The Collections class consists exclusively of static methods that operate on or return collections.
It contains:
fill(), frequency()static <T> void fill(List<? super T> list, T obj): Replaces all of the elements of the specified list with the specified element.static int frequency(Collection<?> c, Object o): Returns the number of elements in the specified collection equal to the specified object.import java.util.ArrayList;
import java.util.Collections;
public class FillFrequencyDemo {
public static void main(String[] args) {
ArrayList<String> list = new ArrayList<>();
list.add("Rick");
list.add("Morty");
list.add("Summer");
System.out.println("Original: " + list);
Collections.fill(list, "Mr. Meeseeks");
System.out.println("Filled: " + list);
int frequency = Collections.frequency(list, "Mr. Meeseeks");
System.out.println("Frequency of \"Mr. Meeseeks\": " + frequency);
}
}Original: [Rick, Morty, Summer]
Filled: [Mr. Meeseeks, Mr. Meeseeks, Mr. Meeseeks]
Frequency of "Mr. Meeseeks": 3sort(), reverse(), shuffle()static <T extends Comparable<? super T>> void sort(List<T> list): Sorts the specified list into ascending order, according to the natural ordering of its elements.static void reverse(List<?> list): Reverses the order of the elements in the specified list.static void shuffle(List<?> list): Randomly permutes the specified list using a default source of randomness.import java.util.ArrayList;
import java.util.Collections;
public class SortReverseShuffleDemo {
public static void main(String[] args) {
ArrayList<String> list = new ArrayList<>();
list.add("Rick");
list.add("Morty");
list.add("Summer");
list.add("Beth");
System.out.println("Original: " + list);
Collections.sort(list);
System.out.println("Sorted: " + list);
Collections.reverse(list);
System.out.println("Reversed: " + list);
Collections.shuffle(list);
System.out.println("Shuffled: " + list);
}
}Original: [Rick, Morty, Summer, Beth]
Sorted: [Beth, Morty, Rick, Summer]
Reversed: [Summer, Rick, Morty, Beth]
Shuffled: [Morty, Beth, Rick, Summer]min(), max()static <T extends Object & Comparable<? super T>> T min(Collection<? extends T> coll): Returns the minimum element of the given collection, according to the natural ordering of its elements.static <T extends Object & Comparable<? super T>> T max(Collection<? extends T> coll): Returns the maximum element of the given collection, according to the natural ordering of its elements.public class MinMaxDemo {
public static void main(String[] args) {
ArrayList<String> list = new ArrayList<>();
list.add("Rick");
list.add("Morty");
list.add("Summer");
list.add("Beth");
System.out.println(list);
String min = Collections.min(list);
System.out.println("Min: " + min);
String max = Collections.max(list);
System.out.println("Max: " + max);
}
}[Rick, Morty, Summer, Beth]
Min: Beth
Max: SummeraddAll(), copy(), disjoint()static <T> boolean addAll(Collection<? super T> c, T... elements): Adds all of the specified elements to the specified collection.static <T> void copy(List<? super T> dest, List<? extends T> src): Copies all of the elements from one list into another.static boolean disjoint(Collection<?> c1, Collection<?> c2): Returns true if the two specified collections have no elements in common.import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
public class AddAllCopyDisjointDemo {
public static void main(String[] args) {
List<String> list = new ArrayList<>();
System.out.println("List: " + list);
Collections.addAll(list, "Rick", "Morty", "Summer");
System.out.println("List: " + list + "\n");
List<String> list2 = List.of("Beth", "Jerry", "Birdperson");
System.out.println("List2: " + list2);
System.out.println("Is list2 disjoint? " + Collections.disjoint(list, list2) + "\n");
List<String> list3 = new ArrayList<>(list2);
System.out.println("List3: " + list3);
System.out.println("Is list3 disjoint? " + Collections.disjoint(list, list3));
Collections.copy(list3, list);
System.out.println("List3: " + list3);
System.out.println("Is list3 disjoint? " + Collections.disjoint(list, list3));
}
}List: []
List: [Rick, Morty, Summer]
List2: [Beth, Jerry, Birdperson]
Is list2 disjoint? true
List3: [Beth, Jerry, Birdperson]
Is list3 disjoint? true
List3: [Rick, Morty, Summer]
Is list3 disjoint? falseempty()static final <T> List<T> emptyList(): Returns an empty list (immutable).static final <T> Set<T> emptySet(): Returns an empty set (immutable).static <E> SortedSet<E> emptySortedSet(): Returns an empty sorted set (immutable).static <T> Iterator<T> emptyIterator(): Returns an iterator that has no elements.import java.util.*;
public class EmptyCollectionFactoryDemo {
public static void main(String[] args) {
List<String> list = Collections.emptyList();
System.out.println("Empty list: " + list);
// These factory methods create immutable collections.
// If you try to add a value, the operation will throw an exception
// list.add("UNIBZ");
Set<String> set = Collections.emptySet();
System.out.println("Empty set: " + set);
SortedSet<String> sortedSet = Collections.emptySortedSet();
System.out.println("Empty sorted set: " + sortedSet);
Iterator<String> iterator = Collections.emptyIterator();
System.out.println("Empty iterator: hasNext()? " + iterator.hasNext());
ListIterator<String> listIterator = Collections.emptyListIterator();
System.out.println("Empty list iterator: hasNext()? " + listIterator.hasNext());
}
}Empty list: []
Empty set: []
Empty sorted set: []
Empty iterator: hasNext()? false
Empty list iterator: hasNext()? falsesingleton(), singletonList()static <T> Set<T> singleton(T o): Returns an immutable set containing only the specified object.static <T> List<T> singletonList(T o): Returns an immutable list containing only the specified object.import java.util.Collections;
import java.util.List;
import java.util.Set;
public class SingletonFactoryDemo {
public static void main(String[] args) {
List<String> list = Collections.singletonList("Rick");
System.out.println("Singleton list: " + list);
// These factory methods create immutable collections.
// If you try to add a value, the operation will throw an exception
// list.add("Potter");
Set<String> set = Collections.singleton("Rick");
System.out.println("Singleton set: " + set);
}
}Singleton list: [Rick]
Singleton set: [Rick]