总结数组列表中对象的值（Java）

Question

I have key/value like this

("A", 2.2);
("A", 1.1);
("B", 4.0);
("B", 2.0);
("C", 2.5);
("A", 2.0);
("A", 2.2);
("A", 1.0);

I expect to have result is

A=3.3
B=6.0
C=2.5
A=5.2

I tried with code

           static LinkedHashMap<String, Double> map = new LinkedHashMap<String, Double>();
    public static void putAndIncrement(String key, double value) {
        Double prev = map.get(key);
        Double newValue = value;
        if (prev != null) {
            newValue += prev;
        }
        double roundOff = Math.round(newValue * 10.0) / 10.0;
        map.put(key,roundOff);

    }

However result is

A=8.5
B=6.0
C=2.5

Hashmap, Map, LinkedHashmap is not right way to get my expecting result. Can you consult me any other method for this situation ? Please help me how to get that or any suggestion is really helpful for me. Thank you

Answer 1

A LinkedHashMap is still a Map , which means that keys are unique . The map simply cannot have two "A" keys at the same time.

If you want to sum up the values of consecutive keys, you need to use a List with a class for storing the key/value pair:

static List<KeyValue> list = new ArrayList<>();
public static void putAndIncrement(String key, double value) {
    KeyValue keyValue = (list.isEmpty() ? null : list.get(list.size() - 1));
    if (keyValue == null || ! key.equals(keyValue.getKey())) {
        list.add(new KeyValue(key, value));
    } else {
        keyValue.addValue(value);
    }
}

public final class KeyValue {
    private final String key;
    private double value;
    public KeyValue(String key, double value) {
        this.key = key;
        this.value = value;
    }
    public void addValue(double value) {
        this.value += value;
    }
    public String getKey() {
        return this.key;
    }
    public double getValue() {
        return this.value;
    }
    @Override
    public String toString() {
        return this.key + "=" + this.value;
    }
}

Test

putAndIncrement("A", 2.2);
putAndIncrement("A", 1.1);
putAndIncrement("B", 4.0);
putAndIncrement("B", 2.0);
putAndIncrement("C", 2.5);
putAndIncrement("A", 2.0);
putAndIncrement("A", 2.2);
putAndIncrement("A", 1.0);
System.out.println(list);

Output

[A=3.3000000000000003, B=6.0, C=2.5, A=5.2]