Java has built-in API for resizing images in java.awt package. The code example below uses the Graphics2D and BufferedImage classes to resize all images in a directory. The ImageIO class supports a number of file formats such as jpeg, png, gif and bmp. The code below uses a percentage value for resizing the image. This ensures that the aspect ratio of the image is maintained during image resize.

When images are written to the destination folder, the extensions of source files are changed to the format of the output files. This means that you cannot have more than one file with same name but different extension in the source folder. If they are present in source folder, change the removeExtension() method to allow for duplicate names.

import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.image.BufferedImage;
import java.io.File;

import javax.imageio.ImageIO;


// Resizes all images in a folder to another folder
// IMPORTANT: Cannot have same file name with different extensions in the source folder.
public class ImageResizer {

    public static void main(String[] args) throws Exception {
        ImageResizer resizer = new ImageResizer();
        ImageOutputConfig config = new ImageOutputConfig();

        // Quality hints to Graphics2D
        RenderingHints hints = new RenderingHints(RenderingHints.KEY_INTERPOLATION,
                RenderingHints.VALUE_INTERPOLATION_BILINEAR);
        hints.put(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY);
        hints.put(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

        config.hints = hints;
        config.formatName = "jpg"; // use jpg, bmp, gif etc.
        config.resizePercentage = 50; // Resize to 50%

        String sourceDir = "/Users/jj/source-images/";
        String destDir = "/Users/jj/dest-images/";

        resizer.resizeAllImages(sourceDir, destDir, config);
    }

    // Java program to resize all images in the specified folder
    // Ignores all hidden files.
    // IMPORTANT: If source folder contains identical file names with different extensions only one appear in output
    public void resizeAllImages(String sourceDirName, String destDirName, ImageOutputConfig config) throws Exception {
        File sourceDir = new File(sourceDirName);
        File destFolder = new File(destDirName);
        destFolder.mkdir(); // create destination if missing
        File[] sourceNames = sourceDir.listFiles();

        BufferedImage sourceImage;
        BufferedImage outputImage;
        File newFile;
        for (int i = 0; i < sourceNames.length; i++) {
            if (sourceNames[i].isFile() && !sourceNames[i].isHidden()) {
                sourceImage = ImageIO.read(new File(sourceDirName + sourceNames[i].getName()));
                outputImage = resizeImage(sourceImage, config);

                newFile = new File(destDirName + removeExtension(sourceNames[i]) + "." + config.formatName);
                ImageIO.write(outputImage, config.formatName, newFile);
            }
        }
    }

    // Resizes the image using the configuration passed in
    private BufferedImage resizeImage(BufferedImage source, ImageOutputConfig config) {
        int newHeight = (int) (source.getHeight() * config.resizePercentage / 100);
        int newWidth = (int) (source.getWidth() * config.resizePercentage / 100);
        
        // PNG supports transparency
        int type = config.formatName.equals("png")?BufferedImage.TYPE_INT_ARGB:BufferedImage.TYPE_INT_RGB;
        
        BufferedImage outputImage = new BufferedImage(newWidth, newHeight, type);
        Graphics2D graphics2D = outputImage.createGraphics();
        if (config.hints != null) {
            graphics2D.setRenderingHints(config.hints);
        }
        graphics2D.drawImage(source, 0, 0, newWidth, newHeight, null);
        graphics2D.dispose();
        return outputImage;
    }

    // Removes file extension. The new extension depends on format name
    private String removeExtension(File file) {
        String name = file.getName();
        int pos = name.lastIndexOf(".");
        if (pos > 0) {
            name = name.substring(0, pos);
        }
        return name;
    }
}

// Wrapper for image ouput parameters
class ImageOutputConfig {
    String formatName;
    double resizePercentage;
    RenderingHints hints;
}

The quality of the output image generated by the above program may not useful for many use cases. In such cases, it is recommended to use a third party library such as this one.