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Java : AbstractMap with Examples

December 21, 2024 June 15, 2022

AbstractMap (Java SE 23 & JDK 23) in Java with Examples.
You will find code samples for most of the AbstractMap<K,V> methods.


Summary

This class provides a skeletal implementation of the Map interface, to minimize the effort required to implement this interface.

AbstractMap (Java SE 23 & JDK 23)

Class diagram

class MyMap extends AbstractMap<String, Integer> {

    private final Set<Entry<String, Integer>> entries;

    MyMap(String key1, int value1, String key2, int value2, String key3, int value3) {
        if (Objects.equals(key1, key2) || Objects.equals(key1, key3) || Objects.equals(key2, key3)) {
            throw new IllegalArgumentException("duplicate element");
        }

        this.entries = Set.of(
                Map.entry(key1, value1),
                Map.entry(key2, value2),
                Map.entry(key3, value3)
        );
    }

    @Override
    public Set<Entry<String, Integer>> entrySet() {
        return entries;
    }
}

final var map = new MyMap("aaa", 10, "bbb", 20, "ccc", 30);

System.out.println(map); // {ccc=30, aaa=10, bbb=20}
System.out.println(map.size()); // 3

System.out.println("-- entries --");
for (final var entry : map.entrySet()) {
    System.out.println(entry.getKey() + " : " + entry.getValue());
}

// Result
// ↓
//-- entries --
//ccc : 30
//aaa : 10
//bbb : 20

Constructors

AbstractMap ()

Sole constructor.

AbstractMap (Java SE 23 & JDK 23) 
class MyMap extends AbstractMap<String, Integer> {

    private final Set<Entry<String, Integer>> entries;

    MyMap(String key1, int value1, String key2, int value2, String key3, int value3) {
        if (Objects.equals(key1, key2) || Objects.equals(key1, key3) || Objects.equals(key2, key3)) {
            throw new IllegalArgumentException("duplicate element");
        }

        this.entries = Set.of(
                Map.entry(key1, value1),
                Map.entry(key2, value2),
                Map.entry(key3, value3)
        );
    }

    @Override
    public Set<Entry<String, Integer>> entrySet() {
        return entries;
    }
}

final var map = new MyMap("aaa", 10, "bbb", 20, "ccc", 30);

System.out.println(map); // {ccc=30, aaa=10, bbb=20}
System.out.println(map.size()); // 3

System.out.println("-- entries --");
for (final var entry : map.entrySet()) {
    System.out.println(entry.getKey() + " : " + entry.getValue());
}

// Result
// ↓
//-- entries --
//ccc : 30
//aaa : 10
//bbb : 20

Methods

void clear ()

Removes all of the mappings from this map (optional operation).

AbstractMap (Java SE 23 & JDK 23) 
final AbstractMap<String, Integer> map = new HashMap<>();
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

protected Object clone ()

Returns a shallow copy of this AbstractMap instance: the keys and values themselves are not cloned.

AbstractMap (Java SE 23 & JDK 23) 
class MyMap extends AbstractMap<String, Integer> implements Cloneable {

    private final Set<Entry<String, Integer>> entries;

    MyMap(String key1, int value1, String key2, int value2, String key3, int value3) {
        if (Objects.equals(key1, key2) || Objects.equals(key1, key3)
                || Objects.equals(key2, key3)) {
            throw new IllegalArgumentException("duplicate element");
        }

        this.entries = Set.of(
                Map.entry(key1, value1),
                Map.entry(key2, value2),
                Map.entry(key3, value3)
        );
    }

    @Override
    public Set<Entry<String, Integer>> entrySet() {
        return entries;
    }

    @Override
    public Object clone() {
        try {
            return super.clone();
        } catch (CloneNotSupportedException e) {
            throw new InternalError(e);
        }
    }
}

final var map = new MyMap("aaa", 10, "bbb", 20, "ccc", 30);
System.out.println(map); // {ccc=30, aaa=10, bbb=20}

final var cloned = map.clone();
System.out.println(cloned); // {ccc=30, aaa=10, bbb=20}

boolean containsKey (Object key)

Returns true if this map contains a mapping for the specified key.

AbstractMap (Java SE 23 & JDK 23) 
final AbstractMap<String, Integer> map = new HashMap<>();
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.containsKey("a")); // true
System.out.println(map.containsKey("b")); // true
System.out.println(map.containsKey("c")); // true
System.out.println(map.containsKey("X")); // false
System.out.println(map.containsKey("")); // false

boolean containsValue (Object value)

Returns true if this map maps one or more keys to the specified value.

AbstractMap (Java SE 23 & JDK 23) 
final AbstractMap<String, Integer> map = new HashMap<>();
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

boolean equals (Object o)

Compares the specified object with this map for equality.

AbstractMap (Java SE 23 & JDK 23) 
final AbstractMap<String, Integer> map1 = new HashMap<>();
map1.put("a", 1);
map1.put("b", 2);
map1.put("c", 3);

System.out.println(map1); // {a=1, b=2, c=3}

final AbstractMap<String, Integer> map2 = new HashMap<>();
map2.put("a", 1);
map2.put("b", 2);
map2.put("c", 3);

System.out.println(map2); // {a=1, b=2, c=3}

final AbstractMap<String, Integer> map3 = new HashMap<>();
map3.put("A", 1);
map3.put("B", 2);
map3.put("C", 3);

System.out.println(map3); // {A=1, B=2, C=3}

final AbstractMap<String, Integer> map4 = new HashMap<>();
map4.put("a", 10);
map4.put("b", 20);
map4.put("c", 30);

System.out.println(map4); // {a=10, b=20, c=30}

System.out.println(map1.equals(map2)); // true
System.out.println(map1.equals(map3)); // false
System.out.println(map1.equals(map4)); // false

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.

AbstractMap (Java SE 23 & JDK 23) 
final AbstractMap<String, Integer> map = new HashMap<>();
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

int hashCode ()

Returns the hash code value for this map.

AbstractMap (Java SE 23 & JDK 23) 
final AbstractMap<String, Integer> map = new HashMap<>();
System.out.println(map); // {}
System.out.println(map.hashCode()); // 0

final AbstractMap<String, Integer> map = new HashMap<>();
map.put("a", 1);
map.put("b", 2);

System.out.println(map); // {a=1, b=2}
System.out.println(map.hashCode()); // 192

final AbstractMap<String, Integer> map = new HashMap<>();
map.put("A", 1);
map.put("B", 2);

System.out.println(map); // {A=1, B=2}
System.out.println(map.hashCode()); // 128

final AbstractMap<String, Integer> map = new HashMap<>();
map.put("a", 10);
map.put("b", 20);

System.out.println(map); // {a=10, b=20}
System.out.println(map.hashCode()); // 225

boolean isEmpty ()

Returns true if this map contains no key-value mappings.

AbstractMap (Java SE 23 & JDK 23) 
final AbstractMap<String, Integer> map = new HashMap<>();
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
System.out.println(map.isEmpty()); // false

map.clear();

System.out.println(map); // {}
System.out.println(map.size()); // 0
System.out.println(map.isEmpty()); // true

Set<K> keySet ()

Returns a Set view of the keys contained in this map.

AbstractMap (Java SE 23 & JDK 23) 
final AbstractMap<String, Integer> map = new HashMap<>();
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]

V put (K key, V value)

Associates the specified value with the specified key in this map (optional operation).

AbstractMap (Java SE 23 & JDK 23) 
final AbstractMap<String, Integer> map = new HashMap<>();
System.out.println(map); // {}

System.out.println(map.put("a", 1)); // null
System.out.println(map); // {a=1}

System.out.println(map.put("b", 2)); // null
System.out.println(map); // {a=1, b=2}

System.out.println(map.put("c", 3)); // null
System.out.println(map); // {a=1, b=2, c=3}

System.out.println(map.put("a", 999)); // 1
System.out.println(map); // {a=999, b=2, c=3}

void putAll (Map<? extends K,? extends V> m)

Copies all of the mappings from the specified map to this map (optional operation).

AbstractMap (Java SE 23 & JDK 23) 
final AbstractMap<String, Integer> map = new HashMap<>();
System.out.println(map); // {}

map.putAll(Map.of());
System.out.println(map); // {}

map.putAll(Map.of("a", 1, "b", 2));
System.out.println(map); // {a=1, b=2}

map.putAll(Map.of("c", 3, "d", 4, "e", 5));
System.out.println(map); // {a=1, b=2, c=3, d=4, e=5}

map.putAll(Map.of("a", 901, "b", 902, "z", 100));
System.out.println(map);  // {a=901, b=902, z=100, c=3, d=4, e=5}

V remove (Object key)

Removes the mapping for a key from this map if it is present (optional operation).

AbstractMap (Java SE 23 & JDK 23) 
final AbstractMap<String, Integer> map = new HashMap<>();
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.remove("b")); // 2
System.out.println(map); // {a=1, c=3}

System.out.println(map.remove("z")); // null
System.out.println(map); // {a=1, c=3}

int size ()

Returns the number of key-value mappings in this map.

AbstractMap (Java SE 23 & JDK 23) 
final AbstractMap<String, Integer> map = new HashMap<>();

System.out.println(map); // {}
System.out.println(map.size()); // 0

map.put("a", 1);
System.out.println(map); // {a=1}
System.out.println(map.size()); // 1

map.put("b", 2);
System.out.println(map); // {a=1, b=2}
System.out.println(map.size()); // 2

map.put("c", 3);
System.out.println(map); // {a=1, b=2, c=3}
System.out.println(map.size()); // 3

String toString ()

Returns a string representation of this map.

AbstractMap (Java SE 23 & JDK 23) 
final AbstractMap<String, Integer> map = new HashMap<>();

final var str1 = map.toString();
System.out.println(str1); // {}

map.put("a", 1);

final var str2 = map.toString();
System.out.println(str2); // {a=1}

map.put("b", 2);

final var str3 = map.toString();
System.out.println(str3); // {a=1, b=2}

map.put("c", 3);

final var str4 = map.toString();
System.out.println(str4); // {a=1, b=2, c=3}

Collection<V> values ()

Returns a Collection view of the values contained in this map.

AbstractMap (Java SE 23 & JDK 23) 
final AbstractMap<String, Integer> map = new HashMap<>();
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 Map

compute, computeIfAbsent, computeIfPresent, entrySet, forEach, getOrDefault, merge, putIfAbsent, remove, replace, replace, replaceAll

Please see the link below.


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