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[java/en] Added more info about Collections initialization (#4200)

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EgorBobrov 2021-08-22 21:21:01 +03:00 committed by GitHub
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1 changed files with 46 additions and 7 deletions
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@ -510,10 +510,12 @@ public class LearnJava {
// toString returns this Object's string representation.
System.out.println("trek info: " + trek.toString());
} // End main method
private static class TestInitialization {
// Double Brace Initialization
// The Java Language has no syntax for how to create static Collections
// in an easy way. Usually you end up in the following way:
// Before Java 11, the Java Language had no syntax for how to create
// static Collections in an easy way. Usually you end up like this:
private static final Set<String> COUNTRIES = new HashSet<String>();
static {
COUNTRIES.add("DENMARK");
@ -521,10 +523,10 @@ public class LearnJava {
COUNTRIES.add("FINLAND");
}
// But there's a nifty way to achieve the same thing in an
// easier way, by using something that is called Double Brace
// Initialization.
private static final Set<String> COUNTRIES = new HashSet<String>() {{
// There's a nifty way to achieve the same thing,
// by using something that is called Double Brace Initialization.
private static final Set<String> COUNTRIES_DOUBLE_BRACE =
new HashSet<String>() {{
add("DENMARK");
add("SWEDEN");
add("FINLAND");
@ -536,7 +538,44 @@ public class LearnJava {
// This does not only work for Collections, it works for all
// non-final classes.
} // End main method
// Another option was to initialize the Collection from an array,
// using Arrays.asList() method:
private static final List<String> COUNTRIES_AS_LIST =
Arrays.asList("SWEDEN", "DENMARK", "NORWAY");
// This has one catch: the list we get is internally backed by the array,
// and since arrays can't change their size, the list backed by the array
// is not resizeable, which means we can't add new elements to it:
public static void main(String[] args) {
COUNTRIES.add("FINLAND"); // throws UnsupportedOperationException!
// However, we can replace elements by index, just like in array:
COUNTRIES.set(1, "FINLAND");
System.out.println(COUNTRIES); // prints [SWEDEN, FINLAND, NORWAY]
}
// The resizing problem can be circumvented
// by creating another Collection from the List:
private static final Set<String> COUNTRIES_SET =
new HashSet<>(Arrays.asList("SWEDEN", "DENMARK", "NORWAY"));
// It's perfectly fine to add anything to the Set of COUNTRIES now.
} // End TestInitialization class
private static class TestJava11Initialization {
// Since Java 11, there is a convenient option to initialize Collections:
// Set.of() and List.of() methods.
private static final Set<String> COUNTRIES =
Set.of("SWEDEN", "DENMARK", "NORWAY");
// There is a massive catch, though: Lists and Sets initialized like this
// 1) are immutable
// 2) can't contain null elements (even check for null elements fails)!
public static void main(String[] args) {
COUNTRIES.add("FINLAND"); // throws UnsupportedOperationException
COUNTRIES.remove("NORWAY"); // throws UnsupportedOperationException
COUNTRIES.contains(null); // throws NullPointerException
}
private static final Set<String> COUNTRIES_WITH_NULL =
Set.of("SWEDEN", null, "NORWAY"); // throws NullPointerException
} // End TestJava11Initialization class
} // End LearnJava class
// You can include other, non-public outer-level classes in a .java file,