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Summary of usage of generics in Java

高洛峰
Release： 2017-01-18 11:19:58
Original
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The examples in this article summarize the usage of generics in Java. Share it with everyone for your reference. The details are as follows:

1 Basic use

public interface List<E> {
 void add(E);
 Iterator<E> iterator();
}
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2 Generics and subclasses

Child is a subclass of Parent, but List is not A subclass of List.
So: List list = new ArrayList() 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，将object转换为string将失败。
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3 wildcards

Because of reason 2, the following implementation cannot be used to unify the output of the collection. 

void printCollection(Collection<Object> c) {
 for (Object o: c) {
 // do something
 }
}
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Therefore, the wildcard ? is needed: 

void printCollection(Collection<?> c) {
 for (Object o: c) { // 1
 // do something
 }
} // ok
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The ? here indicates that the type is unknown, but any object is Object, so 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
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The reason is also very clear, ? extends MyClass indicates that the type is a subclass of MyClass, but the specific type is not known

4. Generic method

The above example can be implemented as: 

<T> add(Collection<T> c, T t) {
 c.add(t);
}
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The compiler will help with type conversion while ensuring semantics.

5. Comparison of generic runtime

List<String> l1 = new ArrayList<String>();
List<Integer> l2 = new ArrayList<Integer>();
System.out.println(l1.getClass() == l2.getClass()); // true
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Because the runtime of generic classes is the same.

6 Generic array (may lead to type insecurity) 

List<String>[] lsa = new ArrayList<String>[10]; // error
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If possible, may 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
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I hope this article will be helpful to everyone’s java programming.

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