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giaunvgiaunv
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Java Collections examples
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‎java-examples/java-core/src/main/java/com/hellokoding/java/collections/ArrayListExample.java‎

Lines changed: 25 additions & 93 deletions
Original file line numberDiff line numberDiff line change
@@ -1,110 +1,42 @@
11
package com.hellokoding.java.collections;
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33

4-
import java.util.*;
4+
import java.util.ArrayList;
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import java.util.Comparator;
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import java.util.List;
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68
public class ArrayListExample {
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public static void main(String args[]) {
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// create and initialize with ArrayList constructors
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List<Integer> lst1 = new ArrayList<>();
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lst1.add(1);
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lst1.add(2);
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lst1.add(3);
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List<Integer> lst2 = new ArrayList<>(20);
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List<Integer> lst3 = new ArrayList<>(List.of(1, 2, 3));
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System.out.println(lst3); // [1, 2, 3]
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List<Integer> lst4 = new ArrayList<>(Set.of(1, 2, 3));
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System.out.println(lst4); // [3, 2, 1]
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// create and initialize with Arrays.asList
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List<Integer> lst5 = Arrays.asList(1, 2, 3);
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// create and initialize with Collections factory methods
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List<Integer> lst6 = Collections.emptyList();
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List<Integer> lst7 = Collections.singletonList(1);
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// create and initialize with Java 9+ List.of, List.copyOf
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List<Integer> lst9 = List.of(1, 2, 3);
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List<Integer> lst10 = List.copyOf(lst9);
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// create and initialize
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List<Integer> lst = new ArrayList<>();
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lst.add(3);
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lst.add(1);
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lst.add(2);
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System.out.println(lst);
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// iterate with Java 8+ forEach(Consumer)
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lst.forEach(e -> System.out.printf("%d ", e));
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lst1.forEach(System.out::println);
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// iterate with for loop
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for(int ele : lst1) {
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System.out.printf("%d ", ele);
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}
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System.out.println();
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for (int i = 0; i < lst1.size(); i++) {
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System.out.printf("%d ", lst1.get(i));
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}
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System.out.println();
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// iterate with iterator
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Iterator<Integer> iter = lst1.iterator();
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while (iter.hasNext()) {
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System.out.printf("%d ", iter.next());
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}
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System.out.println();
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// sort
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lst.sort(Comparator.naturalOrder());
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System.out.println(lst);
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62-
// add elements into a list
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lst1.add(4);
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lst1.add(5);
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lst1.add(5); // add a duplicate element
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lst1.add(null); // add a null element
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System.out.println(lst1); // [1, 2, 3, 4, 5, 5, null]
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// query
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System.out.println(lst.get(0)); // get element by index
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System.out.println(lst.indexOf(3)); // get index by element
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System.out.println(lst.contains(2));
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System.out.println(lst.size());
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System.out.println(lst.isEmpty());
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// update an element at index
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lst1.set(0, 10);
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lst.set(0, 10);
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System.out.println(lst);
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// delete an element by index
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lst1.remove(1);
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lst.remove(1);
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System.out.println(lst);
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// delete an element by value
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lst1.remove(null);
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// get an element by index
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int i1 = lst1.get(0);
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// check if an element existing
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boolean isExist = lst1.contains(3);
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// check the list size
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int size = lst1.size();
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// check if the list is empty
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boolean isEmpty = lst1.isEmpty();
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// sort a list in ascending order
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// NullPointerException will be thrown if the list has null elements
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lst1.sort(Comparator.naturalOrder());
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Collections.sort(lst1);
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System.out.println(lst1);
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// sort a list in descending order
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lst1.sort(Comparator.reverseOrder());
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Collections.sort(lst1, Collections.reverseOrder());
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System.out.println(lst1);
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lst.remove(Integer.valueOf(3));
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System.out.println(lst);
10941
}
11042
}

‎java-examples/java-core/src/main/java/com/hellokoding/java/collections/HashMapExample.java‎

Lines changed: 5 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -13,9 +13,10 @@ public static void main(String args[]) {
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System.out.println(map); // {k1=1, k2=2}
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// iterate
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map.entrySet().forEach(System.out::println);
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map.keySet().forEach(System.out::println);
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map.values().forEach(System.out::println);
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map.entrySet().forEach(e -> System.out.printf("%s=%d ", e.getKey(), e.getValue()));
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map.keySet().forEach(e -> System.out.printf("%s ", e));
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map.values().forEach(e -> System.out.printf("%d ", e));
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System.out.println();
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// sort
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SortedMap<String, Integer> sortedMap = new TreeMap<>(map);
@@ -29,7 +30,7 @@ public static void main(String args[]) {
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3031
// update value
3132
map.replace("k2", 20);
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System.out.println(map); // {k1=1, k2=2}
33+
System.out.println(map); // {k1=1, k2=20}
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3435
// remove entries
3536
map.remove("k2");

‎java-examples/java-core/src/main/java/com/hellokoding/java/collections/HashSetExample.java‎

Lines changed: 38 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,38 @@
1+
package com.hellokoding.java.collections;
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3+
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import java.util.*;
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6+
public class HashSetExample {
7+
public static void main(String args[]) {
8+
// create and initialize
9+
Set<Integer> set = new HashSet<>();
10+
set.add(3);
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set.add(1);
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set.add(2);
13+
System.out.println(set);
14+
15+
// iterate with Java 8+ forEach(Consumer)
16+
set.forEach(e -> System.out.printf("%d ", e));
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System.out.println();
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// sort
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Set<Integer> treeSet = new TreeSet<>(set);
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System.out.println(treeSet);
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// query
24+
System.out.println(set.iterator().next()); // get first element
25+
System.out.println(set.contains(2));
26+
System.out.println(set.size());
27+
System.out.println(set.isEmpty());
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// delete an element
30+
set.remove(1);
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System.out.println(set);
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33+
// update = delete + add
34+
set.remove(2);
35+
set.add(20);
36+
System.out.println(set);
37+
}
38+
}

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