A summary of the usage of generics in Java

This example summarizes the use of generics in Java. Share with you for your reference. The details are as follows:

1 basic use

public interface List<E> {
 void add(E);
 Iterator<E> iterator();
}

Generics and subclasses

Child is a Child of Parent, List< Child> But not List< Parent> The subclass.
Therefore: List< Object> List = new ArrayList< String> () is wrong.
If the above is correct, then:

List<String> ls = new ArrayList<String>(); //1
List<Object> lo = ls; //2
lo.add(new Object()); // 3
String s = ls.get(0); // 4 That will be object convert string Will fail. 

3 wildcards

For 2 reasons, the following implementation does not work for coordinating the output of the set

void printCollection(Collection<Object> c) {
 for (Object o: c) {
 // do something
 }
}

Therefore, you need the wildcard ? :

void printCollection(Collection<?> c) {
 for (Object o: c) { // 1
 // do something
 }
} // ok

Here the & # 63; The type is unknown, but any Object is an Object, so the 1 in the above example is correct. But the following example is wrong:

void add(Collection<? extends MyClass> c) {
 c.add(new MyClass()); // wrong
} // ok

The reason is also clear, ? Extends MyClass indicates that the type is a subclass of MyClass, but the type is not known

4. Generic methods

The above example can be implemented as:

<T> add(Collection<T> c, T t) {
 c.add(t);
}

The compiler will help with the translation of the type without compromising the semantics.

5. Generic runtime comparison

List<String> l1 = new ArrayList<String>();
List<Integer> l2 = new ArrayList<Integer>();
System.out.println(l1.getClass() == l2.getClass()); // true

Because generic classes always run the same.

6 generic array (may cause type insecurity)

List<String>[] lsa = new ArrayList<String>[10]; // error

If possible, this can lead to type insecurity. Such as:

Object o = lsa;
Object []oa = (Object[])o;
List<Integer> li = new ArrayList<Integer>();
li.add(new Integer(3));
oa[1] = li;
String s = lsa[1].get(0); // runtime error

I hope this article has been helpful to your Java programming.