Add an ImageSuffix class to the extractor

The goal of this utility class is to simply store suffixes which need to be
appended to image URLs, in order to get images at the suffix resolution.

This class contains four properties: the suffix (as a string), the height,
the width (as integers) and the estimated resolution level of the image
corresponding to the one represented by the suffix.

extractor/src/main/java/org/schabi/newpipe/extractor/utils/ImageSuffix.java

@@ -0,0 +1,104 @@
+package org.schabi.newpipe.extractor.utils;
+
+import org.schabi.newpipe.extractor.Image.ResolutionLevel;
+
+import javax.annotation.Nonnull;
+import java.io.Serializable;
+import java.util.Objects;
+
+/**
+ * Serializable class representing a suffix (including its format extension, such as {@code .jpg})
+ * which needs to be added to get an image/thumbnail URL with its corresponding height, width and
+ * estimated resolution level.
+ *
+ * <p>
+ * This class is used to construct {@link org.schabi.newpipe.extractor.Image Image}
+ * instances from a single base URL/path, in order to get all or most image resolutions provided,
+ * depending of the service and the resolutions provided.
+ * </p>
+ *
+ * <p>
+ * Note that this class is not intended to be used externally and so should only be used when
+ * interfacing with the extractor.
+ * </p>
+ */
+public final class ImageSuffix implements Serializable {
+    @Nonnull
+    private final String suffix;
+    private final int height;
+    private final int width;
+    @Nonnull
+    private final ResolutionLevel resolutionLevel;
+
+    /**
+     * Create a new {@link ImageSuffix} instance.
+     *
+     * @param suffix                   the suffix string
+     * @param height                   the height corresponding to the image suffix
+     * @param width                    the width corresponding to the image suffix
+     * @param estimatedResolutionLevel the {@link ResolutionLevel} of the image suffix, which must
+     *                                 not be null
+     * @throws NullPointerException if {@code estimatedResolutionLevel} is {@code null}
+     */
+    public ImageSuffix(@Nonnull final String suffix,
+                       final int height,
+                       final int width,
+                       @Nonnull final ResolutionLevel estimatedResolutionLevel)
+            throws NullPointerException {
+        this.suffix = suffix;
+        this.height = height;
+        this.width = width;
+        this.resolutionLevel = Objects.requireNonNull(estimatedResolutionLevel,
+                "estimatedResolutionLevel is null");
+    }
+
+    /**
+     * @return the suffix which needs to be appended to get the full image URL
+     */
+    @Nonnull
+    public String getSuffix() {
+        return suffix;
+    }
+
+    /**
+     * @return the height corresponding to the image suffix, which may be unknown
+     */
+    public int getHeight() {
+        return height;
+    }
+
+    /**
+     * @return the width corresponding to the image suffix, which may be unknown
+     */
+    public int getWidth() {
+        return width;
+    }
+
+    /**
+     * @return the estimated {@link ResolutionLevel} of the suffix, which is never null.
+     */
+    @Nonnull
+    public ResolutionLevel getResolutionLevel() {
+        return resolutionLevel;
+    }
+
+    /**
+     * Get a string representation of this {@link ImageSuffix} instance.
+     *
+     * <p>
+     * The representation will be in the following format, where {@code suffix}, {@code height},
+     * {@code width} and {@code resolutionLevel} represent the corresponding properties:
+     * <br>
+     * <br>
+     * {@code ImageSuffix {url=url, height=height, width=width, resolutionLevel=resolutionLevel}'}
+     * </p>
+     *
+     * @return a string representation of this {@link ImageSuffix} instance
+     */
+    @Nonnull
+    @Override
+    public String toString() {
+        return "ImageSuffix {" + "suffix=" + suffix + ", height=" + height + ", width="
+                + width + ", resolutionLevel=" + resolutionLevel + "}";
+    }
+}