Core Java Volume I--Fundamentals (Core Series)

by Horstmann Cay S.

Java has the distinction of being the first and only programming language that had a ten-minute story on National Public Radio. A $100,000,000 venture capital fund was set up solely for products using a specific computer language.

Java was never just a language. There are lots of programming languages out there, but few of them make much of a splash. Java is a whole platform, with a huge library, containing lots of reusable code, and an execution environment that provides services such as security, portability across operating systems, and automatic garbage collection.

We wanted to build a system that could be programmed easily without a lot of esoteric training and which leveraged today’s standard practice. So even though we found that C++ was unsuitable, we designed Java as closely to C++ as possible in order to make the system more comprehensible. Java omits many rarely used, poorly understood, confusing features of C++ that, in our experience, bring more grief than benefit.

The syntax for Java is, indeed, a cleaned-up version of C++ syntax. There is no need for header files, pointer arithmetic (or even a pointer syntax), structures, unions, operator overloading, virtual base classes, and so on.

Header files are equivalent to import statements. Pointers were fun. Structures are there because C++ is a superclass of C. Don't know about unions and virtual base class.

Another aspect of being simple is being small. One of the goals of Java is to enable the construction of software that can run stand-alone on small machines. The size of the basic interpreter and class support is about 40K; the basic standard libraries and thread support (essentially a self-contained microkernel) add another 175K.

There is now a separate Java Micro Edition with a smaller library, suitable for embedded devices.

However, the security model of Java is complex. Not long after the first version of the Java Development Kit was shipped, a group of security experts at Princeton University found subtle bugs that allowed untrusted code to attack the host system. Initially, security bugs were fixed quickly. Unfortunately, over time, hackers got quite good at spotting subtle flaws in the implementation of the security architecture. Sun, and then Oracle, had a tough time keeping up with bug fixes.

Even though in hindsight, the Java security model was not as successful as originally envisioned, Java was well ahead of its time. A competing code delivery mechanism from Microsoft relied on digital signatures alone for security. Clearly this was not sufficient—as any user of Microsoft’s own products can confirm, programs from well-known vendors do crash and create damage.

Generating code for a “virtual machine” was not a new idea at the time. Programming languages such as Lisp, Smalltalk, and Pascal had employed this technique for many years.

Unlike C and C++, there are no “implementation-dependent” aspects of the specification. The sizes of the primitive data types are specified, as is the behavior of arithmetic on them.

Shortly after the initial success of Java, Microsoft released a product called J++ with a programming language and virtual machine that were almost identical to Java. At this point, Microsoft is no longer supporting J++ and has instead introduced another language called C# that also has many similarities with Java but runs on a different virtual machine.

“All along, the language was a tool, not the end.”

Gosling decided to call his language “Oak”

The people at Sun later realized that Oak was the name of an existing computer language, so they changed the name to Java.

You can specify floating-point literals in hexadecimal. For example, 0.125 = 2–3 can be written as 0x1.0p-3. In hexadecimal notation, you use a p, not an e, to denote the exponent. (An e is a hexadecimal digit.) Note that the mantissa is written in hexadecimal and the exponent in decimal. The base of the exponent is 2, not 10.

System.out.println(2.0 - 1.1) prints 0.8999999999999999, not 0.9 as you would expect. Such roundoff errors are caused by the fact that floating-point numbers are represented in the binary number system. There is no precise binary representation of the fraction 1/10, just as there is no accurate representation of the fraction 1/3 in the decimal system. If you need precise numerical computations without roundoff errors, use the BigDecimal class, which is introduced later in this chapter.

public static void main(String\u005B\u005D args)

A code point is a code value that is associated with a character in an encoding scheme.

In Java, the char type describes a code unit in the UTF-16 encoding.

Our strong recommendation is not to use the char type in your programs unless you are actually manipulating UTF-16 code units.

A variable name must begin with a letter and must be a sequence of letters or digits. Note that the terms “letter” and “digit” are much broader in Java than in most languages. A letter is defined as 'A'–'Z', 'a'–'z', '_', '$', or any Unicode character that denotes a letter in a language. For example, German users can use umlauts such as 'ä' in variable names; Greek speakers could use a π. Similarly, digits are '0'–'9' and any Unicode characters that denote a digit in a language. Symbols like '+' or '©' cannot be used inside variable names, nor can spaces.

The double type uses 64 bits to store a numeric value, but some processors use 80-bit floating-point registers.

methods tagged with the strictfp keyword must use strict floating-point operations that yield reproducible results.

Be careful about testing for equality of floating-point numbers in loops. A for loop like this one

Because of roundoff errors, the final value might not be reached exactly. In this example, x jumps from 9.99999999999998 to 10.09999999999998 because there is no exact binary representation for 0.1.

Although the designers of Java kept goto as a reserved word, they decided not to include it in the language. In general, goto statements are considered poor style. Some programmers feel the anti-goto forces have gone too far (see, for example, the famous article of Donald Knuth called “Structured Programming with goto statements”). They argue that unrestricted use of goto is error-prone but that an occasional jump out of a loop is beneficial. The Java designers agreed and even added a new statement, the labeled break, to support this programming style.

The designers of the Java language considered using keywords, such as foreach and in. But this loop was a late addition to the Java language, and in the end nobody wanted to break the old code that already contained methods or variables with these names (such as System.in).

Java has no multidimensional arrays at all, only one-dimensional arrays. Multidimensional arrays are faked as “arrays of arrays.”

We say that the plusDays method does not mutate the object on which it is invoked.

In contrast, methods that only access objects without modifying them are sometimes called accessor methods.

Be careful not to write accessor methods that return references to mutable objects.

The Java programming language always uses call by value. That means that the method gets a copy of all parameter values. In particular, the method cannot modify the contents of any parameter variables passed to it.

But in a class, if you don’t initialize a field, it is automatically initialized to a default (0, false, or null).

in a class, if you don’t initialize a field, it is automatically initialized to a default (0, false, or null).

Please keep in mind that you get a free no-argument constructor only when your class has no other constructors.

There is a third mechanism in Java, called an initialization block. Class declarations can contain arbitrary blocks of code. These blocks are executed whenever an object of that class is constructed.

This mechanism is never necessary and is not common. It is usually more straightforward to place the initialization code inside a constructor.

If the subclass constructor does not call a superclass constructor explicitly, the no-argument constructor of the superclass is invoked. If the superclass does not have a no-argument constructor and the subclass constructor does not call another superclass constructor explicitly, the Java compiler reports an error.

the super keyword has two meanings: to invoke a superclass method and to invoke a superclass constructor.

The compiler enumerates all methods called f in the class C and all accessible methods called f in the superclasses of C.

If among all the methods called f there is a unique method whose parameter types are a best match for the supplied arguments, that method is chosen to be called. This process is called overloading resolution.

A subclass may change the return type to a subtype of the original type.

When you override a method, the subclass method must be at least as visible as the superclass method. In particular, if the superclass method is public, the subclass method must also be declared public. It is a common error to accidentally omit the public specifier for the subclass method. The compiler then complains that you try to supply a more restrictive access privilege.

You can cast only within an inheritance hierarchy.

The test x instanceof C does not generate an exception if x is null. It simply returns false.

Actually, converting the type of an object by a cast is not usually a good idea.

it takes only one uncaught ClassCastException to terminate your program. In general, it is best to minimize the use of casts and the instanceof operator.

a class with one or more abstract methods must itself be declared abstract.

Some programmers don’t realize that abstract classes can have concrete methods. You should always move common fields and methods (whether abstract or not) to the superclass (whether abstract or not).

There are times, however, when you want to restrict a method to subclasses only or, less commonly, to allow subclass methods to access a superclass field. In that case, you declare a class feature as protected.