an abstract
class.This class loader is responsible for loading classes. Every Class object contains a
reference to the ClassLoader that defines. getClass() method returns the runtime class of an
object. It fetches the java instance of the given fully qualified type name. The code we have
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