单例模式

饿汉式

class Demo {
    private static Demo instance;
    
    private Demo() {}
    
    static {
        instance = new Demo();
    }
    
    public static Demo getInstance() {
        return instance;
    }
}

注：在类加载的时候就吃资源。

懒汉式

class Demo {
    private static volatile Demo instance;
    
    private Demo() {}
    
    public static Demo getInstance() {
        if (instance==null){
            synchronized (Demo.class) {
                if (instance==null){
                    instance = new Demo();
                }
            }
        }
        return instance;
    }
}

注：在调用时才实例化对象。

静态内部类

class Demo {
    private static volatile Demo instance;
    
    private Demo() {}
    
    private static class DemoInstance {
        private static final Demo INSTANCE = new Demo();
    }
    
    public static Demo getInstance() {
        return DemoInstance.INSTANCE;
    }
}

枚举

enum Demo {
    INSTANCE;
    public void echo() {
        System.out.println("echo");
    }
}