Guava Group By Key - guava

Question

Still learning all the features of the Guava API. Wanted to shoot this out there to see if there were any suggestions on how to go about doing this. The current picture looks like this: List < ObjectA >, List< ObjectB >, List< ObjectC >
ObjectA {
Integer attribute1;
String attribute2;
….
}
ObjectB {
Integer attribute1;
String attribute2;
….
}
ObjectC {
Integer attribute1;
String attribute2;
….
}
I would like to take the 3 lists and group the objects together by attribute1 and attribute2 thus creating something like this:
123|”Whatever” : { Object A, ObjectB, ObjectC}
456|”Something” : { Object A, ObjectB, ObjectC}
An obvious *Multimap is the return but the process of building this could have multiple solutions. Thoughts on how to approach this?

Answer 1

I would use Multimaps.index() with a custom pair class to represent the combination of attribute1 and attribute2.
/**
* A "pair" class that contains an {#code attribute1} and an {#code attribute2}. Mainly used as a {#link Map} key.
*/
#Immutable
public class Attribute1AndAttribute2 {
#Nullable
private final Integer attribute1;
#Nullable
private final String attribute2;
public Attribute1AndAttribute2(#Nullable Integer attribute1,
#Nullable String attribute2) {
this.attribute1 = attribute1;
this.attribute2 = attribute2;
}
#Nullable
public Integer getAttribute1() {
return attribute1;
}
#Nullable
public String getAttribute2() {
return attribute2;
}
#Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (obj instanceof Attribute1AndAttribute2) {
Attribute1AndAttribute2 that = (Attribute1AndAttribute2) obj;
return Objects.equal(this.attribute1, that.attribute1)
&& Objects.equal(this.attribute2, that.attribute2);
}
return false;
}
#Override
public int hashCode() {
return Objects.hashCode(attribute1, attribute2);
}
}
/**
* Static utility methods pertaining to {#link ObjectA}, {#link ObjectB}, and {#link ObjectC}'s {#code attribute1} and
* {#code attribute2}.
*/
public final class Attribute1AndAttribute2Utils {
private Attribute1AndAttribute2Utils() { /* prevents instantiation */ }
public static Multimap<Attribute1AndAttribute2, Object> groupedByAttribute1AndAttribute2(List<ObjectA> as, List<ObjectB> bs, List<ObjectC> cs) {
Iterable<Object> abcs = Iterables.concat(as, bs, cs);
return Multimaps.index(abcs, Attribute1AndAttribute2ForObjectFunction.INSTANCE);
}
// enum singleton pattern
private enum Attribute1AndAttribute2ForObjectFunction implements Function<Object, Attribute1AndAttribute2> {
INSTANCE;
#Override
public Attribute1AndAttribute2 apply(#Nullable Object object) {
if (object instanceof ObjectA) {
ObjectA objectA = (ObjectA) object;
return new Attribute1AndAttribute2(objectA.attribute1, objectA.attribute2);
} else if (object instanceof ObjectB) {
ObjectB objectB = (ObjectB) object;
return new Attribute1AndAttribute2(objectB.attribute1, objectB.attribute2);
} else if (object instanceof ObjectC) {
ObjectC objectC = (ObjectC) object;
return new Attribute1AndAttribute2(objectC.attribute1, objectC.attribute2);
} else {
throw new RuntimeException("Object must be ObjectA, ObjectB, or ObjectC, but was " + object);
}
}
}
}
Note that the code would a be a lot cleaner / better if you had an interface such as:
public interface HasAttribute1AndAttribute2 {
Integer getAttribute1();
String getAttribute2();
}
Also, I assumed that attribute1 and attribute2 may be null. If they are never null, consider changing the pair class to:
/**
* A "pair" class that contains an {#code attribute1} and an {#code attribute2}. Mainly used as a {#link Map} key.
*/
#Immutable
public class Attribute1AndAttribute2 {
#Nonnull
private final Integer attribute1;
#Nonnull
private final String attribute2;
public Attribute1AndAttribute2(#Nonnull Integer attribute1, #Nonnull String attribute2) {
this.attribute1 = checkNotNull(attribute1);
this.attribute2 = checkNotNull(attribute2);
}
#Nonnull
public Integer getAttribute1() {
return attribute1;
}
#Nonnull
public String getAttribute2() {
return attribute2;
}
#Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (obj instanceof Attribute1AndAttribute2) {
Attribute1AndAttribute2 that = (Attribute1AndAttribute2) obj;
return this.attribute1.equals(that.attribute1)
&& this.attribute2.equals(that.attribute2);
}
return false;
}
#Override
public int hashCode() {
return Objects.hashCode(attribute1, attribute2);
}
}