Java编程基础：面向对象选项详解

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《Java编程基础：面向对象选项 - 卡林·加莱利乌》是一本专为国际文凭计算机科学考试（IBCS Exam）中的面向对象编程（OOP）选项编写的教材。作者卡林·加莱利乌博士通过这本书，为初学者系统地讲解了Java编程的基础知识。该书于2020年首次出版，可在Lulu网站获取。在本书的开篇，作者首先介绍了运行HelloWorld Java程序的三种选择： 1. **在线Java编译器**：这是最简单的方式，通过网络平台可以直接编译并运行Java代码。 2. **NetBeans集成开发环境（IDE）**：提供了全面的开发工具，适合初学者逐步熟悉Java环境和项目管理。 3. **命令行接口（CLI）**：使用javac.exe和java.exe命令行工具，对于理解和底层原理很有帮助。接着，作者探讨了为何看似简单的HelloWorld程序在Java中显得复杂，通过对比BASIC、C语言版本和Java版本的HelloWorld程序，解释了语言特性和语法差异。这有助于读者理解Java的独特之处和设计哲学。第三部分深入到变量的概念，包括基本数据类型（如int, float等）、字符串（text strings）以及常量（constants）。这些是编程的基本构建块，对理解程序逻辑至关重要。第四章讨论如何从键盘接收用户输入（data input），这对于交互式程序设计和用户界面开发非常重要。这通常涉及到Scanner类的使用，演示了如何处理用户输入并将其与程序逻辑结合。此外，书中还会涵盖其他关键主题，如类（Classes）、对象（Objects）、封装（Encapsulation）、继承（Inheritance）和多态（Polymorphism）等面向对象编程的核心概念。每个章节都可能包括示例代码、练习题和深入的理论解释，旨在帮助读者逐步掌握Java编程语言及其在OOP环境下的应用。通过阅读《Foundations of Java Programming for the OOP Option of the IBCS Exam》，学习者不仅能够掌握基础知识，还能为IBCS考试做好准备，同时培养出良好的编程习惯和面向对象的思维方式。

Călin Galeriu Foundations of Java Programming for the OOP Option of the IB CS Exam

the kbdInput object, which is done with the help of the new keyword and the Scanner(System.in)

method (function) . The Scanner() function, with the same name as the Scanner class, is called the

constructor of this class. System.in is an input parameter, a Java object that identifies the computer

keyboard as the source of the alphanumeric stream of data.

Once the kbdInput object is created (and therefore exists in RAM), we can use some of its methods to

input data. The program demonstrates the use of the nextByte() , nextShort() , nextInt() , nextLong() ,

nextFloat() , nextDouble() , next() , and nextBoolean() methods, which return variables of type byte ,

short , int , long , float , double , an object of type String , and a variable of type boolean . Please

notice how kbdInput , the name of the object, shows up in front of each method, and how the two are

connected by a dot. The object returned by the kbdInput.next() function is of type String , and the

charAt(0) method of the String class selects the first alphanumeric character of a given text string. This

character is what the kbdInput.next().charAt(0) expression returns as a value.

What happens if instead of typing 100<ENTER> you type 100 3000<ENTER> ? This shows that

everything you type on the computer keyboard is saved in a buffer, which is a dedicated memory space.

This keyboard input buffer is then checked by the methods of the Scanner class.

What happens if instead of typing 100<ENTER> you type z<ENTER> ? There are ways to prevent the

program from crashing due to invalid input. The Scanner class has methods that verify the type of the

alphanumeric input received. Nonetheless, in order to keep the Java programs short and simple, we will

assume that the users know what they are supposed to do, and that they always input some valid

information.

The next program, Java Example 6, uses the Scanner class to read a text string (a word) from the

keyboard. The word consists of all the alphanumeric characters that show up in the typed text string

before the special delimiter character (usually the empty space character) or before the end of the text

string, signaled by <ENTER> . The program demonstrates the use of the next() method, which returns

an object of type String .

// Java Example 6

import java.util.Scanner;

public class Main

{

public static void main(String[] args)

{

Scanner kbdInput = new Scanner(System.in);

System.out.print("String word = ? "); // input : Happy New Year!

String word = kbdInput.next();

System.out.println("word = " + word); // output : Happy

}

What happens if you try to enter an empty text string, or an empty space?

The next program, Java Example 7, uses the Scanner class to read a text string (a sentence) from the

keyboard. The sentence consists of all the alphanumeric characters that show up in the typed text string

before the end of the text string, signaled by <ENTER>. The program demonstrates the use of the

nextLine() method, which returns an object of type String .

Călin Galeriu Foundations of Java Programming for the OOP Option of the IB CS Exam

The Java logical operators return a result (of type boolean) that can only be true or false.

Warning! It is important to realize that the logical operators, just like the arithmetic operators, are

evaluated according to a predetermined order of operations: NOT is evaluated first, AND is evaluated

next, and OR is evaluated last.

Since using the order of operations for logical operators is not something very intuitive to humans, it is

good programming practice to use parentheses in order to clarify the meaning of more complex boolean

expressions. For example,

write X = [(NOT A) AND B] OR [A AND (NOT B)] (in Java: x = ((!A) && B) || (A && (!B)); )

instead of X = NOT A AND B OR A AND NOT B (in Java: x = !A && B || A && !B; )

even though, from the computer's perspective, it is the same thing.

The next program, Java Example 11, evaluates some simple boolean expressions (logical statements

that can only be true or false) that use logical operators.

// Java Example 11

public class Main

{

public static void main(String[] args)

{

boolean a = false;

boolean b = true;

System.out.println(a && b); // output : false

System.out.println(a || b); // output : true

System.out.println(!a); // output : true

}

The input-output tables below give the boolean values of all the simple boolean expressions with logical

operators. These tables have to be memorized. One way to memorize these tables is to think of false as

0, to think of true as 1 (or 2), to think of AND as multiplication, and to think of OR as addition.

Java has more operators (bitwise operators, assignment operators, etc.) but they are not required for the

IB Computer Science exam. For a list of all the Java operators and their order of operations, please go

to: https://introcs.cs.princeton.edu/java/11precedence/

X AND Y

false

true

false

true

false

true

X OR Y

false

true

false

true

NOT X

false

true

false

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