Java for C++ Programmers

Contents

• Introduction
• A First Example
• Values, Objects, and Pointers
• Garbage Collection
• Static, Final, Public, and Private
• Arrays
• Strings
• Constructors and Overloading
• Inheritance, Interfaces, and Casts
• Exceptions
• Threads
• Input and Output
• Other Goodies

The purpose of these notes is to help 537 students learn enough about Java to do the course projects. All of the students have experience programming in C++, but practically none of whom have used Java.

What is Java?

Before we turn to the details, it is worth considering why Java has created such a stir. Much of it has nothing to do with the language per se but with the way it is intended to be used.

To understand this point, you have to know a little bit about the World-Wide Web. The Web consists of a set of servers scattered around the Internet, each of which maintains a set of pages. Each page has a ``name'' called a Universal Resource Locator (URL) that uniquely distinguishes it from any other page in the world. The URL starts with the name of the server. For example, the URL of this document, http://www.cs.wisc.edu/~cs537-1/java-tutorial.html, says that it can be retrieved by using the HyperText Transfer Protocol (http) to connect to server www.cs.wisc.edu and ask it for the page it calls /~cs537-1/java-tutorial.html.

Each page is written in a combination of text and formatting directives called the HyperText Markup Language (HTML). A browser is a program that runs on a personal computer or workstation that knows how to display HTML documents ( Netscape Navigator is currently the most popular browser by far, but Microsoft and others are trying to change that). HTML is fairly simple. For example, the directive <p> is used to start a paragraph, and text is set in Italics by surrounding it with <i> and </i>. The most important directive precedes some text with <a name="URL> and follows it with </a>. The browser underlines the text, and when the user clicks on it with the mouse, the browser goes and gets the page named by the URL and displays it. To see more examples of HTML, take a look at this document. If you are using Netscape Navigator, select ``Document Source'' from the ``View'' pull-down menu at the top of the window.

No sooner had HTML caught on that people wanted to extend it. Directives were added to show text in different colors and sizes, or make it blink. A set of directives was added to display tables and images .

Next came sound and movies. The possibilities seemed endless.

There was one big problem, however: If you created a document using a new feature, it couldn't be displayed by a browser that didn't ``understand'' that feature. Thus for a feature to be useful, it had to be implemented by all (or at least a substantial fraction of) the millions of browsers ``out there''.

Then someone got a bright idea: How about including with each document a program capable of displaying it? The browser wouldn't have to ``understand'' the document. It could just run the executable code that came with it, and that code would know how to display the document. Unfortunately, this idea has two very serious problems.

The first problem is that browsers run on all sorts of computers, running all sorts of operating systems. If I include with my document a program designed to run on a SparcStation under Solaris, it will be useless on a Pentium running Windows 95. The second program concerns security. Running programs from random untrusted sources on your own computer is asking for trouble. That's what viruses are all about. Instead of displaying the document, the program may clear your disk or look for private information and send it back over the network to a bad guy.

Fortunately, there is another idea that solves both of these problems. Instead of including a machine-language program with a document, send a program written for a virtual machine that is simulated by a software interpreter. Arrange to have each browser include an interpreter for the virtual machine. In a sense, this just re-introduces the previous problem of requiring a new feature to be implemented by everyone's browser, but only one last time. Once you've convinced all the browser vendors to implement an interpreter for the virtual machine, they won't have to implement any more extensions.

The interpreter can also include security safeguards. For example, if it doesn't implement any operation to open files, you don't have to worry about a mischievous program stealing your data. It is very difficult, if not impossible, to build such safeguards around machine-language programs, because such programs can contain ``bugs'' that are actually devious tricks to circumvent security restrictions. A subscript out of bounds may actually be an attempt to read some data stored outside the memory space allocated to the program.

The Java language was specially designed as a ``cleaned up'' version of C++ with these loopholes removed. If you assign random garbage to a C++ pointer variable and then dereference it, you will get whatever is stored in memory at that address. For example, the following fetches whatever happens to be store at location 1000 in memory:

    int *p = (int *)1000;
    int x = *p;

Java prevents you from doing this sort of thing, whether accidentally or on purpose, by a combination of strong type checking and runtime checks. A pointer variable is always initialized to null, and you can only assign other pointer values (not integers or other data) to it. If you attempt to dereference a null pointer, you get a runtime exception.

Because Java was designed primarily to build active, multimedia, interactive documents, its standard runtime library has lots of features to aid in creating user interfaces. There are standard classes to create scrollbars, pop-up menus, etc. There are special facilities for manipulating URL's and network connections. We will not be using any of these features. On the other hand, there is one thing operating systems and user interfaces have in common: They both require multiple, cooperating threads of control. We will be using these features in this course.

Applications vs Applets

The first thing you have to decide when writing a Java program is whether you are writing an application or an applet. An applet is piece of code designed to display a part of a document. It is run by a browser in response to an HTML directive in the document. We will not be writing any applets in this course.

An application is a stand-alone program. All of our programs will be applications.

A First Example

Large parts of Java are identical to C++. For example, the following procedure, which sorts an array of integers using insertion sort, is exactly the same in C++ or Java.

/** Sort the array a[] in ascending order
 ** using an insertion sort.
 */
void sort(int a[], int size) {
    for (int i = 1; i < size; i++) {
        // a[0..i-1] is sorted
        // insert a[i] in the proper place
        int x = a[i];
        int j;
        for (j = i-1; j >=0; --j) {
            if (a[j] <= x)
                break;
            a[j+1] = a[j];
        }
        // now a[0..j] are all <= x
        // and a[j+2..i] are > x
        a[j+1] = x;
    }
}

[This example could have been in either green or blue!].

Note that the syntax of control structures (such as for and if), assignment statements, variable declarations, and comments are all the same in Java as in C++.

To test this procedure in a C++ program, we might use a ``main program'' like this:

#include <iostream.h>
#include <stdlib.h>
extern "C" int random();

/** Test program to test sort */
int main(int argc, char *argv[]) {
    if (argc != 2) {
        cerr << "usage: sort array-size" << endl;
        exit(1);
    }
    int size = atoi(argv[1]);
    int *test = new int[size];
    for (int i = 0; i < size; i++)
        test[i] = random() % 100;
    cout << "before" << endl;
    for (int i = 0; i < size; i++)
        cout << " " << test[i];
    cout << endl;

    sort(test, size);

    cout << "after" << endl;
    for (int i = 0; i < size; i++)
        cout << " " << test[i];
    cout << endl;
    return 0;
}

A Java program to test the sort procedure is different in a few ways. Here is a complete Java program using the sort procedure.

import java.io.*;
import java.util.Random;

class SortTest {
    //+ sort
    /** Sort the array a[] in ascending order
     ** using an insertion sort.
     */
    static void sort(int a[], int size) {
        for (int i = 1; i < size; i++) {
            // a[0..i-1] is sorted
            // insert a[i] in the proper place
            int x = a[i];
            int j;
            for (j = i-1; j >=0; --j) {
                if (a[j] <= x)
                    break;
                a[j+1] = a[j];
            }
            // now a[0..j] are all <= x
            // and a[j+2..i] are > x
            a[j+1] = x;
        }
    }
    //- sort

    //+ main
    /** Test program to test sort */
    public static void main(String argv[]) {
        if (argv.length != 1) {
            System.out.println("usage: sort array-size");
            System.exit(1);
        }
        int size = Integer.parseInt(argv[0]);
        int test[] = new int[size];
        Random r = new Random();

        for (int i = 0; i < size; i++)
            test[i] = (int)(r.nextFloat() * 100);
        System.out.println("before");
        for (int i = 0; i < size; i++)
            System.out.print(" " + test[i]);
        System.out.println();

        sort(test, size);

        System.out.println("after");
        for (int i = 0; i < size; i++)
            System.out.print(" " + test[i]);
        System.out.println();

        System.exit(0);
    }
    //- main
}

A copy of this program is available in ~cs537-1/public/examples/SortTest.java. To try it out, create a new directory and copy the example to a file named SortTest.java in that directory. (The file must be called SortTest.java!)

    mkdir test1
    cd test1
    cp ~cs537-1/public/examples/SortTest.java SortTest.java
    javac SortTest.java
    java SortTest 10

The javac command invokes the Java compiler on the source file SortTest.java. If all goes well, it will create a file named SortTest.class, which contains code for the Java virtual machine. The java command invokes the Java interpreter to run the code for class SortTest. Note that the first parameter is SortTest, not SortTest.class or SortTest.java because it is the name of a class, not a file.

There are several things to note about this program. First, Java has no ``top-level'' or ``global'' variables or functions. A Java program is always a set of class definitions. Thus, we had to make sort and main member functions (called ``methods'' in Java) of a class, which we called SortTest.

Second, the main function is handled somewhat differently in Java from C++. In C++, the first function to be executed is always a function called main, which has two arguments and returns an integer value. The return value is the ``exit status'' of the program; by convention, a status of zero means ``normal termination'' and anything else means something went wrong. The first argument is the number of words on the command-line that invoked the program, and the second argument is a an array of character strings (denoted char *argv[] in C++) containing those words. If we invoke the program by typing

    sort 10

In Java, the first thing executed is the method called main of the indicated class (in this case SortTest). The main method does not return any value (it is of type void). For now, ignore the words ``public static'' preceding void. We will return to these later. The main method takes only one parameter, an array of strings (denoted String argv[] in Java). This array will have one element for each word on the command line following the name of the class being executed. Thus in our example call,

    java SortTest 10

The third difference to note is the way I/O is done in Java. System.out in Java is roughly equivalent to cout in C++ (or stdout in C), and

    System.out.println(whatever);

    cout << whatever << endl;

Our C++ program used three functions from the standard library, atoi, random, and exit. Integer.parseInt does the same thing as atoi: It converts the character-string "10" to the integer value ten, and System.exit(1) does the same thing as exit(1): It immediately terminates the program, returning an exit status of 1 (meaning something's wrong). The library class Random defines random-number generators. The statement Random r = new Random() create an instance of this class, and r.nextFloat() uses it to generate a floating point number between 0 and 1. The cast (int) means the same thing in Java as in C++. It converts its floating-point argument to an integer, throwing away the fraction.

Finally, note that the #include directives from C++ have been replaced by import declarations. Although they have roughly the same effect, the mechanisms are different. In C++, #include <iostream.h> pulls in a source file called iostream.h from a source library and compiles it along with the rest of the program. #include is usually used to include files containing declarations of library functions and classes, but the file could contain any C++ source code whatever. The Java declaration import java.util.Random brings in the pre-compiled class Random from a package called java.util. The declaration import java.io.* bring in all of the classes defined in the package java.util. A package is sort of like a library. You won't have to worry much about packages for this course.

Values, Objects, and Pointers

It is sometimes said that Java doesn't have pointers. That is not true. In fact, objects can only be referenced with pointers. More precisely, variables can hold primitive values (such as integers or floating-point numbers) or pointers to objects. A variable cannot hold an object, and you cannot make a pointer to a primitive value. Since you don't have a choice, Java doesn't have a special notation like C++ does to indicate when you want to use a pointer.

There are exactly eight primitive types in Java, boolean, char, byte, short, int, long, float, and double. Most of these are similar to types with the same name in C++. We mention only the differences.

A boolean value is either true or false. You cannot use an integer where a boolean is required (e.g. in an if or while statement) nor is there any automatic conversion between boolean and integer.

A char value is 16 bits rather than 8 bits, as it is in C or C++, to allow for all sorts of international alphabets. As a practical matter, however, you are unlikely to notice the difference. The byte type is 8 bits.

A short is 16 bits and an int is 32 bits, just as in C or C++ on most modern machines (in C++ the size is machine-dependent, but in Java it is guaranteed to be 32 bits). A Java long is not the same as in C++; it is 64 bits long--twice as big as a normal int--so it can hold any value from -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807. The types float and double are just like in C++: 32-bit and 64-bit floating point.

As in C++, objects are instances of classes. There is no prefix * or & operator or infix -> operator.

As an example, consider the class declaration (which is the same in C++ and in Java)

class Point { int x, y; }

As in C or C++, arguments to a Java procedure are passed ``by value'':

    void f() {
        int n = 1;
        Point p = new Point();
        p.x = 2; p.y = 3;
        System.out.println(n);    // prints 1
        System.out.println(p.x);  // prints 2
        g(n,p);
        System.out.println(n);    // still prints 1
        System.out.println(p.x);  // prints 100
    }
    void g(int num, Point ptr) {
        System.out.println(num);  // prints 1
        num = 17;                 // changes only the local copy
        System.out.println(num);  // prints 17

        System.out.println(ptr.x);// prints 2
        ptr.x = 100;              // changes the x field of caller's Point
        ptr = null;               // changes only the local ptr
    }

Unlike C++, Java has no way of declaring reference parameters, and unlike C++ or C, Java has no way of creating a pointer to a (non-object) value, so you can't do something like this

    /* C or C++ */
    void swap1(int *xp, int *yp) {
        int tmp;
        tmp = *xp;
        *xp = *yp;
        *yp = tmp;
    }
    int foo = 10, bar = 20;
    swap1(&foo, &bar);    /* now foo==20 and bar==10 */

    // C++ only
    void swap2(int &xp, int &yp) {
        int tmp;
        tmp = xp;
        xp = yp;
        yp = tmp;
    }
    int this_one = 88, that_one = 99;
    swap2(this_one, that_one);    // now this_one==99 and that_one==88

New objects are create by the new operator in Java just like C++ (except that an argument list is required after the class name, even if the constructor for the class doesn't take any arguments so the list is empty). However, there is no delete operator. The Java system automatically deletes objects when no references to them remain. This is a much more important convenience than it may at first seem. delete operator is extremely error-prone. Deleting objects too early can lead to dangling reference, as in

    p = new Point();
    // ...
    q = p;
    // ... much later
    delete p;
    q -> x  = 5; // oops!

Just as in C++, it is possible to restrict access to members of a class by declaring them private, but the syntax is different.
In C++:

    class C {
        private:
            int i;
            double d;
        public:
            int j;
            void f() { /*...*/ }
    }

    class C {
        private    int i;
        public     int j;
        private    double d;
        public     void f() { /* ... */ }
    }

The keyword static also means the same thing in Java as C++, which not what the word implies: Ordinary members have one copy per instance, whereas a static member has only one copy, which is shared by all instances. In effect, a static member lives in the class itself, rather than instances.

    class C {
        int x = 1;  // by the way, this is ok in Java but not C++
        static int y = 1;
        void f(int n) { x += n; }
        static int g() { return ++y; }
    }
    C p = new C();
    C q = new C();
    p.f(3);
    q.f(5);
    System.out.println(p.x);  // prints 4
    System.out.println(q.x);  // prints 6
    System.out.println(C.y);  // prints 1
    System.out.println(p.y);  // means the same thing
    System.out.println(C.g());// prints 2
    System.out.println(q.g());// prints 3

    Math.tan(x);            // tan is a static method of class Math
    Math.PI;                // a static "field" of class Math with value 3.14159...
    Integer.parseInt("10"); // used in the sorting example

    class Card {
        int suit = CLUBS;     // default
        final static int CLUBS = 1;
        final static int DIAMONDS = 2;
        final static int HEARTS = 3;
        final static int SPADES = 4;
    }
    Card c = new Card();
    c.suit = Card.SPADES;

In Java, arrays are objects. Like all objects in Java, you can only point to them, but unlike a C++ variable, which is treated like a pointer to the first element of the array, a Java array variable points to the whole object. There is no way to point to a particular slot in an array.

Each array has a read-only (final) field length that tells you how many elements it has. The elements are numbered starting at zero as in C++: a[0] ... a[a.length-1]. Once you create an array (using new), you can't change its size. If you need more space, you have to create a new (larger) array and copy over the elements (but see the library class Vector below).

    int x = 3;   // a value 
    int a[];     // a pointer to an array object; initially null
    int[] a;     // means exactly the same thing
    a = new int[10]; // now a points to an array object
    a[3] = 17;   // accesses one of the slots in the array
    a = new int[5]; // assigns a different array to a
                 // the old array is inaccessible (and so
                 // is garbage-collected)
    int[] b = a; // a and b share the same array object
    System.out.println(a.length); // prints 5

.

Since you can make an array of anything, you can make an an array of char or an an array of byte, but Java has something much better: the type String. The + operator is overloaded on Strings to mean concatenation. What's more, you can concatenate anything with a string; Java automatically converts it to a string. Built-in types such as numbers are converted in the obvious way. Objects are converted by calling their toString() methods. Library classes all have toString methods that do something reasonable. You can do likewise for classes you define. This is great for debugging.

    String s = "hello";
    String t = "world";
    System.out.println(s + ", " + t);       // prints "hello, world"
    System.out.println(s + "1234");         // "hello1234"
    System.out.println(s + (12*100 + 34));  // "hello1234"
    System.out.println(s + 12*100 + 34);    // "hello120034" (why?)
    System.out.println("The value of x is " + x);  // will work for any x
    System.out.println("System.out = " + System.out);
                // "System.out = java.io.PrintStream@80455198"
    String numbers = "";
    for (int i=0; i<5; i++)
        numbers += " " + i;
    System.out.println(numbers);            // " 1 2 3 4 5"

    String s = "whatever", t = "whatnow";
    s.charAt(0);            // 'w'
    s.charAt(3);            // 't'
    t.substring(4);         // "now"
    t.substring(4,6);       // "no"
    s.substring(0,4);       // "what"
    t.substring(0,4);       // "what"
    s.compareTo(t);         // a value less than zero
                            // s precedes t in "lexicographic"
                            // (dictionary) order
    t.compareTo(s);         // a value greater than zero (t follows s)
    t.compareTo("whatnow"); // zero
    t.substring(0,4) == s.substring(0,4);
                            // false (they are different String objects)
    t.substring(0,4).equals(s.substring(0,4));
                            // true (but they are both equal to "what")
    t.indexOf('w');         // 0
    t.indexOf('t');         // 3
    t.indexOf("now");       // 4
    t.lastIndexOf('w');     // 6
    t.endsWith("now");      // true

You can't modify a string, but you can make a string variable point to a new string (as in numbers += " " + i;). See StringBuffer if you want a string you can scribble on.

Constructors and Overloading

A constructor is like in C++: a method with the same name as the class. If a constructor has arguments, you supply corresponding values when using new. Even if it has no arguments, you still need the parentheses (unlike C++). There can be multiple constructors, with different numbers or types of arguments. The same is true for other methods. This is called overloading. Unlike C++, you cannot overload operators. The operator `+' is overloaded for strings and (various kinds of) numbers, but user-defined overloading is not allowed.

    class Point {
        int x, y;
        Point(int u, int v) {
            x = u;  // the same as this.x = u
            y = v;
        }
        Point(int x) {
            this.x = x;  // not the same as x = x!
            y = 0;
        }
        Point() {
            x = 0;
            y = 0;
        }
    }
    class Test {
        public static void main(String[] argv) {
            Point p1 = new Point(3,4);
            Point p2 = new Point();  // same as new Point(0,0)
            Point p3 = new Point;    // error!
        }
    }

    class Foo {
        double square(double d) { return d*d; }
    };

    class Foo {
        double square(double);
    };
    double Foo::square(double d) { return d*d; }
        // ok in C++ but not in Java

In C++, when we write

    class Derived : public Base { ... }

• A Derived can do anything a Base can, and perhaps more.
• A Derived does things the way a Base does them, unless specified otherwise.

Method inheritance is specified with the keyword extends.

    class Base {
        int f() { /* ... */ }
        void g(int x) { /* ... */ }
    }
    class Derived extends Base {
        void g(int x) { /* ... */ }
        double h() { /* ... */ }
    }

Interface inheritance is specified with implements. A class implements an Interface, which is like a class, except that the methods don't have bodies. Two examples are given by the built-in interfaces Runnable and Enumeration.

    interface Runnable {
        public void run();
    }
    interface Enumeration {
        public boolean hasMoreElements();
        public Object nextElement();
    }

    class Words extends StringTokenizer implements Enumeration, Runnable {
        public void run() {
            for (;;) {
                String s = nextToken();
                if (s == null) return;
                System.out.println(s);
            }
        }
        Words(String s) {
            super(s);
            // perhaps do something else with s as well
        }
    }

    w.run();

    if (w.hasMoreElements()) ...

By the way, constructors are not inherited. The call super(s) in class Words calls the constructor of StringTokenizer that takes one String argument. If you don't explicitly call super, Java automatically calls the super class constructor with no arguments (such a constructor must exist in this case). Note the call nextToken() in Words.run, which is short for this.nextToken(). Since this is an instance of Words, it has a nextToken method -- the one it inherited from StringTokenizer.

A cast in Java looks just like a cast in C++: It is a type name in parentheses preceding an expression. We have already seen an example of a cast used to convert between primitive types. A cast can also be used to convert an object reference to a super class or subclass. For example,

    Words w = new Words("this is a test");
    Object o = w.nextElement();
    String s = (String)o;
    System.out.println("The first word has length " + s.length());

    if (o instanceof String)
        n = ((String)o).length();
    else
        System.err.println("Bad type " + o.getClass().getName());

A Java program should never ``core dump,'' no matter how buggy it is. If the compiler excepts it and something goes wrong at run time, Java throws an exception. By default, an exception causes the program to terminate with an error message, but you can also catch an exception.

    try {
        // ...
        foo.bar();
        // ...
        a[i] = 17;
        // ...
    }
    catch (IndexOutOfBoundsException e) {
        System.err.println("Oops: " + e);
    }

The catch clause actually catches IndexOutOfBoundsException or any of its subclasses, including ArrayIndexOutOfBoundsException , StringIndexOutOfBoundsException , and others. An Exception is just another kind of object, and the same rules for inheritance hold for exceptions as any other king of class.

You can define and throw your own exceptions.

    class SytaxError extends Exception {
        int lineNumber;
        SytaxError(String reason, int line) {
            super(reason);
            lineNumber = line;
        }
        public String toString() {
            return "Syntax error on line " + lineNumber + ": " + getMessage();
        }
    }
    class SomeOtherClass {
        public void parse(String line) throws SyntaxError {
            // ...
            if (...)
                throw new SyntaxError("missing comma", currentLine);
            //...
        }
        public void parseFile(String fname) {
            //...
            try {
                // ...
                nextLine = in.readLine();
                parse(nextLine);
                // ...
            }
            catch (SyntaxError e) {
                System.err.println("" + e);
            }
        }
    }

There can be several catch clauses at the end of a try statement, to catch various kinds of exceptions. The first one that ``matches'' the exception (i.e., is a super class of it) is executed. You can also add a finally clause, which will always be executed, no matter how the program leaves the try clause (whether by falling through the bottom, executing a return, break, or continue, or throwing an exception).

Threads

Java lets you do several things at once by using threads. If your computer has more than one CPU, it may actually run two or more threads simultaneously. Otherwise, it will switch back and forth among the threads at times that are unpredictable unless you take special precautions to control it.

There are two different ways to create threads. I will only describe one of them here.

    Thread t = new Thread(command); // 
    t.start();  // t start running command, but we don't wait for it to finish
    // ... do something else (perhaps start other threads?)
    // ... later:
    t.join();  // wait for t to finish running command

In project 1, you are supposed to run each command in a separate thread. Thus you might declare something like this:

    class Command implements Runnable {
        String theCommand;
        Command(String c) {
            theCommand = c;
        }
        public void run() {
            // Do what the command says to do
        }
    }

The main program loop reads a command line, breaks it up into commands, runs all of the commands concurrently (each in a separate thread), and waits for them to all finish before issuing the next prompt. In outline, it may look like this.

    for (;;) {
        System.out.print("% "); System.out.flush();
        String line = inputStream.readLine();
        int numberOfCommands = // count how many comands there are on the line
        Thread t[] = new Thread[numberOfCommands];
        for (int i=0; i<numberOfCommands; i++) {
            String c = // next command on the line
            t[i] = new Thread(new Command(c));
            t[i].start();
        }
        for (int i=0; i<numberOfCommands; i++) {
            t[i].join();
        }
    }

Although you won't need it for project 1, the next project will require to to synchronize thread with each other. There are two reasons why you need to do this: to prevent threads from interferring with each other, and to allow them to cooperate. You use synchronized methods to prevent interference, and the built-in methods Object.wait() , Object.notify() , Object.notifyAll() , and Thread.yield() to support cooperation.

Any method can be preceded by the word synchronized (as well as public, static, etc.). The rule is:

The Java system enforces this rule by associating a monitor lock with each object. When a thread calls a synchronized method of an object, it tries to grab the object's monitor lock. If another thread is holding the lock, it waits until that thread releases it. A thread releases the monitor lock when it leaves the synchronized method. If one synchronized method of a calls contains a call to another, a thread may have the same lock ``multiple times.'' Java keeps track of that correctly. For example,

    class C {
        public synchronized void f() {
            // ...
            g();
            // ...
        }
        public synchronized void g() { /* ... */ }
    }

Sometimes a thread needs to wait for another thread to do something before it can continue. The methods wait() and notify(), which are defined in class Object and thus inherited by all classes, are made for this purpose. They can only be called from within synchronized methods. A call to wait() releases the monitor lock and puts the calling thread to sleep (i.e., it stops running). A subsequent call to notify on the same object wakes up a sleeping thread and lets it start running again. If more than one thread is sleeping, one is chosen arbitrarily (as a practical matter, it's probably the one that has been sleeping the longest) and if no threads are sleeping in this object, notify() does nothing. The awakened thread has to wait for the monitor lock before it starts; it completes on an equal basis with other threads trying to get into the monitor. The method notifyAll is similar, but wakes up all threads sleeping in the object.

    class Buffer {
        private Queue q;
        public synchronized void put(Object o) {
            q.enqueue(o);
            notify();
        }
        public synchronized Object get() {
            while (q.isEmpty())
                wait();
            return q.dequeue();
        }
    }

This example is not correct as it stands (and the Java compiler will reject it). The wait() method can throw an InterruptedException exception, so the get() method must either catch it or declare that it throws InterruptedException as well. The simplest solution is just to catch the exception and ignore it:

    class Buffer {
        private Queue q;
        public synchronized void put(Object o) {
            q.enqueue(o);
            notify();
        }
        public synchronized Object get() {
            while (q.isEmpty()) {
                try { wait(); }
                catch (InterruptedException e) {}
            }
            return q.dequeue();
        }
    }

You may wonder why Buffer.get() uses while (q.isEmpty()) rather than if (q.isEmpty()). In this particular case, either would work. However, in more complicated situations, a sleeping thread might be awakened for the ``wrong'' reason. Thus it is always a good idea when you wake up to recheck the condition that made to decide to go to sleep before you continue.

Input and Output

Input/Output, as described in Chapter 11 of the Java book, is not as complicated as it looks. You can get pretty far just writing to System.out (which is of type PrintStream ) with methods println and print . For input, you probably want to wrap the standard input System.in in a BufferedReader , which provides the handy method readLine()

    BufferedReader in =
        new BufferedReader(new InputStreamReader(System.in));
    for(;;) {
        String line = in.readLine();
        if (line == null)
            break;
        // do something with the next line
    }

If you want to read from a file, rather than from the keyboard (standard input), you can use FileReader , probably wrapped in a BufferedReader.

    BufferedReader in =
        new BufferedReader(new FileReader("somefile"));
    for (;;) {
        String line = in.readLine();
        if (line == null)
            break;
        // do something with the next line
    }

NOTE The information in Chapter 11 of the Java book is somewhat out of date. It tells you to use DataInputStream().readLine() instead of BufferedReader.readLine() . BufferedReader is new in Java 1.1, and DataInputStream.readLine() is ``deprecated,'' which means that if you use it, you will get a warning from the compiler. For now, it will work ok, but the warning is there to warn you that this feature will eventually be removed from the Java library, so you'd better update your programs to stop using it. (It is being removed because it is ASCII-specific and doen't work well with internationalization.) The Java book has an appendix describing the changes from version 1.0 to 1.1 of Java, but the body of the book hasn't been updated.

The library of pre-defined classes has several other handy tools. See the online manual , particularly java.lang and java.util for more details.

Integer, Character, etc.

    int i = Integer.MAX_VALUE;          // 2147483648, the largest possible int
    int i = Integer.parseInt("123");    // the int value 123
    String s = Integer.toHexString(123);// "7b" (123 in hex)
    double x = 
        new Double("123e-2").doubleValue();
                                // the double value 1.23
                                // see also page 150 of the Java Book
    Character.isDigit('3')      // true
    Character.isUpperCase('a')  // false
    Character.toUpperCase('a')  // 'A'

A Vector is like an array, but it grows as necessary to allow you to add as many elements as you like. Unfortunately, there is only one kind of Vector--a vector of Object. Thus you can insert objects of any type into it, but when you take objects out, you have to use a cast to recover the original type.

    Vector v = new Vector();     // an empty vector
    for (int i=0; i<100; i++)
        v.addElement(new Integer(i));
    // now it contains 100 Integer objects

    // print their squares
    for (int i=0; i<100; i++) {
        Integer N = (Integer)(v.elementAt(i));
        int n = N.intvalue();
        System.out.println(n*n);
    }
    v.setElementAt("hello", 5);  // like v[5] = "hello"
    Object o = v.elementAt(3);   // like o = v[3];
    v.insertElement("world", 6); // set v[6] = "world" after first shifting
                                 // element v[7], v[8], ... to the right to make room
    v.removeElementAt(3);        // remove v[3] and shift v[4], ... to the
                                 // left to fill in the gap

    v.addElement(47);                   // WRONG!
    sum += v.elementAt(i);              // WRONG!
    v.addElement(new Integer(47));      // right
    sum += ((Integer)v.elementAt(i)).intValue(); // ugly, but right

Don't forget to import java.util.Vector; (or import java.util.*; ) if you are using Vectors.

Hashtable

A Hashtable implements a mapping from objects called ``keys'' to objects called ``values.''

    Hashtable h = new Hashtable();    // an empty table
    h.put("seven", new Integer(7));   // key is the String "seven";
                                      // value is an Integer object
    h.put("seven", 7);                // WRONG! (7 is not an object)
    Object o = h.put("seven", new Double(7.0));
                                      // binds "seven" to a double object
                                      // and returns the previous value
    int n = ((Integer)o).intValue();  // n = 7
    h.containsKey("seven")            // true
    h.containsKey("twelve")           // false
    o = h.get("seven");               // get current binding (a Double)
    o = h.remove("seven");            // get current binding and remove it
    h.clear();                        // remove all bindings

StringTokenizer

A StringTokenizer is handy in breaking up a string into words separated by white space (or other separator characters). The following example is from the Java book:

    String str = "Gone, and forgotten";
    StringTokenizer tokens = new StringTokenizer(str, " ,");
    while (tokens.hasMoreTokens())
        System.out.println(tokens.nextToken());

    Gone
    and
    forgotten

There is a much more complicated class StreamTokenizer for breaking up an input stream into tokens. Many of its features seem to be designed to aid in parsing the Java langauge itself (which is not a surprise, considering that the Java compiler is written in Java).

Other Utilities

The random-number generator Random was presented above. See Chapter 12 of the Java book for information about other handy classes.