February 4, 2025
June 26, 2022
LinkedHashMap (Java SE 23 & JDK 23) in Java with Examples.
You will find code samples for most of the LinkedHashMap<K,V> methods.
Summary
Hash table and linked list implementation of the Map interface, with well-defined encounter order. This implementation differs from HashMap in that it maintains a doubly-linked list running through all of its entries.
final var map = new LinkedHashMap<String, Integer>();
System.out.println(map); // {}
map.put("a", 10);
map.put("b", 20);
map.put("c", 30);
System.out.println(map); // {a=10, b=20, c=30}
System.out.println(map.get("a")); // 10
System.out.println(map.get("b")); // 20
System.out.println(map.get("X")); // null
map.remove("b");
System.out.println(map); // {a=10, c=30}
final var linkedHashMap = new LinkedHashMap<String, Integer>();
linkedHashMap.put("xxx", 10);
linkedHashMap.put("yyy", 20);
linkedHashMap.put("aaa", 30);
linkedHashMap.put("bbb", 40);
System.out.println(linkedHashMap); // {xxx=10, yyy=20, aaa=30, bbb=40}
final var hashMap = new HashMap<String, Integer>();
hashMap.put("xxx", 10);
hashMap.put("yyy", 20);
hashMap.put("aaa", 30);
hashMap.put("bbb", 40);
System.out.println(hashMap); // {aaa=30, bbb=40, yyy=20, xxx=10}
Constructors
LinkedHashMap ()
Constructs an empty insertion-ordered LinkedHashMap instance with the default initial capacity (16) and load factor (0.75).
final var map = new LinkedHashMap<String, Integer>();
System.out.println(map); // {}
System.out.println(map.isEmpty()); // true
map.put("a", 10);
map.put("b", 20);
map.put("c", 30);
System.out.println(map); // {a=10, b=20, c=30}
System.out.println(map.size()); // 3
LinkedHashMap (int initialCapacity)
Constructs an empty insertion-ordered LinkedHashMap instance with the specified initial capacity and a default load factor (0.75).
final var map = new LinkedHashMap<String, Integer>(10000000);
System.out.println(map); // {}
final var startTime = System.nanoTime();
for (int i = 0; i < 1000000; i++) {
map.put(String.valueOf(i), i);
}
final var endTime = System.nanoTime();
// 0.076968 sec.
System.out.printf("%f sec.%n", (endTime - startTime) / 1000000000.0);
final var map = new LinkedHashMap<String, Integer>(1);
System.out.println(map); // {}
final var startTime = System.nanoTime();
for (int i = 0; i < 1000000; i++) {
map.put(String.valueOf(i), i);
}
final var endTime = System.nanoTime();
// 0.119630 sec.
System.out.printf("%f sec.%n", (endTime - startTime) / 1000000000.0);
LinkedHashMap (int initialCapacity, float loadFactor)
Constructs an empty insertion-ordered LinkedHashMap instance with the specified initial capacity and load factor.
final var map = new LinkedHashMap<String, Integer>(16, 0.05f);
System.out.println(map); // {}
final var startTime = System.nanoTime();
for (int i = 0; i < 1000000; i++) {
map.put(String.valueOf(i), i);
}
final var endTime = System.nanoTime();
// 0.160612 sec.
System.out.printf("%f sec.%n", (endTime - startTime) / 1000000000.0);
final var map = new LinkedHashMap<String, Integer>(16, 0.75f);
System.out.println(map); // {}
final var startTime = System.nanoTime();
for (int i = 0; i < 1000000; i++) {
map.put(String.valueOf(i), i);
}
final var endTime = System.nanoTime();
// 0.117338 sec.
System.out.printf("%f sec.%n", (endTime - startTime) / 1000000000.0);
LinkedHashMap (int initialCapacity, float loadFactor, boolean accessOrder)
Constructs an empty LinkedHashMap instance with the specified initial capacity, load factor and ordering mode.
// accessOrder = false
final var map = new LinkedHashMap<String, Integer>(16, 0.75f, false);
map.put("a", 10);
map.put("b", 20);
map.put("c", 30);
System.out.println(map); // {a=10, b=20, c=30}
System.out.println(map.get("a")); // 10
System.out.println(map); // {a=10, b=20, c=30}
System.out.println(map.get("b")); // 20
System.out.println(map); // {a=10, b=20, c=30}
// accessOrder = true
final var map = new LinkedHashMap<String, Integer>(16, 0.75f, true);
map.put("a", 10);
map.put("b", 20);
map.put("c", 30);
System.out.println(map); // {a=10, b=20, c=30}
System.out.println(map.get("a")); // 10
System.out.println(map); // {b=20, c=30, a=10}
System.out.println(map.get("b")); // 20
System.out.println(map); // {c=30, a=10, b=20}
LinkedHashMap (Map<? extends K,? extends V> m)
Constructs an insertion-ordered LinkedHashMap instance with the same mappings as the specified map.
final var m = Map.of("a", 10, "b", 20, "c", 30);
System.out.println(m); // {b=20, a=10, c=30}
final var map = new LinkedHashMap<>(m);
System.out.println(map); // {b=20, a=10, c=30}
System.out.println(map.size()); // 3
Methods
void clear ()
Removes all of the mappings from this map.
final var map = new LinkedHashMap<String, Integer>();
map.put("a", 1);
map.put("b", 2);
map.put("c", 3);
System.out.println(map); // {a=1, b=2, c=3}
System.out.println(map.size()); // 3
map.clear();
System.out.println(map); // {}
System.out.println(map.size()); // 0
boolean containsValue (Object value)
Returns true if this map maps one or more keys to the specified value.
final var map = new LinkedHashMap<String, Integer>();
map.put("a", 1);
map.put("b", 2);
map.put("c", 3);
map.put("X", 3);
System.out.println(map); // {a=1, b=2, c=3, X=3}
System.out.println(map.containsValue(0)); // false
System.out.println(map.containsValue(1)); // true
System.out.println(map.containsValue(2)); // true
System.out.println(map.containsValue(3)); // true
System.out.println(map.containsValue(4)); // false
Set<Map.Entry<K,V>> entrySet ()
Returns a Set view of the mappings contained in this map.
final var map = new LinkedHashMap<String, Integer>();
map.put("a", 1);
map.put("b", 2);
map.put("c", 3);
System.out.println(map); // {a=1, b=2, c=3}
final var entries = map.entrySet();
System.out.println(entries); // [a=1, b=2, c=3]
map.replace("b", 20);
System.out.println(map); // {a=1, b=20, c=3}
System.out.println(entries); // [a=1, b=20, c=3]
entries.remove(Map.entry("a", 1));
System.out.println(map); // {b=20, c=3}
System.out.println(entries); // [b=20, c=3]
for (final var entry : entries) {
entry.setValue(entry.getValue() * 10);
}
System.out.println(map); // {b=200, c=30}
System.out.println(entries); // [b=200, c=30]
V get (Object key)
Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.
final var map = new LinkedHashMap<String, Integer>();
map.put("a", 1);
map.put("b", 2);
map.put("c", 3);
System.out.println(map); // {a=1, b=2, c=3}
System.out.println(map.get("a")); // 1
System.out.println(map.get("b")); // 2
System.out.println(map.get("c")); // 3
System.out.println(map.get("X")); // null
System.out.println(map.get("")); // null
Set<K> keySet ()
Returns a Set view of the keys contained in this map.
final var map = new LinkedHashMap<String, Integer>();
map.put("a", 1);
map.put("b", 2);
map.put("c", 3);
System.out.println(map); // {a=1, b=2, c=3}
final var keys = map.keySet();
System.out.println(keys); // [a, b, c]
map.put("d", 4);
System.out.println(map); // {a=1, b=2, c=3, d=4}
System.out.println(keys); // [a, b, c, d]
keys.remove("a");
System.out.println(map); // {b=2, c=3, d=4}
System.out.println(keys); // [b, c, d]
static <K, V> LinkedHashMap<K,V> newLinkedHashMap (int numMappings)
Creates a new, empty, insertion-ordered LinkedHashMap suitable for the expected number of mappings.
final var map = LinkedHashMap.<String, Integer>newLinkedHashMap(3);
System.out.println(map); // {}
System.out.println(map.isEmpty()); // true
map.put("a", 10);
map.put("b", 20);
map.put("c", 30);
System.out.println(map); // {a=10, b=20, c=30}
System.out.println(map.size()); // 3
V putFirst (K k, V v)
Inserts the given mapping into the map if it is not already present, or replaces the value of a mapping if it is already present (optional operation).
final var map = new LinkedHashMap<>();
map.putFirst("a", 10);
System.out.println(map); // {a=10}
map.putFirst("b", 20);
System.out.println(map); // {b=20, a=10}
map.putFirst("c", 30);
System.out.println(map); // {c=30, b=20, a=10}
V putLast (K k, V v)
Inserts the given mapping into the map if it is not already present, or replaces the value of a mapping if it is already present (optional operation).
final var map = new LinkedHashMap<>();
map.putLast("a", 10);
System.out.println(map); // {a=10}
map.putLast("b", 20);
System.out.println(map); // {a=10, b=20}
map.putLast("c", 30);
System.out.println(map); // {a=10, b=20, c=30}
protected boolean removeEldestEntry (Map.Entry<K,V> eldest)
Returns true if this map should remove its eldest entry.
@SuppressWarnings("serial")
class MyMap extends LinkedHashMap<String, Integer> {
private static final int MAX_ENTRIES = 3;
@Override
protected boolean removeEldestEntry(Map.Entry<String, Integer> eldest) {
System.out.println("removeEldestEntry eldest : " + eldest);
return size() > MAX_ENTRIES;
}
}
final var map = new MyMap();
System.out.println("-- put a --");
map.put("a", 10);
System.out.println("map : " + map);
System.out.println("-- put b --");
map.put("b", 20);
System.out.println("map : " + map);
System.out.println("-- put c --");
map.put("c", 30);
System.out.println("map : " + map);
System.out.println("-- put d --");
map.put("d", 40);
System.out.println("map : " + map);
System.out.println("-- put e --");
map.put("e", 50);
System.out.println("map : " + map);
// Result
// ↓
//-- put a --
//removeEldestEntry eldest : a=10
//map : {a=10}
//-- put b --
//removeEldestEntry eldest : a=10
//map : {a=10, b=20}
//-- put c --
//removeEldestEntry eldest : a=10
//map : {a=10, b=20, c=30}
//-- put d --
//removeEldestEntry eldest : a=10
//map : {b=20, c=30, d=40}
//-- put e --
//removeEldestEntry eldest : b=20
//map : {c=30, d=40, e=50}
SequencedMap<K,V> reversed ()
Returns a reverse-ordered view of this map.
final var map = new LinkedHashMap<>();
map.putLast("a", 10);
map.putLast("b", 20);
map.putLast("c", 30);
System.out.println(map); // {a=10, b=20, c=30}
final var reversedMap = map.reversed();
System.out.println(reversedMap); // {c=30, b=20, a=10}
System.out.println(reversedMap.reversed()); // {a=10, b=20, c=30}
SequencedSet<Map.Entry<K,V>> sequencedEntrySet ()
Returns a SequencedSet view of this map's entrySet.
final var map = new LinkedHashMap<>();
map.putLast("a", 10);
map.putLast("b", 20);
map.putLast("c", 30);
System.out.println(map); // {a=10, b=20, c=30}
final var entries = map.sequencedEntrySet();
System.out.println(entries); // [a=10, b=20, c=30]
System.out.println(entries.reversed()); // [c=30, b=20, a=10]
SequencedSet<K> sequencedKeySet ()
Returns a SequencedSet view of this map's keySet.
final var map = new LinkedHashMap<>();
map.putLast("a", 10);
map.putLast("b", 20);
map.putLast("c", 30);
System.out.println(map); // {a=10, b=20, c=30}
final var keys = map.sequencedKeySet();
System.out.println(keys); // [a, b, c]
System.out.println(keys.reversed()); // [c, b, a]
SequencedCollection<V> sequencedValues ()
Returns a SequencedCollection view of this map's values collection.
final var map = new LinkedHashMap<>();
map.putLast("a", 10);
map.putLast("b", 20);
map.putLast("c", 30);
System.out.println(map); // {a=10, b=20, c=30}
final var values = map.sequencedValues();
System.out.println(values); // [10, 20, 30]
System.out.println(values.reversed()); // [30, 20, 10]
Collection<V> values ()
Returns a Collection view of the values contained in this map.
final var map = new LinkedHashMap<String, Integer>();
map.put("a", 1);
map.put("b", 2);
map.put("c", 3);
System.out.println(map); // {a=1, b=2, c=3}
final var values = map.values();
System.out.println(values); // [1, 2, 3]
map.replace("b", 20);
System.out.println(map); // {a=1, b=20, c=3}
System.out.println(values); // [1, 20, 3]
values.remove(1);
System.out.println(map); // {b=20, c=3}
System.out.println(values); // [20, 3]
//values.add(4); // UnsupportedOperationException
Methods declared in HashMap
clear, clone, compute, computeIfAbsent, computeIfPresent, containsKey, isEmpty, merge, newHashMap, put, putAll, remove, size
Please see the link below.
Methods declared in AbstractMap
Methods declared in Map
compute, computeIfAbsent, computeIfPresent, containsKey, equals, forEach, getOrDefault, hashCode, isEmpty, merge, put, putAll, putIfAbsent, remove, remove, replace, replace, replaceAll, size
Please see the link below.
Methods declared in SequencedMap
firstEntry, lastEntry, pollFirstEntry, pollLastEntry
Please see the link below.
Related posts
- API Examples