单例模式

懒汉模式

public class Demo2 {
    private static Demo1 demo = null;
    public Demo2() {
    }
    private static Demo1 getInstance() {
        if (demo == null) {
            synchronized (Demo2.class) {
                if (demo == null) {
                    demo = new Demo1();
                }
            }
        }
        return demo;
    }
}

饿汉模式

public class Demo3 {
    public Demo3() {
    }
    private static Demo1 demo = new Demo1();
    private static Demo1 getInstance() {
        return demo;
    }
}

静态内部类模式

public class Demo4 {
    public Demo4() {
    }
    private static class DemoHolder {
        private static final Demo1 DEMO = new Demo1();
    }
    private static Demo1 getInstance() {
        return DemoHolder.DEMO;
    }
}

枚举模式

public class Demo5 {
    public static void main(String[] args) {
        DemoEnum.DEMO.getDemo();
    }
    public enum DemoEnum {
        DEMO;
        private Demo1 demo;
        public Demo1 getDemo() {
            return demo;
        }
        DemoEnum() {
            this.demo = new Demo1();
            System.out.println("111");
        }
    }
}