java.lang.Object
  ⇒ java.awt.Image
      ⇒ java.awt.image.BufferedImage
public class BufferedImage extends Image implements WritableRenderedImage, Transparency

BufferedImage
public BufferedImage(int width, int height, int imageType)

BufferedImage bi = new BufferedImage(100, 100, BufferedImage.TYPE_INT_BGR);

java.lang.Object
  ⇒ javax.imageio.ImageIO
public final class ImageIO extends Object

read
public static BufferedImage read(File input) throws IOException

BufferedImage readImage = null;

try {
  readImage = ImageIO.read(new File("sample.png"));
} catch (Exception e) {
  e.printStackTrace();
  readImage = null;
}

write
public static boolean write(RenderedImage im, String formatName, File output)
                     throws IOException

BufferedImage writeImage;
...

boolean result = false;

try {
  result = ImageIO.write(writeImage, "jpeg", new File("sample.jpeg"));
} catch (Exception e) {
  e.printStackTrace();
  result = false;
}

利用可能なformatNameについて

import javax.imageio.ImageIO;

class FormatNamePrint {
  public static void main(String[] args) throws Exception {
    String[] formatNames = ImageIO.getReaderFormatNames();
    System.out.println("ReaderFormatNames:");
    for(int i = 0; i < formatNames.length; i++) {
       System.out.println(formatNames[i]);
    }

    formatNames = ImageIO.getWriterFormatNames();
    System.out.println("WriterFormatNames:");
    for(int i = 0; i < formatNames.length; i++) {
       System.out.println(formatNames[i]);
    }
  }
}

ReaderFormatNames:
JPG
jpg
bmp
BMP
gif
GIF
WBMP
png
PNG
wbmp
jpeg
JPEG

WriterFormatNames:
JPG
jpg
bmp
BMP
gif
GIF
WBMP
png
PNG
wbmp
jpeg
JPEG

背景に読み込んだ画像を表示させてみる

Graphics2D g2 = (Graphics2D)g;

BufferedImage readImage = null;
try {
  readImage = ImageIO.read(new File("sample.png"));
} catch (Exception e) {
  e.printStackTrace();
  readImage = null;
}

if (readImage != null){
  g2.drawImage(readImage, 0, 0, this);
}

/**
 * ImageIOクラスで画像ファイルを読み書きする
 */
import java.awt.*;
import java.awt.geom.*;
import java.awt.event.*;
import javax.swing.*;
import java.awt.image.BufferedImage;
import java.io.File;
import javax.imageio.ImageIO;
import java.io.IOException;

public class Java2dTest1 extends JFrame{
  public static void main(String[] args){
    Java2dTest1 test = new Java2dTest1();

    test.addWindowListener(new WindowAdapter(){
      public void windowClosing(WindowEvent e){System.exit(0);}
    });

    test.setBounds( 0, 0, 200, 200);
    test.setVisible(true);
  }

  public void paint(Graphics g){
    Graphics2D g2 = (Graphics2D)g;

    BufferedImage readImage = null;
    try {
//      readImage = ImageIO.read(new File("hana02.png"));
      readImage = ImageIO.read(new File("sample.png"));
    } catch (Exception e) {
      e.printStackTrace();
      readImage = null;
    }

    if (readImage != null){
      g2.drawImage(readImage, 0, 0, this);
    }

    g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, 
    RenderingHints.VALUE_ANTIALIAS_ON);

    BasicStroke wideStroke = new BasicStroke(4.0f);
    g2.setStroke(wideStroke);

    g2.setPaint(Color.black);
    g2.draw(new Ellipse2D.Double(30, 40, 50, 50));

    g2.setPaint(Color.blue);
    g2.draw(new Ellipse2D.Double(70, 40, 50, 50));

    g2.setPaint(Color.red);
    g2.draw(new Ellipse2D.Double(110, 40, 50, 50));

    g2.setPaint(Color.yellow);
    g2.fill(new Arc2D.Double(50, 100, 110, 110, 330, 100, Arc2D.PIE));
    g2.setPaint(Color.gray);
    g2.draw(new Arc2D.Double(50, 100, 110, 110, 330, 100, Arc2D.PIE));
  }
}

BufferedImageへの描画

BufferedImage readImage = null;

if (readImage == null){
  readImage = 
    new BufferedImage(getWidth(), getHeight(), BufferedImage.TYPE_INT_BGR);
}

Graphics2D off = readImage.createGraphics();

off.setPaint(Color.white);
off.draw(new Ellipse2D.Double(30, 40, 50, 50));

if (readImage != null){
  g2.drawImage(readImage, 0, 0, this);
}

/**
 * BufferedImageへの描画
 */
import java.awt.*;
import java.awt.geom.*;
import java.awt.event.*;
import javax.swing.*;
import java.awt.image.BufferedImage;
import java.io.File;
import javax.imageio.ImageIO;
import java.io.IOException;

public class Java2dTest2 extends JFrame{
  public static void main(String[] args){
    Java2dTest2 test = new Java2dTest2();

    test.addWindowListener(new WindowAdapter(){
      public void windowClosing(WindowEvent e){System.exit(0);}
    });

    test.setBounds( 0, 0, 200, 200);
    test.setVisible(true);
  }

  public void paint(Graphics g){
    Graphics2D g2 = (Graphics2D)g;

    BufferedImage readImage = null;
    try {
//      readImage = ImageIO.read(new File("hana02.png"));
      readImage = ImageIO.read(new File("sample.png"));
    } catch (Exception e) {
      e.printStackTrace();
      readImage = null;
    }

    if (readImage == null){
      readImage = new BufferedImage(getWidth(), getHeight(), 
        BufferedImage.TYPE_INT_BGR);
    }

    Graphics2D off = readImage.createGraphics();

    off.setRenderingHint(RenderingHints.KEY_ANTIALIASING, 
    RenderingHints.VALUE_ANTIALIAS_ON);

    BasicStroke wideStroke = new BasicStroke(4.0f);
    off.setStroke(wideStroke);

    off.setPaint(Color.white);
    off.draw(new Ellipse2D.Double(30, 40, 50, 50));

    off.setPaint(Color.blue);
    off.draw(new Ellipse2D.Double(70, 40, 50, 50));

    off.setPaint(Color.red);
    off.draw(new Ellipse2D.Double(110, 40, 50, 50));

    off.setPaint(Color.yellow);
    off.fill(new Arc2D.Double(50, 100, 110, 110, 330, 100, Arc2D.PIE));
    off.setPaint(Color.gray);
    off.draw(new Arc2D.Double(50, 100, 110, 110, 330, 100, Arc2D.PIE));

    if (readImage != null){
      g2.drawImage(readImage, 0, 0, this);
    }
  }
}

ファイルへの書き出し

/**
 * ファイルへの書き出し
 */
import java.awt.*;
import java.awt.geom.*;
import java.awt.event.*;
import javax.swing.*;
import java.awt.image.BufferedImage;
import java.io.File;
import javax.imageio.ImageIO;
import java.io.IOException;

public class Java2dTest3 extends JFrame{
  public static void main(String[] args){
    Java2dTest3 test = new Java2dTest3();

    test.addWindowListener(new WindowAdapter(){
      public void windowClosing(WindowEvent e){System.exit(0);}
    });

    test.setBounds( 0, 0, 200, 200);
    test.setVisible(true);
  }

  public void paint(Graphics g){
    Graphics2D g2 = (Graphics2D)g;

    BufferedImage readImage = null;
    try {
//      readImage = ImageIO.read(new File("hana03.png"));
      readImage = ImageIO.read(new File("sample.png"));
    } catch (Exception e) {
      e.printStackTrace();
      readImage = null;
    }

    if (readImage == null){
      readImage = new BufferedImage(getWidth(), getHeight(), 
        BufferedImage.TYPE_INT_BGR);
    }

    Graphics2D off = readImage.createGraphics();

    off.setRenderingHint(RenderingHints.KEY_ANTIALIASING, 
    RenderingHints.VALUE_ANTIALIAS_ON);

    BasicStroke wideStroke = new BasicStroke(4.0f);
    off.setStroke(wideStroke);

    off.setPaint(Color.white);
    off.draw(new Ellipse2D.Double(30, 40, 50, 50));

    off.setPaint(Color.blue);
    off.draw(new Ellipse2D.Double(70, 40, 50, 50));

    off.setPaint(Color.red);
    off.draw(new Ellipse2D.Double(110, 40, 50, 50));

    off.setPaint(Color.yellow);
    off.fill(new Arc2D.Double(50, 100, 110, 110, 330, 100, Arc2D.PIE));
    off.setPaint(Color.gray);
    off.draw(new Arc2D.Double(50, 100, 110, 110, 330, 100, Arc2D.PIE));

    if (readImage != null){
      g2.drawImage(readImage, 0, 0, this);
    }

    try {
      boolean result = 
       ImageIO.write(readImage, "png", new File("sample3.png"));
    } catch (Exception e) {
        e.printStackTrace();
    }
  }
}