Distributed Query Projections Projections API

The API of the Projection looks as follows:

package com.hazelcast.projection;

import java.io.Serializable;

/**
 * Enables transforming object into other objects.
 * Exemplary usage scenario is the project() method of the {@link com.hazelcast.core.IMap}
 * <p>
 * Only 1:1 transformations allowed. Use an Aggregator to perform N:1 aggregations.
 *
 * @param <I> input type
 * @param <O> output type
 */
public abstract class Projection<I, O> implements Serializable {

    /**
     * Transforms the input object into the output object.
     *
     * @param input object.
     * @return the output object.
     */
    public abstract O transform(I input);

}

The transform() method is called on each result object. Its result is then gathered as the final query result entity.

Projections and Map Interfaces

Projections are available on com.hazelcast.core.IMap only.

There are two methods that enable using the Projections:

/**
     * Applies the projection logic on all map entries and returns the result
     *
     * @param projection projection to transform the entries with. May return null.
     * @param <R>        type of the result
     * @return the result of the given type
     */
    <R> Collection<R> project(Projection<Map.Entry<K, V>, R> projection);

    /**
     * Applies the projection logic on map entries filtered with the Predicated and returns the result
     *
     * @param projection projection to transform the entries with. May return null.
     * @param predicate  predicate to filter the entries with
     * @param <R>        type of the result
     * @return the result of the given type
     */
    <R> Collection<R> project(Projection<Map.Entry<K, V>, R> projection, Predicate<K, V> predicate);