第 57 条：将局部变量的作用域最小化

Iterator<Element> i = c.iterator();
while (i.hasNext()) {
    doSomething(i.next());
}
...
Iterator<Element> i2 = c2.iterator();
while (i.hasNext()) { // BUG!
    doSomethingElse(i2.next());
}

for (Iterator<Element> i = c.iterator(); i.hasNext(); ) {
    Element e = i.next();
    ... // Do something with e and i
}
...
// Compile-time error - cannot find symbol i
for (Iterator<Element> i2 = c2.iterator(); i.hasNext(); ) {
    Element e2 = i2.next();
    ... // Do something with e2 and i2
}