How to use java generic container Collection

Let’s start with a simple example:

<code>public void testGenerics() {<br>    Collection<Number> numbers = new ArrayList<>();<br>    numbers.add(1); // ok<br>    numbers.add(0.1); // ok<br><br>    Collection<? extends Number> numbers2 = new ArrayList<>();<br>    // don't work, you don't know which subtype 'numbers2' exactly contains<br>    numbers2.add(1); // oops!<br>}<br></code>

This example is actually a bit anti-human. It is estimated that the first reaction of most people (including me) to this conversion must be "of course it is right" (this is I fell into a trap), let me tell you my understanding:

• Collection: Indicates that this Collection contains all Number type objects, which can be Integer/Long/Float, because The compiler can determine that obj instanceof Number == true;

• Collection: Indicates that this Collection is a Collection instance of "a subtype" of the Number type, which can be Collection< Integer>/Collection, so calling numbers2.add(1) will not work, because the compiler does not know which subtype of Number the element contained in numbers2 is, and the compiler cannot determine the result of obj instanceof UnknownType;

• Collection, this E type is "a" specific type, and cannot be a placeholder representing multiple subtypes of a parent;

Another example:

<code>public void testGenerics() {<br>    Collection<Number> numbers = new ArrayList<>();<br>    Collection<Integer> integers = new ArrayList<>();<br>    Collection<? extends Number> numbers2 = new ArrayList<>();<br>    <br>    numbers2 = integers; // ok<br>    numbers2 = numbers; // ok<br>    <br>    // don't work, Collection<Number> != Collection<Integer><br>    numbers = integers; // oops!<br>}<br></code>

Integer obviously inherits Number, so why

• Collection == Collection

is not established, let’s look at an example:

<code>public void testGenerics() {<br>    Collection<Integer> profits = new ArrayList<>();<br>    <br>    insertSomething(profits); // line 1<br>    <br>    Integer profit = profits.iterator().next(); // oops! crash<br>}<br><br>private void insertSomething(Collection<Number> numbers) {<br>    numbers.add(Long.MAX_VALUE);<br>}<br></code>

If line 1 is established, then the next profit will be a negative number, and a series of subsequent calculations will sound abnormal. If the code If it is not robust enough, it may even throw some unexpected RuntimeException, causing the method to end abnormally or even the program to crash.

So in one sentence, Collection != Collection is for runtime safety, and possible type conversion exceptions will be resolved at compile time.

Now let’s talk about Collection