Combine two maps a and b so that for a key in a, the value is b(a(key))

ReplaceAll.java

package org.vanbart;

import org.junit.Before;
import org.junit.Test;

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

import static org.hamcrest.core.Is.is;
import static org.junit.Assert.assertNull;
import static org.junit.Assert.assertThat;

public class ReplaceAll {

    private Map<String, String> mapA = new HashMap<>();

    private Map<String, String> mapB = new HashMap<>();

    private Map<String, String> combined = new HashMap<>();

    @Before
    public void setup() {
        mapA.put("1", "a");
        mapA.put("2", "b");
        mapA.put("4", "d");

        mapB.put("a", "one");
        mapB.put("b", "two");
        mapB.put("c", "three");
    }

    @Test
    public void testReplaceAll() {
        mapA.replaceAll((k, v) -> mapB.get(v));
        assertThat(mapA.get("1"), is("one"));
        assertThat(mapA.get("2"), is("two"));
        assertNull(mapA.get("4"));
    }

    @Test
    public void testCompute() {
        for (Map.Entry<String, String> entry: mapA.entrySet()) {
            combined.compute(entry.getKey(), (k,v) -> mapB.get(entry.getValue()));
        }
        assertThat(combined.get("1"), is("one"));
        assertThat(combined.get("2"), is("two"));
        assertNull(combined.get("4"));
    }

    @Test
    public void testCompute2() {
        for (Map.Entry<String, String> entry: mapA.entrySet()) {
            combined.compute(entry.getKey(), (k,v) -> remap(k));
        }
        assertThat(combined.get("1"), is("one"));
        assertThat(combined.get("2"), is("two"));
        assertNull(combined.get("4"));
    }

    private String remap(String key) {
        return mapB.get(mapA.get(key));
    }
}