用Java输入5个学生的信息（包含学号、姓名、3科成绩），统计各学生的总分，然后将学生信息和统计结果存入二进制数据文件STUDENT.DAT中。从建立的STUDENT.DAT文件中读取数据，寻找平均分最高的学生，并输出该学生的所有信息。

以下是用Java实现该功能的代码：

import java.io.*;

public class StudentDemo {
    public static void main(String[] args) {
        Student[] students = new Student[5];
        DataOutputStream dos = null;
        DataInputStream dis = null;
        try {
            dos = new DataOutputStream(new FileOutputStream("STUDENT.DAT"));
            dis = new DataInputStream(new FileInputStream("STUDENT.DAT"));
            for (int i = 0; i < 5; i++) {
                System.out.println("请输入第" + (i + 1) + "个学生的信息：");
                System.out.print("学号：");
                int id = Integer.parseInt(System.console().readLine());
                System.out.print("姓名：");
                String name = System.console().readLine();
                System.out.print("成绩1：");
                double score1 = Double.parseDouble(System.console().readLine());
                System.out.print("成绩2：");
                double score2 = Double.parseDouble(System.console().readLine());
                System.out.print("成绩3：");
                double score3 = Double.parseDouble(System.console().readLine());
                Student student = new Student(id, name, score1, score2, score3);
                students[i] = student;
                dos.writeInt(student.getId());
                dos.writeUTF(student.getName());
                dos.writeDouble(student.getScore1());
                dos.writeDouble(student.getScore2());
                dos.writeDouble(student.getScore3());
            }
            double maxAvg = 0;
            int maxIndex = 0;
            for (int i = 0; i < 5; i++) {
                double totalScore = students[i].getScore1() + students[i].getScore2() + students[i].getScore3();
                double avgScore = totalScore / 3;
                students[i].setTotalScore(totalScore);
                students[i].setAvgScore(avgScore);
                if (avgScore > maxAvg) {
                    maxAvg = avgScore;
                    maxIndex = i;
                }
            }
            System.out.println("学生信息和统计结果已存入STUDENT.DAT文件中。");
            System.out.println("平均分最高的学生是：");
            System.out.println("学号：" + students[maxIndex].getId());
            System.out.println("姓名：" + students[maxIndex].getName());
            System.out.println("成绩1：" + students[maxIndex].getScore1());
            System.out.println("成绩2：" + students[maxIndex].getScore2());
            System.out.println("成绩3：" + students[maxIndex].getScore3());
            System.out.println("总分：" + students[maxIndex].getTotalScore());
            System.out.println("平均分：" + students[maxIndex].getAvgScore());
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                if (dos != null) {
                    dos.close();
                }
                if (dis != null) {
                    dis.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

class Student {
    private int id;
    private String name;
    private double score1;
    private double score2;
    private double score3;
    private double totalScore;
    private double avgScore;

    public Student(int id, String name, double score1, double score2, double score3) {
        this.id = id;
        this.name = name;
        this.score1 = score1;
        this.score2 = score2;
        this.score3 = score3;
    }

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public double getScore1() {
        return score1;
    }

    public double getScore2() {
        return score2;
    }

    public double getScore3() {
        return score3;
    }

    public double getTotalScore() {
        return totalScore;
    }

    public void setTotalScore(double totalScore) {
        this.totalScore = totalScore;
    }

    public double getAvgScore() {
        return avgScore;
    }

    public void setAvgScore(double avgScore) {
        this.avgScore = avgScore;
    }
}

请将代码保存为StudentDemo.java文件，并在命令行中输入以下命令来编译和运行程序：

javac StudentDemo.java
java StudentDemo

注意：在实现过程中，需要注意异常处理和资源释放，以保证程序的稳定性和可靠性。