1、某班需要班长主持班会，应用单例模式（分别用懒汉式单例和饿汉式单例两种实现形式）产生班长，模拟班会举行。

懒汉式单例实现：

public class ClassLeader {
    private static ClassLeader instance;
    private ClassLeader() {}
    public static synchronized ClassLeader getInstance() {
        if (instance == null) {
            instance = new ClassLeader();
        }
        return instance;
    }
    public void presideMeeting() {
        System.out.println("班长主持班会");
    }
}

public class Main {
    public static void main(String[] args) {
        ClassLeader leader = ClassLeader.getInstance();
        leader.presideMeeting();
    }
}

饿汉式单例实现：

public class ClassLeader {
    private static final ClassLeader instance = new ClassLeader();
    private ClassLeader() {}
    public static ClassLeader getInstance() {
        return instance;
    }
    public void presideMeeting() {
        System.out.println("班长主持班会");
    }
}

public class Main {
    public static void main(String[] args) {
        ClassLeader leader = ClassLeader.getInstance();
        leader.presideMeeting();
    }
}