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面试准备

Core Java 面试题与答案

Test your skills through the online practice test: Core Java Quiz Online Practice Test

问题 186. What restrictions are placed on the location of a package statement within a source code file?

A package statement must appear as the first line in a source code file (excluding blank lines and comments).

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问题 187. Which package is always imported by default?

The java.lang package is always imported by default.

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问题 188. What is a package?

To group set of classes into a single unit is known as packaging. Packages provides wide namespace ability.

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问题 189. Explain the usage of Java packages.

This is a way to 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.

e.g.

java.lang — basic language functionality and fundamental types

java.util — collection data structure classes

java.io — file operations

java.math — multiprecision arithmetics

java.nio — the New I/O framework for Java

java.net — networking operations, sockets, DNS lookups

java.security — key generation, encryption and decryption

java.sql — Java Database Connectivity (JDBC) to access databases

java.awt — basic hierarchy of packages for native GUI components

javax.swing — hierarchy of packages for platform-independent rich GUI components

java.applet — classes for creating an applet

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问题 190. If a class is located in a package, what do you need to change in the OS environment to be able to use it?

You need to add a directory or a jar file that contains the package directories to the CLASSPATH environment variable. Lets say a class Employee belongs to a package com.xyz.hr; and is located in the file c:/dev/com.xyz.hr.Employee.java. In this case, you'd need to add c:/dev to the variable CLASSPATH. If this class contains the method main(), you could test it from a command prompt window as follows:
c:>java com.xyz.hr.Employee

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