Update image save workflow

* throw new checked exception if format not found
* add `saveAsPng` method

build.gradle

@@ -8,7 +8,7 @@ plugins {
 }
 
 group 'com.docutools'
-version = '4.2.8'
+version = '5.0.0'
 
 java {
     toolchain {

src/main/java/com/docutools/jocument/image/DefaultImageReference.java

@@ -40,19 +40,28 @@ public BufferedImage getImage() {
   }
 
   @Override
-  public Path saveAsJpeg() throws IOException {
+  public Path saveAsJpeg() throws IOException, NoWriterFoundException {
+    return saveAs(".jpg", "JPEG");
+  }
+
+  @Override
+  public Path saveAsPng() throws IOException, NoWriterFoundException {
+    return saveAs(".png", "PNG");
+  }
+
+  private Path saveAs(String suffix, String formatName) throws IOException, NoWriterFoundException {
     if (image == null) {
       throw new ImageReferenceClosedException("Image was already closed.");
     }
 
-    var filePath = Files.createTempFile("jocument-", ".jpg");
+    var filePath = Files.createTempFile("jocument-", suffix);
     log.trace("Saving image {} to '{}'...", id, filePath);
 
-    boolean foundWriter = ImageIO.write(image, "JPEG", filePath.toFile());
+    boolean foundWriter = ImageIO.write(image, formatName, filePath.toFile());
     if (!foundWriter) {
-      throw new IOException("Found no JPG writer");
+      throw new NoWriterFoundException(formatName);
     }
-    log.trace("Successfully saved image {} to '{}'!", id, filePath);
+    log.trace("Successfully saved image {} to '{}'", id, filePath);
 
     return filePath;
   }

src/main/java/com/docutools/jocument/image/ImageReference.java

@@ -24,7 +24,7 @@ public abstract class ImageReference implements AutoCloseable {
    * @param width width in pixel
    * @param height height in pixel
    */
-  public ImageReference(int width, int height) {
+  protected ImageReference(int width, int height) {
     this.id = UUID.randomUUID();
     this.width = width;
     this.height = height;
@@ -62,7 +62,9 @@ public int getHeight() {
    *
    * @return the {@link Path} to the JPEG
    */
-  public abstract Path saveAsJpeg() throws IOException;
+  public abstract Path saveAsJpeg() throws IOException, NoWriterFoundException;
+
+  public abstract Path saveAsPng() throws IOException, NoWriterFoundException;
 
   @Override
   public abstract void close();

src/main/java/com/docutools/jocument/image/NoWriterFoundException.java

@@ -0,0 +1,7 @@
+package com.docutools.jocument.image;
+
+public class NoWriterFoundException extends Exception {
+  public NoWriterFoundException(String type) {
+    super("Found no writer for type %s".formatted(type));
+  }
+}