Add an Image class to the extractor

Objects of this serializable class contains four properties: a URL (as a
string), a width, a height (represented as integers) and an estimated
resolution level, which can be constructed from a given height.

Possible resolution levels are:
- UNKNOWN: for unknown heights or heights <= 0;
- LOW: for heights > 0 & < 175;
- MEDIUM: for heights >= 175 & < 720;
- HIGH: for heights >= 720.

Getters of these properties are available and the constructor needs these four
properties.

extractor/src/main/java/org/schabi/newpipe/extractor/Image.java

@@ -0,0 +1,211 @@
+package org.schabi.newpipe.extractor;
+
+import javax.annotation.Nonnull;
+import java.io.Serializable;
+import java.util.Objects;
+
+/**
+ * Class representing images in the extractor.
+ *
+ * <p>
+ * An image has four properties: its URL, its height, its width and its estimated quality level.
+ * </p>
+ *
+ * <p>
+ * Depending of the services, the height, the width or both properties may be not known.
+ * Implementations <b>must use</b> the relevant unknown constants in this case
+ * ({@link #HEIGHT_UNKNOWN} and {@link #WIDTH_UNKNOWN}), to ensure properly the lack of knowledge
+ * of one or both of these properties to extractor clients.
+ * </p>
+ *
+ * <p>
+ * They should also respect the ranges defined in the estimated image resolution levels as much as
+ * possible, to ensure consistency to extractor clients.
+ * </p>
+ */
+public final class Image implements Serializable {
+
+    /**
+     * Constant representing that the height of an {@link Image} is unknown.
+     */
+    public static final int HEIGHT_UNKNOWN = -1;
+
+    /**
+     * Constant representing that the width of an {@link Image} is unknown.
+     */
+    public static final int WIDTH_UNKNOWN = -1;
+
+    @Nonnull
+    private final String url;
+    private final int height;
+    private final int width;
+    @Nonnull
+    private final ResolutionLevel estimatedResolutionLevel;
+
+    /**
+     * Construct an {@link Image} instance.
+     *
+     * @param url                      the URL to the image, which should be not null or empty
+     * @param height                   the image's height
+     * @param width                    the image's width
+     * @param estimatedResolutionLevel the image's estimated resolution level, which must not be
+     *                                 null
+     * @throws NullPointerException if {@code estimatedResolutionLevel} is null
+     */
+    public Image(@Nonnull final String url,
+                 final int height,
+                 final int width,
+                 @Nonnull final ResolutionLevel estimatedResolutionLevel)
+            throws NullPointerException {
+        this.url = url;
+        this.height = height;
+        this.width = width;
+        this.estimatedResolutionLevel = Objects.requireNonNull(
+                estimatedResolutionLevel, "estimatedResolutionLevel is null");
+    }
+
+    /**
+     * Get the URL of this {@link Image}.
+     *
+     * @return the {@link Image}'s URL.
+     */
+    @Nonnull
+    public String getUrl() {
+        return url;
+    }
+
+    /**
+     * Get the height of this {@link Image}.
+     *
+     * <p>
+     * If it is unknown, {@link #HEIGHT_UNKNOWN} is returned instead.
+     * </p>
+     *
+     * @return the {@link Image}'s height or {@link #HEIGHT_UNKNOWN}
+     */
+    public int getHeight() {
+        return height;
+    }
+
+    /**
+     * Get the width of this {@link Image}.
+     *
+     * <p>
+     * If it is unknown, {@link #WIDTH_UNKNOWN} is returned instead.
+     * </p>
+     *
+     * @return the {@link Image}'s width or {@link #WIDTH_UNKNOWN}
+     */
+    public int getWidth() {
+        return width;
+    }
+
+    /**
+     * Get the estimated resolution level of this image.
+     *
+     * <p>
+     * If it is unknown, {@link ResolutionLevel#UNKNOWN} is returned instead.
+     * </p>
+     *
+     * @return the estimated resolution level, which is never {@code null}
+     * @see ResolutionLevel
+     */
+    @Nonnull
+    public ResolutionLevel getEstimatedResolutionLevel() {
+        return estimatedResolutionLevel;
+    }
+
+    /**
+     * Get a string representation of this {@link Image} instance.
+     *
+     * <p>
+     * The representation will be in the following format, where {@code url}, {@code height},
+     * {@code width} and {@code estimatedResolutionLevel} represent the corresponding properties:
+     * <br>
+     * <br>
+     * {@code Image {url=url, height='height, width=width,
+     * estimatedResolutionLevel=estimatedResolutionLevel}'}
+     * </p>
+     *
+     * @return a string representation of this {@link Image} instance
+     */
+    @Nonnull
+    @Override
+    public String toString() {
+        return "Image {" + "url=" + url + ", height=" + height + ", width=" + width
+                + ", estimatedResolutionLevel=" + estimatedResolutionLevel + "}";
+    }
+
+    /**
+     * The estimated resolution level of an {@link Image}.
+     *
+     * <p>
+     * Some services don't return the size of their images, but we may know for a specific image
+     * type that a service returns, according to real data, an approximation of the resolution
+     * level.
+     * </p>
+     */
+    public enum ResolutionLevel {
+
+        /**
+         * The high resolution level.
+         *
+         * <p>
+         * This level applies to images with a height greater than or equal to 720px.
+         * </p>
+         */
+        HIGH,
+
+        /**
+         * The medium resolution level.
+         *
+         * <p>
+         * This level applies to images with a height between 175px inclusive and 720px exclusive.
+         * </p>
+         */
+        MEDIUM,
+
+        /**
+         * The low resolution level.
+         *
+         * <p>
+         * This level applies to images with a height between 1px inclusive and 175px exclusive.
+         * </p>
+         */
+        LOW,
+
+        /**
+         * The unknown resolution level.
+         *
+         * <p>
+         * This value is returned when the extractor doesn't know what resolution level an image
+         * could have, for example if the extractor loops in an array of images with different
+         * resolution levels without knowing the height.
+         * </p>
+         */
+        UNKNOWN;
+
+        /**
+         * Get a {@link ResolutionLevel} based from the given height.
+         *
+         * @param heightPx the height from which returning the good {@link ResolutionLevel}
+         * @return the {@link ResolutionLevel} corresponding to the height provided. See the
+         * {@link ResolutionLevel} values for details about what value is returned.
+         */
+        public static ResolutionLevel fromHeight(final int heightPx) {
+            if (heightPx <= 0) {
+                return UNKNOWN;
+            }
+
+            if (heightPx < 175) {
+                return LOW;
+            }
+
+            if (heightPx < 720) {
+                return MEDIUM;
+            }
+
+            return HIGH;
+        }
+    }
+}