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sphere-java-client
There is a newer version: 0.72.1
package io.sphere.client.model;

import com.google.common.base.Strings;
import io.sphere.client.ReferenceException;
import org.codehaus.jackson.annotate.JsonValue;

import javax.annotation.Nonnull;

/** A null object signalling that a reference field in some parent object was empty,
 *  to prevent NullPointerExceptions and provide better error messages. */
public class EmptyReference extends Reference {
    @Nonnull private String fieldName;

    private EmptyReference(String fieldName) {
        if (Strings.isNullOrEmpty(fieldName)) throw new IllegalArgumentException("EmptyReference.fieldName can't be empty.");
        this.fieldName = fieldName;
    }

    /** A field name hinting where this reference was used. */
    @Nonnull public String getFieldName() { return fieldName; }

    @JsonValue
    /** Tells JSON serializer to serialize this class as null. */
    protected Object toJson() {
        return null;
    }

    /** Creates a dummy empty reference that serves as a null object to prevent NullPointerExceptions.
     * @param fieldName Name of the attribute in the parent object where the reference is used, for better error messages. */
    public static  Reference create(String fieldName) {
        return new EmptyReference(fieldName);
    }

    private ReferenceException emptyReferenceException() {
        throw new ReferenceException("This object has no: " + fieldName + ".");
    }

    /** Checks whether a reference has been expanded.
     *
     * User code should always check for {@link #isExpanded() isExpanded} before calling {@link #get()}.
     * This way, the user won't get an exception even if the reference was empty in the parent object. */
    @Override public boolean isExpanded() { return false; }

    @Override public boolean isEmpty() { return true; }

    @Override public T get() throws ReferenceException { throw emptyReferenceException(); }

    @Override public String getId() throws ReferenceException { throw emptyReferenceException(); }

    @Override public String getTypeId() throws ReferenceException { throw emptyReferenceException(); }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof EmptyReference)) return false;

        EmptyReference that = (EmptyReference) o;

        if (!fieldName.equals(that.fieldName)) return false;

        return true;
    }

    @Override
    public int hashCode() {
        return fieldName.hashCode();
    }

    @Override
    public String toString() {
        return "EmptyReference{" +
                "fieldName='" + fieldName + '\'' +
                '}';
    }

    @Override
    public ReferenceId toReferenceIdOrNull() {
        return null;
    }
}