[Chapter 7] Basic Utility Classes

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| Chapter 7 |

# 7. Basic Utility Classes

Contents:
Strings
Math Utilities
Dates
Vectors and Hashtables
Properties
The Security Manager
Internationalization

If you've been reading this book sequentially, you've read all about the core Java language constructs, including the object-oriented aspects of the language and the use of threads. Now it's time to shift gears and talk about the Java Application Programming Interface (API), the collection of classes that comes with every Java implementation. The Java API encompasses all the public methods and variables in the classes that comprise the core Java packages, listed in Table 7.1. This table also lists the chapters in this book that describe each of the packages.

Table 7.1: Packages of the Java API
Package | Contents | Chapter(s)
--- | --- | ---
`java.lang` | Basic language classes | 4, 5, 6, 7
`java.io` | Input and output | 8
`java.util` | Utilities and collections classes | 7
`java.text` | International text classes | 7
`java.net` | Sockets and URLs | 9
`java.applet` | The applet API | 10
`java.awt` | The Abstract Windowing Toolkit | 10, 11, 12, 13, 14
`java.awt.image` | AWT image classes | 13, 14
`java.beans` | Java Beans API |
`java.rmi` | RMI classes |
`java.security` | Encryption and signing |
`java.sql` | JDBC classes |

As you can see in Table 7.1, we've already examined some of the classes in `java.lang` in earlier chapters on the core language constructs. Starting with this chapter, we'll throw open the Java toolbox and begin examining the rest of the classes in the API.

We'll begin our exploration with some of the fundamental language classes in `java.lang`, including strings and math utilities. Figure 7.1 shows the class hierarchy of the `java.lang` package.

#### Figure 7.1: The java.lang package

We cover some of the classes in `java.util`, such as classes that support date and time values, random numbers, vectors, and hashtables. Figure 7.2 shows the class hierarchy of the `java.util` package.

#### Figure 7.2: The java.util package

## 7.1 Strings

In this section, we take a closer look at the Java `String` class (or more specifically, `java.lang.String`). Because strings are used so extensively throughout Java (or any programming language, for that matter), the Java `String` class has quite a bit of functionality. We'll test drive most of the important features, but before you go off and write a complex parser or regular expression library, you should probably refer to a Java class reference manual for additional details.

Strings are immutable; once you create a `String` object, you can't change its value. Operations that would otherwise change the characters or the length of a string instead return a new `String` object that copies the needed parts of the original. Because of this feature, strings can be safely shared. Java makes an effort to consolidate identical strings and string literals in the same class into a shared string pool.

### String Constructors

To create a string, assign a double-quoted constant to a `String` variable:

    String quote = "To be or not to be";

Java automatically converts the string literal into a `String` object. If you're a C or C++ programmer, you may be wondering if `quote` is null-terminated. This question doesn't make any sense with Java strings. The `String` class actually uses a Java character array internally. It's `private` to the `String` class, so you can't get at the characters and change them. As always, arrays in Java are real objects that know their own length, so `String` objects in Java don't require special terminators (not even internally). If you need to know the length of a `String`, use the `length()` method:

    int length = quote.length();

Strings can take advantage of the only overloaded operator in Java, the `+` operator, for string concatenation. The following code produces equivalent strings:

    String name = "John " + "Smith";
    String name = "John ".concat("Smith");

Literal strings can't span lines in Java source files, but we can concatenate lines to produce the same effect:

    String poem =
        "'Twas brillig, and the slithy toves\n" +
        "   Did gyre and gimble in the wabe:\n" +
        "All mimsy were the borogoves,\n" +
        "   And the mome raths outgrabe.\n";

Of course, embedding lengthy text in source code should now be a thing of the past, given that we can retrieve a `String` from anywhere on the planet via a URL. In Chapter 9, Network Programming, we'll see how to do things like:

    String poem =
        (String) new URL
           ("http://server/~dodgson/jabberwocky.txt").getContent();

In addition to making strings from literal expressions, we can construct a `String` from an array of characters:

    char [] data = { 'L', 'e', 'm', 'm', 'i', 'n', 'g' };
    String lemming = new String( data );

Or from an array of bytes:

    byte [] data = { 97, 98, 99 };
    String abc = new String(data, "8859_5");

The second argument to the `String` constructor for byte arrays is the name of an encoding scheme. It's used to convert the given bytes to the string's Unicode characters. Unless you know something about Unicode, you can probably use the form of the constructor that accepts only a byte array; the default encoding scheme will be used.

### Strings from Things

We can get the string representation of most things with the static `String.valueOf()` method. Various overloaded versions of this method give us string values for all of the primitive types:

    String one = String.valueOf( 1 );
    String two = String.valueOf( 2.0f );
    String notTrue = String.valueOf( false );

All objects in Java have a `toString()` method, inherited from the `Object` class (see Chapter 5, Objects in Java). For class-type references, `String.valueOf()` invokes the object's `toString()` method to get its string representation. If the reference is `null`, the result is the literal string "null":

    String date = String.valueOf( new Date() );
    System.out.println( date );
    // Sun Dec 19 05:45:34 CST 1999

    date = null;
    System.out.println( date );
    // null

### Things from Strings

Producing primitives like numbers from `String` objects is not a function of the `String` class. For that we need the primitive wrapper classes; they are described in the next section on the `Math` class. The wrapper classes provide `valueOf()` methods that produce an object from a `String`, as well as corresponding methods to retrieve the value in various primitive forms. Two examples are:

    int i = Integer.valueOf("123").intValue();
    double d = Double.valueOf("123.0").doubleValue();

In the above code, the `Integer.valueOf()` call yields an `Integer` object that represents the value 123. An `Integer` object can provide its primitive value in the form of an `int` with the `intValue()` method.

Although the techniques above may work for simple cases, they will not work internationally. Let's pretend for a moment that we are programming Java in the rolling hills of Tuscany. We would follow the local customs for representing numbers and write code like the following.

    double d = Double.valueOf("1.234,56").doubleValue();    // oops!

Unfortunately, this code throws a `NumberFormatException`. The `java.text` package, which we'll discuss later, contains the tools we need to generate and parse strings in different countries and languages.

The `charAt()` method of the `String` class lets us get at the characters of a `String` in an array-like fashion:

    String s = "Newton";

    for ( int i = 0; i < s.length(); i++ )     System.out.println( s.charAt( i ) );

This code prints the characters of the string one at a time. Alternately, we can get the characters all at once with `toCharArray()`. Here's a way to save typing a bunch of single quotes:

    char [] abcs = "abcdefghijklmnopqrstuvwxyz".toCharArray();

### Comparisons

Just as in C, you can't compare strings for equality with "`==`" because as in C, strings are actually references. If your Java compiler doesn't happen to coalesce multiple instances of the same string literal to a single string pool item, even the expression `"foo"` `==` `"foo"` will return `false`. Comparisons with `<`, `>`, `<=`, and `>=` don't work at all, because Java can't convert references to integers.

Use the `equals()` method to compare strings:

    String one = "Foo";

    char [] c = { 'F', 'o', 'o' };
    String two = new String ( c );

    if ( one.equals( two ) )                // yes

An alternate version, `equalsIgnoreCase()`, can be used to check the equivalence of strings in a case-insensitive way:

    String one = "FOO";
    String two = "foo";

    if ( one.equalsIgnoreCase( two ) )      // yes

The `compareTo()` method compares the lexical value of the `String` against another `String`. It returns an integer that is less than, equal to, or greater than zero, just like the C routine `string()`:

    String abc = "abc";
    String def = "def";
    String num = "123";

    if ( abc.compareTo( def ) < 0 )         // yes
    if ( abc.compareTo( abc ) == 0 )        // yes
    if ( abc.compareTo( num ) > 0 )         // yes

On some systems, the behavior of lexical comparison is complex, and obscure alternative character sets exist. Java avoids this problem by comparing characters strictly by their position in the Unicode specification.

In Java 1.1, the `java.text` package provides a sophisticated set of classes for comparing strings, even in different languages. German, for example, has vowels with umlauts (those funny dots) over them and a weird-looking beta character that represents a double-s. How should we sort these? Although the rules for sorting these characters are precisely defined, you can't assume that the lexical comparison we used above works correctly for languages other than English. Fortunately, the `Collator` class takes care of these complex sorting problems. In the following example, we use a `Collator` designed to compare German strings. (We'll talk about `Locale`s in a later section.) You can obtain a default `Collator` by calling the `Collator.getInstance()` method that has no arguments. Once you have an appropriate `Collator` instance, you can use its `compare()` method, which returns values just like `String`'s `compareTo()` method. The code below creates two strings for the German translations of "fun" and "later," using Unicode constants for these two special characters. It then compares them, using a `Collator` for the German locale; the result is that "later" (spaeter) sorts before "fun" (spass).

    String fun = "Spa\u00df";
    String later = "sp\u00e4ter";
    Collator german = Collator.getInstance(Locale.GERMAN);
    if (german.compare(later, fun) < 0)  // yes

Using collators is essential if you're working with languages other than English. In Spanish, for example, "ll" and "ch" are treated as separate characters, and alphabetized separately. A collator handles cases like these automatically.

### Searching

The `String` class provides several methods for finding substrings within a string. The `startsWith()` and `endsWith()` methods compare an argument `String` with the beginning and end of the `String`, respectively:

    String url = "http://foo.bar.com/";
    if ( url.startsWith("http:") )
        // do HTTP

Overloaded versions of `indexOf()` search for the first occurrence of a character or substring:

    int i = abcs.indexOf( 'p' );            // i = 15
    int i = abcs.indexOf( "def" );          // i = 3

Correspondingly, overloaded versions of `lastIndexOf()` search for the last occurrence of a character or substring.

### Editing

A number of methods operate on the `String` and return a new `String` as a result. While this is useful, you should be aware that creating lots of strings in this manner can affect performance. If you need to modify a string often, you should use the `StringBuffer` class, as I'll discuss shortly.

`trim()` is a useful method that removes leading and trailing white space (i.e., carriage return, newline, and tab) from the `String`:

    String abc = "   abc   ";
    abc = abc.trim();                       // "abc"

In the above example, we have thrown away the original `String` (with excess white space), so it will be garbage collected.

The `toUpperCase()` and `toLowerCase()` methods return a new `String` of the appropriate case:

    String foo = "FOO".toLowerCase();
    String FOO = foo.toUpperCase();

`substring()` returns a specified range of characters. The starting index is inclusive; the ending is exclusive:

    String abcs = "abcdefghijklmnopqrstuvwxyz";
    String cde = abcs.substring(2, 5);      // "cde"

### String Method Summary

Many people complain when they discover the Java `String` class is `final` (i.e., it can't be subclassed). There is a lot of functionality in `String`, and it would be nice to be able to modify its behavior directly. Unfortunately, there is also a serious need to optimize and rely on the performance of `String` objects. As I discussed in Chapter 5, Objects in Java, the Java compiler can optimize `final` classes by inlining methods when appropriate. The implementation of `final` classes can also be trusted by classes that work closely together, allowing for special cooperative optimizations. If you want to make a new string class that uses basic `String` functionality, use a `String` object in your class and provide methods that delegate method calls to the appropriate `String` methods.

Table 7.2 summarizes the methods provided by the `String` class.

Table 7.2: String Methods
Method | Functionality
--- | ---
`charAt()` | Gets at a particular character in the string
`compareTo()` | Compares the string with another string
`concat()` | Concatenates the string with another string
`copyValueOf()` | Returns a string equivalent to the specified character array
`endsWith()` | Checks if the string ends with a suffix
`equals()` | Compares the string with another string
`equalsIgnoreCase()` | Compares the string with another string and ignores case
`getBytes()` | Copies characters from the string into a byte array
`getChars()` | Copies characters from the string into a character array
`hashCode()` | Returns a hashcode for the string
`indexOf()` | Searches for the first occurrence of a character or substring in the string
`intern()` | Fetches a unique instance of the string from a global shared string pool
`lastIndexOf()` | Searches for the last occurrence of a character or substring in a string
`length()` | Returns the length of the string
`regionMatches()` | Checks whether a region of the string matches the specified region of another string
`replace()` | Replaces all occurrences of a character in the string with another character
`startsWith()` | Checks if the string starts with a prefix
`substring()` | Returns a substring from the string
`toCharArray()` | Returns the array of characters from the string
`toLowerCase()` | Converts the string to uppercase
`toString()` | Converts the string to a string
`toUpperCase()` | Converts the string to lowercase
`trim()` | Removes the leading and trailing white space from the string
`valueOf()` | Returns a string representation of a value

### java.lang.StringBuffer

The `java.lang.StringBuffer` class is a growable buffer for characters. It's an efficient alternative to code like the following:

    String ball = "Hello";
    ball = ball + " there.";
    ball = ball + " How are you?";

The above example repeatedly produces new `String` objects. This means that the character array must be copied over and over, which can adversely affect performance. A more economical alternative is to use a `StringBuffer` object and its `append()` method:

    StringBuffer ball = new StringBuffer("Hello");
    ball.append(" there.");
    ball.append(" How are you?");

The `StringBuffer` class actually provides a number of overloaded `append()` methods, for appending various types of data to the buffer.

We can get a `String` from the `StringBuffer` with its `toString()` method:

    String message = ball.toString();

`StringBuffer` also provides a number of overloaded `insert()` methods for inserting various types of data at a particular location in the string buffer.

The `String` and `StringBuffer` classes cooperate, so that even in this last operation, no copy has to be made. The string data is shared between the objects, unless and until we try to change it in the `StringBuffer`.

So, when should you use a `StringBuffer` instead of a `String`? If you need to keep adding characters to a string, use a `StringBuffer`; it's designed to efficiently handle such modifications. You'll still have to convert the `StringBuffer` to a `String` when you need to use any of the methods in the `String` class. You can print a `StringBuffer` directly using `System.out.println()`because `println()` calls the `toString()` for you.

Another thing you should know about `StringBuffer` methods is that they are thread-safe, just like all public methods in the Java API. This means that any time you modify a `StringBuffer`, you don't have to worry about another thread coming along and messing up the string while you are modifying it. If you recall our discussion of synchronization in Chapter 6, Threads, you know that being thread-safe means that only one thread at a time can change the state of a `StringBuffer` instance.

On a final note, I mentioned earlier that strings take advantage of the single overloaded operator in Java, `+`, for concatenation. You might be interested to know that the compiler uses a `StringBuffer` to implement concatenation. Consider the following expression:

    String foo = "To " + "be " + "or";

This is equivalent to:

    String foo = new
      StringBuffer().append("To ").append("be ").append("or").toString();

This kind of chaining of expressions is one of the things operator overloading hides in other languages.

### java.util.StringTokenizer

A common programming task involves parsing a string of text into words or "tokens" that are separated by some set of delimiter characters. The `java.util.StringTokenizer` class is a utility that does just this. The following example reads words from the string `text`:

    String text = "Now is the time for all good men (and women)...";
    StringTokenizer st = new StringTokenizer( text );

    while ( st.hasMoreTokens() )  {
        String word = st.nextToken();
        ...
    }

First, we create a new `StringTokenizer` from the `String`. We invoke the `hasMoreTokens()` and `nextToken()` methods to loop over the words of the text. By default, we use white space (i.e., carriage return, newline, and tab) as delimiters.

The `StringTokenizer` implements the `java.util.Enumeration` interface, which means that `StringTokenizer` also implements two more general methods for accessing elements: `hasMoreElements()` and `nextElement()`. These methods are defined by the `Enumeration` interface; they provide a standard way of returning a sequence of values, as we'll discuss a bit later. The advantage of `nextToken()` is that it returns a `String`, while `nextElement()` returns an `Object`. The `Enumeration` interface is implemented by many items that return sequences or collections of objects, as you'll see when we talk about hashtables and vectors later in the chapter. Those of you who have used the C `strtok()`function should appreciate how useful this object-oriented equivalent is.

You can also specify your own set of delimiter characters in the `StringTokenizer` constructor, using another `String` argument to the constructor. Any combination of the specified characters is treated as the equivalent of white space for tokenizing:

    text = "http://foo.bar.com/";
    tok = new StringTokenizer( text, "/:" );

    if ( tok.countTokens() < 2 )            // bad URL

    String protocol = tok.nextToken();      // protocol = "http" String host = tok.nextToken();          // host = "foo.bar.com"

The example above parses a URL specification to get at the protocol and host components. The characters "/" and ":" are used as separators. The `countTokens()` method provides a fast way to see how many tokens will be returned by `nextToken()`, without actually creating the `String` objects.

An overloaded form of `nextToken()` accepts a string that defines a new delimiter set for that and subsequent reads. And finally, the `StringTokenizer` constructor accepts a flag that specifies that separator characters are to be returned individually as tokens themselves. By default, the token separators are not returned.

Scheduling and Priority |  | Math Utilities