public class CPU {
    public void startup() {
        System.out.println("cpu startup!");
    }
    public void shutdown() {
        System.out.println("cpu shutdown!");
    }
}

public class Memory {
    public void startup(){
        System.out.println("memory startup!");
    }
    public void shutdown(){
        System.out.println("memory shutdown!");
    }
}

public class Disk {
    public void startup(){
        System.out.println("disk startup!");
    }
    public void shutdown(){
        System.out.println("disk shutdown!");
    }
}

public class Computer {
    private CPU cpu;
    private Memory memory;
    private Disk disk;
    public Computer() {
        cpu = new CPU();
        memory = new Memory();
        disk = new Disk();
    }
    public void startup() {
        System.out.println("start the computer!");
        cpu.startup();
        memory.startup();
        disk.startup();
        System.out.println("start computer finished!");
    }
    public void shutdown() {
        System.out.println("begin to close the computer!");
        cpu.shutdown();
        memory.shutdown();
        disk.shutdown();
        System.out.println("computer closed!");
    }
}

User 类如下:
public class User {
    public static void main(String[] args) {
        Computer computer = new Computer();
        computer.startup();
        computer.shutdown();
    }
}

输出:
start the computer! cpu startup! memory startup! disk startup!
start computer finished! begin to close the computer! cpu shutdown!  
memory shutdown!
disk shutdown!
computer closed!