You never need to destroy an object

Bruce Eckel's Thinking in Java Contents | Prev | Next

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.

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 */
  }
}

Scope of objects

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 handle 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 handle to it is out of scope. In later chapters you’ll see how the handle 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 programming problems simply vanish in C++ and 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 importantly, in C++ you must make sure that you destroy the objects when you’re done with them.



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