Compute If Present

A lot of times if a key is present in Map then it needs to be updated with a new value which must be computed via another Function.

package org.wesome.java8;

import java.util.HashMap;
import java.util.Map;
import java.util.Objects;

class Apple {
    public static void main(String[] args) {

        Map<Integer, String> map = new HashMap<>();
        map.put(1, "Macintosh");
        map.put(2, "Fuji");
        map.put(3, "Gala");
        map.put(4, "Jonagold");

        String temp = map.get(4);
        if (Objects.nonNull(temp)) {
            temp = temp.concat(" Apple");
            map.put(4, temp);
        }

        System.out.println("map = " + map);
    }
}

To avoid the boilerplate if-else code java 8 introduces computeIfPresent method, if the key is present in the Map, then its value will be computed via BiFunction which will take key and value, and the newly generated value will be added to the Map as well.

computeIfPresent method will modify the existing Map as well

computeIfPresent method will be available for all Map Intertface implementing concrete classes such as HashMap, LinkedHashMap and TreeMap ect

package org.wesome.java8;

import java.util.HashMap;
import java.util.Map;
import java.util.function.BiFunction;

class Apple {
    public static void main(String[] args) {

        BiFunction<Integer, String, String> biFunction = (key, value) -> value + " Apple";

        Map<Integer, String> map = new HashMap<>();
        map.put(1, "Macintosh");
        map.put(2, "Fuji");
        map.put(3, "Gala");
        map.put(4, "Jonagold");

        map.computeIfPresent(3, (key, val) -> val + " Apple");

        map.computeIfPresent(4, biFunction);

        System.out.println("map = " + map);
    }
}