综合利用Swing界面和自己写音乐播放类，设计音乐播放器如下：（1）纯Swing给定的组件可以设计出一个普通的音乐播放器界面。（2）音乐播放方面，不能调用其他第三方jar包，只能用JDK底层自带的API类，例如JavaSound。使用JavaSound API，可以实现各种基于声音的应用，例如声音录制、音乐播放、网络电话、音乐编辑等。JavaSound API又以各种解码和合成器SPI（服务提供者接口， Service Provider Interface）为基础，实现各种音乐格式的解码与转码。 给出java代码

好的，下面是一个基于JavaSound API的音乐播放器的示例代码。代码中包含了一个基本的Swing界面和一个自定义的音乐播放类，实现了音乐的播放、暂停、停止、上一曲、下一曲等功能。

import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.File;
import java.io.IOException;

import javax.sound.sampled.AudioFormat;
import javax.sound.sampled.AudioInputStream;
import javax.sound.sampled.AudioSystem;
import javax.sound.sampled.DataLine;
import javax.sound.sampled.LineUnavailableException;
import javax.sound.sampled.SourceDataLine;
import javax.swing.JButton;
import javax.swing.JFileChooser;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class MusicPlayer extends JFrame implements ActionListener {

    private static final long serialVersionUID = 1L;

    private JButton btnOpen;
    private JButton btnPlay;
    private JButton btnPause;
    private JButton btnStop;
    private JButton btnPrev;
    private JButton btnNext;

    private AudioInputStream audioInputStream;
    private SourceDataLine line;
    private boolean playing;
    private boolean paused;
    private int currentFrame;

    private File[] playlist;
    private int currentIndex;

    public MusicPlayer() throws LineUnavailableException {
        super("Music Player");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setSize(400, 100);
        setResizable(false);

        JPanel panel = new JPanel(new BorderLayout());

        btnOpen = new JButton("Open");
        btnOpen.addActionListener(this);
        panel.add(btnOpen, BorderLayout.WEST);

        btnPlay = new JButton("Play");
        btnPlay.addActionListener(this);
        panel.add(btnPlay, BorderLayout.CENTER);

        btnPause = new JButton("Pause");
        btnPause.addActionListener(this);
        panel.add(btnPause, BorderLayout.EAST);

        btnStop = new JButton("Stop");
        btnStop.addActionListener(this);
        panel.add(btnStop, BorderLayout.SOUTH);

        btnPrev = new JButton("Prev");
        btnPrev.addActionListener(this);
        panel.add(btnPrev, BorderLayout.NORTH);

        btnNext = new JButton("Next");
        btnNext.addActionListener(this);
        panel.add(btnNext, BorderLayout.SOUTH);

        add(panel);

        playing = false;
        paused = false;
        currentFrame = 0;
        currentIndex = 0;
    }

    public static void main(String[] args) throws LineUnavailableException {
        MusicPlayer player = new MusicPlayer();
        player.setVisible(true);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == btnOpen) {
            JFileChooser fileChooser = new JFileChooser();
            fileChooser.setMultiSelectionEnabled(true);
            if (fileChooser.showOpenDialog(this) == JFileChooser.APPROVE_OPTION) {
                playlist = fileChooser.getSelectedFiles();
                currentIndex = 0;
                playFile(playlist[currentIndex]);
            }
        } else if (e.getSource() == btnPlay) {
            if (paused) {
                paused = false;
            } else if (!playing) {
                playFile(playlist[currentIndex]);
            }
        } else if (e.getSource() == btnPause) {
            paused = true;
        } else if (e.getSource() == btnStop) {
            stopPlaying();
        } else if (e.getSource() == btnPrev) {
            if (currentIndex > 0) {
                currentIndex--;
                stopPlaying();
                playFile(playlist[currentIndex]);
            }
        } else if (e.getSource() == btnNext) {
            if (currentIndex < playlist.length - 1) {
                currentIndex++;
                stopPlaying();
                playFile(playlist[currentIndex]);
            }
        }
    }

    private void playFile(File file) {
        try {
            audioInputStream = AudioSystem.getAudioInputStream(file);
            AudioFormat format = audioInputStream.getFormat();
            DataLine.Info info = new DataLine.Info(SourceDataLine.class, format);
            line = (SourceDataLine) AudioSystem.getLine(info);
            line.open(format);

            byte[] buffer = new byte[4096];
            int n = 0;
            line.start();
            playing = true;
            while (n != -1 && playing) {
                n = audioInputStream.read(buffer, 0, buffer.length);
                if (n > 0) {
                    currentFrame += n;
                    line.write(buffer, 0, n);
                    if (paused) {
                        line.stop();
                        line.flush();
                    }
                }
            }

            line.stop();
            line.flush();
            line.close();
            audioInputStream.close();
            playing = false;
            paused = false;
        } catch (IOException | LineUnavailableException e) {
            e.printStackTrace();
        }
    }

    private void stopPlaying() {
        if (playing) {
            playing = false;
            paused = false;
            line.stop();
            line.flush();
            line.close();
            currentFrame = 0;
        }
    }

}

这段代码中，我们定义了一个名为MusicPlayer的类，继承自JFrame，实现了ActionListener接口。在构造函数中，我们创建了一个Swing界面，并初始化了播放器的状态。在actionPerformed方法中，我们实现了各个按钮的响应事件。

在playFile方法中，我们使用JavaSound API读取音乐文件并创建SourceDataLine，然后通过循环读取音乐数据并写入到SourceDataLine中，实现了音乐的播放功能。在stopPlaying方法中，我们停止了当前正在播放的音乐。

希望这个示例代码能够帮助你完成音乐播放器的设计。