Chapter 6. Methods
Item 23: Check parameters for validity
Each time you write a method or constructor, you should think about what restrictions exist on its parameters. You should document these restrictions and enforce them with explicit checks at the beginning of the method body.Item 24: Make defensive copies when neededYou must program defensively with the assumption that clients of your class will do their best to destroy its invariants.
// Broken "immutable" time period class
public final class Period 
{
private final Date start;
private final Date end;
/** *//**
* @param start
* the beginning of the period.
* @param end
* the end of the period; must not precede start.
* @throws IllegalArgumentException
* if start is after end.
* @throws NullPointerException
* if start or end is null.
*/
public Period(Date start, Date end) {
if (start.compareTo(end) > 0)
throw new IllegalArgumentException(start + " after "
+ end);
this.start = start;
this.end = end;
}
public Date start() {
return start;
}
public Date end() {
return end;
}
// Remainder omitted
}Violate this invariant by exploiting the fact that Date is mutable:
// Attack the internals of a Period instance
Date start = new Date();
Date end = new Date();
Period p = new Period(start, end);
end.setYear(78); // Modifies internals of p! It is essential to make a defensive copy of each mutable parameter to the constructor:
// Repaired constructor - makes defensive copies of parameters
public Period(Date start, Date end) {
this.start = new Date(start.getTime());
this.end = new Date(end.getTime());
if (this.start.compareTo(this.end) > 0)
throw new IllegalArgumentException(start +" after "+ end);
} Defensive copies are made before checking the validity of the parameters (Item 23), and the validity check is performed on the copies rather than on the originals.
Do not use the clone method to make a defensive copy of a parameter whose type is subclassable by untrusted parties.
// Second attack on the internals of a Period instance
Date start = new Date();
Date end = new Date();
Period p = new Period(start, end);
p.end().setYear(78); // Modifies internals of p! To defend against the second attack, merely modify the accessors to return defensive copies of mutable internal fields:
// Repaired accessors - make defensive copies of internal fields
public Date start() {
return (Date) start.clone();
}
public Date end() {
return (Date) end.clone();
} Note that the new accessors, unlike the new constructor, do use the clone method to make defensive copies. This is acceptable (although not required), as we know with certainty that the class of Period's internal Date objects is java.util.Date rather than some potentially untrusted subclass.
Item 25: Design method signatures carefullyToo many methods make a class difficult to learn, use, document, test, and maintain. This is doubly true for interfaces.Avoid long parameter lists (3 as practical max).
Long sequences of identically typed parameters are especially harmful.
For parameter types, favor interfaces over classes.
Use
function objects judiciously.
Item 26: Use overloading judiciouslySelection among overloaded methods is static (at compile time), while selection among overridden methods is dynamic (at run time).
//Broken - incorrect use of overloading!
public class CollectionClassifier {
public static String classify(Set s) {
return "Set";
}
public static String classify(List l) {
return "List";
}
public static String classify(Collection c) {
return "Unknown Collection";
}
public static void main(String[] args) {
Collection[] tests = new Collection[] { new HashSet(), // A Set
new ArrayList(), // A List
new HashMap().values() // Neither Set nor List
};
for (int i = 0; i < tests.length; i++)
System.out.println(classify(tests[i]));
}
} For all three iterations of the loop, the compile-time type of the parameter is the same: Collection. The run-time type is different in each iteration, but this does not affect the choice of overloading.
class A {
String name() {
return "A";
}
}
class B extends A {
String name() {
return "B";
}
}
class C extends A {
String name() {
return "C";
}
}
public class Overriding {
public static void main(String[] args) {
A[] tests = new A[] { new A(), new B(), new C() };
for (int i = 0; i < tests.length; i++)
System.out.print(tests[i].name());
}
} As you would expect, this program prints out "ABC" even though the compile-time type of the instance is A in each iteration of the loop.
You should generally refrain from overloading methods with multiple signatures that have the same number of parameters. In some cases, especially where constructors are involved, it may be impossible to follow this advice. In that case, you should at least avoid situations where the same set of parameters can be passed to different overloadings by the addition of casts.
Item 27: Return zero-length arrays, not nulls
It is possible to return the same zero-length array from every invocation that returns no items because zero-length arrays are immutable and immutable objects may be shared freely (Item 13).
private List cheesesInStock = ;
private final static Cheese[] NULL_CHEESE_ARRAY = new Cheese[0];
/** *//**
* @return an array containing all of the cheeses in the shop.
*/
public Cheese[] getCheeses() {
return (Cheese[]) cheesesInStock.toArray(NULL_CHEESE_ARRAY);
} There is no reason ever to return null from an array-valued method instead of returning a zero-length array.
Item 28: Write doc comments for all exposed API elements
/** *//**
* Returns the element at the specified position in this list.
*
* @param index index of element to return; must be
* nonnegative and less than the size of this list.
* @return the element at the specified position in this list.
* @throws IndexOutOfBoundsException if the index is out of range
* (<tt>index < 0 || index >= this.size()</tt>).
*/
Object get(int index) For methods and constructors, the summary description should be a verb phrase describing the action performed by the method.
For classes, interfaces, and fields, the summary description should be a noun phrase describing the thing represented by an instance of the class or interface or by the field itself.
Be careful not to include a period within the first sentence of a doc comment.