final String s = "forever";
final int i = 1;
final Map<String, Integer> m = new HashMap<String, Integer>();

s = "never"; // compilation error!
i++; // ditto
m.put(s, i); // fine

public void doSomething(String input) {
    input = input.toLowerCase();
    // do a few things with input
}

public void doSomething(final String input) {
    final String lowercaseInput = input.toLowerCase();
    // do a few things with lowercaseInput
}

public void doSomething(final String input) {
    // do a few things with input.toLowerCase()
}

public void doSomething(final String input) {
    doSomethingPrivate(input.toLowerCase());
}

// expects lower case
private void doSomethingPrivate(final String input) {
    if (!input.equals(input.toLowerCase()) {
        throw new IllegalArgumentException("input not lowercase");
    }
    // do a few things with input
}

public void foo(final Date date) {
    date.setTime(0);
    // code that uses date
}