java对象英文文献及翻译 第5页
BigDecimal is for arbitrary-precision fixed-point numbers; you can use these for accurate monetary calculations, for example.
Consult the JDK documentation for details about the constructors and methods you can call for these two classes.
Arrays in Java
Virtually all programming languages support arrays. Using arrays in C and C++ is perilous because those arrays are only blocks of memory. If a program accesses the array outside of its memory block or uses the memory before initialization (common programming errors), there will be unpredictable results.
One of the primary goals of Java is safety, so many of the problems that plague programmers in C and C++ are not repeated in Java. A Java array is guaranteed to be initialized and cannot be accessed outside of its range. The range checking comes at the price of having a small amount of memory overhead on each array as well as verifying the index at run time, but the assumption is that the safety and increased productivity is worth the expense.
When you create an array of objects, you are really creating an array of references, and each of those references is automatically initialized to a special value with its own keyword: null. When Java sees null, it recognizes that the reference in question isn’t pointing to an object. You must assign an object to each reference before you use it, and if you try to use a reference that’s still null, the problem will be reported at run time. Thus, typical array errors are prevented in Java.
You can also create an array of primitives. Again, the compiler guarantees initialization because it zeroes the memory for that array.
Arrays will be covered in detail in later chapters.
You never need to
destroy an object
In most programming languages, the concept of the lifetime of a variable occupies a significant portion of the programming effort. How long does the variable last? If you are supposed to destroy it, when should you? Confusion over variable lifetimes can lead to a lot of bugs, and this section shows how Java greatly simplifies the issue by doing all the cleanup work for you.
Scoping
Most procedural languages have the concept of scope. This determines both the visibility and lifetime of the names defined within that scope. In C, C++, and Java, scope is determined by the placement of curly braces {}. So for example:
{ int x = 12;
// Only x available
{ int q = 96;
// Both x & q available
}
// Only x available
// q “out of scope”
}
A variable defined within a scope is available only to the end of that scope.
Any text after a ‘//’ to the end of a line is a comment.
Indentation makes Java code easier to read. Since Java is a free-form language, the extra spaces, tabs, and carriage returns do not affect the resulting program.
Note that you cannot do the following, even though it is legal in C and C++:
{int x = 12;
{int x = 96; // Illegal
}}
The compiler will announce that the variable x has already been defined. Thus the C and C++ ability to “hide” a variable in a larger scope is not allowed, because the Java designers thought that it led to confusing programs.
Scope of objects优"文-论'文,网
http://www.youerw.com Java objects do not have the same lifetimes as primitives. When you create a Java object using new, it hangs around past the end of the scope. Thus if you use:
{
String s = new String("a string");
} // End of scope
the reference s vanishes at the end of the scope. However, the String object that s was pointing to is still occupying memory. In this bit of code, there is no way to access the object, because the only reference to it is out of scope. In later chapters you’ll see how the reference to the object can be passed around and duplicated during the course of a program.
It turns out that because objects created with new stay around for as long as you want them, a whole slew of C++ programming problems simply vanish in Java. The hardest problems seem to occur in C++ because you don’t get any help from the language in making sure that the objects are available when they’re needed. And more important, in C++ you must make sure that you destroy the objects when you’re done with them.
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