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|---|
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public class MixedTypes {
@SuppressWarnings("unchecked")
private static void addToList(List list, Object obj) {
list.add(obj); // "unchecked" warning
}
private static void print() {
List<String> list = new ArrayList<String> ();
addToList(list, 1);
System.out.println(list.get(0));
}
privatepublic static void main(String[] args) {
MixedTypes.print();
}
}
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...
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class Parameterized {
private static void addToList(List<String> list, String str) {
list.add(str); // "unchecked" warning
}
private static void print() {
List<String> list = new ArrayList<String> ();
addToList(list, "1");
System.out.println(list.get(0));
}
privatepublic static void main(String[] args) {
Parameterized.print();
}
}
|
...
This code suffers from related pitfalls. It compiles and runs cleanly. The method printOne is intended to print the value one either as an int or as a double depending on the type of the variable type. However, despite list being correctly parameterized, this method will always print '1' and never '1.0' because the int value 1 is always added to list without being type checked.
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|---|
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import java.util.*;
public class BadListAdder {
@SuppressWarnings("unchecked")
publicprivate static void addToList(List list, Object obj) {
list.add(obj); // "unchecked" warning
}
publicprivate static <T> void printOne(T type) {
if (!(type instanceof Integer || type instanceof Double))
System.out.println("Cannot print in the supplied type");
List<T> list = new ArrayList<T> ();
addToList(list, 1);
System.out.println(list.get(0));
}
public static void main(String[] args) {
double d = 1;
int i = 1;
System.out.println(d);
BadListAdder.printOne(d);
System.out.println(i);
BadListAdder.printOne(i);
}
}
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|---|
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class GoodListAdder {
publicprivate static void addToList(List<Integer> list, Integer i) {
list.add(i);
}
publicprivate static void addToList(List<Double> list, Double d) {
list.add(d);
}
publicprivate static <T> void printOne(T type) {
if (type instanceof Integer) {
List<Integer> list = new ArrayList<Integer> ();
addToList(list, 1);
System.out.println(list.get(0));
}
else if (type instanceof Double) {
List<Double> list = new ArrayList<Double> ();
// This will not compile if addToList(list, 1) is used
addToList(list, 1.0);
System.out.println(list.get(0));
}
else {
System.out.println("Cannot print in the supplied type");
}
}
public static void main(String[] args) {
double d = 1;
int i = 1;
System.out.println(d);
GoodListAdder.printOne(d);
System.out.println(i);
GoodListAdder.printOne(i);
}
}
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...
