Java 1.3 & 1.4精要：快速掌握详细语言与类库指南

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《Java Precisely》是由Peter Sestoft编著的一本由麻省理工学院出版社出版的书籍，专为已经或正在学习Java编程语言，特别是Java 1.3和1.4版本的读者设计。该书旨在提供一种快速参考指南，帮助读者深入了解Java语言的细节。它不仅全面介绍了Java编程语言本身，还涵盖了类库中的关键部分，即集合类和输入输出类。本书的独特之处在于其结构：通用规则主要在每一页的左侧呈现，右侧则展示相应的示例。所有示例都是合法Java程序片段，以便读者理解概念的实际应用。此外，书中还提供了完整的可运行示例程序，这些程序可以直接从官方网站<http://www.dina.kvl.dk/~sestoft/javaprecisely/>获取。版权方面，未经麻省理工学院出版社书面许可，任何电子或机械形式的复制（包括复印、录音和信息存储与检索）均受到限制。此书采用Times字体，并在美国印刷和装订。本书分为多个章节，内容详尽： 1. **运行Java：编译、加载与执行** - 这部分将引导读者了解如何准备和执行Java代码，包括编译过程以及Java程序的生命周期。 2. **名称与保留名称** - 讨论Java语言的关键标识符命名规范，以及保留给特定用途的关键字。 3. **Java命名约定** - 探讨Java中的命名策略，确保代码的清晰和一致性。 4. **注释与程序布局** - 强调良好的编程习惯，如编写注释以提高代码可读性，以及代码组织结构的重要性。 5. **类型系统** - 深入解析Java的变量、数据类型和它们在编程中的角色。 6. **变量、参数、字段与作用域** - 解释变量的生命周期和不同类型的访问权限，以及函数参数和成员变量的管理。 7. **字符串处理** - 提供字符串操作的基础知识，包括字符串常量、字符串类以及常用的字符串操作方法。 8. **数组** - 学习如何创建、操作和理解Java数组，包括动态数组和多维数组。 9. **类和对象** - 阐述面向对象编程的核心概念，如类的设计、构造函数、继承、封装和多态等。通过《Java Precisely》，读者能够系统地掌握Java语言的各个方面，并通过实际例子加深理解。无论是初学者还是进阶开发者，这本书都是扩展Java知识库的理想资源。

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Chapter 7: Strings

A string is an object of the predefined class String. A string literal is a sequence of characters within

double quotes: "New York", "A38", "", and so on. Internally, a character is stored as a number using

the Unicode character encoding, whose character codes 0–127 coincide with the old ASCII character

encoding. String literals and character literals may use character escape sequences:

Escape Code Meaning

backspace

horizontal tab

newline

form feed (page break)

carriage return

the double quote character

the single quote character

the backslash character

\ddd

the character whose character code is the three-

digit octal number ddd

\udddd

the character whose character code is the four-digit

hexadecimal number dddd

A character escape sequence represents a single character. Since the letter A has code 65 (decimal),

which is written 101 in octal and 0041 in hexadecimal, the string literal "A\101\u0041" is the same as

"AAA". If s1 and s2 are expressions of type String and v is an expression of any type, then

 s1.length () of type int is the length of s1, that is, the number of characters in s1.

 s1.equals (s2) of type boolean is true if s1 and s2 contain the same sequence of

characters, and false otherwise; equalsIgnoreCase is similar but does not distinguish

lowercase and uppercase.

 s1.charAt (i) of type char is the character at position i in s1, counting from 0. If the

index i is less than 0, or greater than or equal to s1.length (), then

StringIndexOutOfBoundsException is thrown.

 s1.toString () of type String is the same object as s1.

 String.valueOf (v) returns the string representation of v, which can have any primitive

type (section 5.1

) or reference type. When v has reference type and is not null, then it is

converted using v.toString(); if it is null, then it is converted to the string "null". Any

class C inherits from Object a default toString method that produces strings of the form

C@2a5734, where 2a5734 is some memory address, but toString may be overridden to

produce more useful strings.

 s1 + s2 has the same meaning as s1.concat (s2): it constructs the concatenation of s1

and s2, a new String consisting of the characters of s1 followed by the characters of s2.

 s1 + v and v + s1 are evaluated by converting v to a string with String.valueOf (v),

thus using v.toString () when v has reference type, and then concatenating the

resulting strings.

 s1.compareTo(s2) returns a negative integer, zero, or a positive integer, according as s1

precedes, equals, or follows s2 in the usual lexicographical ordering based on the Unicode

character encoding. If s1 or s2 is null, then the exception NullPointerException is thrown.

Method compareToIgnoreCase is similar but does not distinguish lowercase and

uppercase.

 More String methods are described in the Java class library documentation [3].

Example 5: Equality of Strings, and the Subtlety of the (+) Operator

String s1 = "abc";

String s2 = s1 + ""; // New object, but contains same text as s1

String s3 = s1; // Same object as s1

Chapter 8: Arrays

An array is an indexed collection of variables, called elements. An array has a given length ℓ

≥

0 and a

given element type t. The elements are indexed by the integers 0, 1,...,ℓ - 1. The value of an

expression of array type u[] is either null or a reference to an array whose element type t is a

subtype of u. If u is a primitive type, then t must equal u.

8.1 Array Creation and Access

A new array of length ℓ with element type t is created (allocated) using an array creation expression:

new t[ℓ]

where ℓ is an expression of type int. If type t is a primitive type, all elements of the new array are

initialized to 0 (when t is byte, char, short, int, or long) or 0.0 (when t is float or double) or

false (when t is boolean). If t is a reference type, all elements are initialized to null.

If ℓ is negative, then the exception NegativeArraySizeException is thrown.

Let a be a reference of array type u[], to an array with length ℓ and element type t. Then

 a.length of type int is the length

ℓ

of a, that is, the number of elements in a.

 The array access expression a[i] denotes element number i of a, counting from 0; this

expression has type u. The integer expression i is called the array index. If the value of i

is less than 0 or greater than or equal to a.length, then exception

ArrayIndexOutOfBoundsException is thrown.

 When t is a reference type, every array element assignment a[i] = e checks that the

value of e is null or a reference to an object whose class C is a subtype of the element

type t. If this is not the case, then the exception ArrayStoreException is thrown. This

check is made before every array element assignment at run-time, but only for reference

types.

8.2 Array Initializers

A variable or field of array type may be initialized at declaration, using an existing array or an array

initializer for the initial value. An array initializer is a comma-separated list of zero or more expressions

enclosed in braces { ... }:

t[] x = { expression, ..., expression }

The type of each expression must be a subtype of t. Evaluation of the initializer causes a distinct new

array, whose length equals the number of expressions, to be allocated. Then the expressions are

evaluated from left to right and their values are stored in the array, and finally the array is bound to x.

Hence x cannot occur in the expressions: it has not been initialized when they are evaluated.

Array initializers may also be used in connection with array creation expressions:

new t[] { expression, ..., expression }

Multidimensional arrays can have nested initializers (example 14

). Note that there are no array

constants: a new distinct array is created every time an array initializer is evaluated.

Example 10: Creating and Using One-Dimensional Arrays

The first half of this example rolls a die one thousand times, then prints the frequencies of the outcomes.

The second half creates and initializes an array of String objects.

int[] freq = new int[6]; // All initialized to 0

for (int i=0; i<1000; i++) { // Roll dice, count frequencies

int die = (int) (1 + 6 * Math.random());

freq[die-1] += 1;

}

for (int c=1; c<=6; c++)

System.out.println(c + " came up " + freq[c-1] + " times");

String[] number = new String[20]; // Create array of null elements

for (int i=0; i<number.length; i++) // Fill with strings "A0", ..., "A19"

number[i] "A" + i;

for (int i=0; i<number.length; i++) // Print strings

System.out.println(number[i]);

Example 11: Array Element Assignment Type Check at Run-Time

This program compiles, but at run-time a[2] =d throws ArrayStoreException, since the class of the object

bound to d (that is, Double) is not a subtype of a's element type (that is, Integer).

Number[] a = new Integer[10]; // Length 10, element type Integer

Double d = new Double(3.14); // Type Double, class Double

Integer i = new Integer(117); // Type Integer, class Integer

Number n = i; // Type Number, class Integer

a[0] = i; // OK, Integer is subtype of Integer

a[1] = n; // OK, Integer is subtype of Integer

a[2] = d; // No, Double not subtype of Integer

Example 12: Using an Initialized Array

Method checkdate here behaves the same as checkdate in example 2

. The array should be declared

outside the method, otherwise a distinct new array is created for every call to the method.

static int[] days = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };

static boolean checkdate(int mth, int day)

{ return (mth >= 1) && (mth <= 12) && (day >= 1) && (day <= days[mth-1]); }

Example 13: Creating a String from a Character Array

When replacing character cl by character c2 in a string, the result can be built in a character array

because its length is known. This is 50 percent faster than example 85

which uses a string buffer.

static String replaceCharChar(String s, char cl, char c2) {

char[] res = new char[s.length()];

for (int i=0; i<s.length(); i++)

if (s.charAt(i) == cl)

res [i] = c2;

else

res[i] = s.charAt(i);

return new String(res); // A string containing the characters of res

}

8.3 Multidimensional Arrays

The types of multidimensional arrays are written t[] [], t[] [] [], and so on. A rectangular n-

dimensional array of size ℓ

ℓ

...

ℓ

is created (allocated) using the array creation expression

new t[ℓ

] [ℓ

] ... [ℓ

]

A multidimensional array a of type t[] [] is in fact a one-dimensional array of arrays; its component

arrays have type t[]. Hence a multidimensional array need not be rectangular, and one need not

create all the dimensions at once. To create only the first k dimensions of size ℓ

ℓ

...

ℓ

of an n-

dimensional array, leave the (n

–

k) last brackets empty:

new t[ℓ

] [ℓ

] ... [ℓ

] [] ... []

To access an element of an n-dimensional array a, use n index expressions: a[i

] [i

] ... [i

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