instanceof | 易学教程

How instanceof will work on an interface

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问题 instanceof can be used to test if an object is a direct or descended instance of a given class. instanceof can also be used with interfaces even though interfaces can't be instantiated like classes. Can anyone explain how instanceof works? 回答1: First of all, we can store instances of classes that implements a particular interface in an interface reference variable like this. package com.test; public class Test implements Testeable { public static void main(String[] args) { Testeable testeable

why instanceof does not work with Generic? [duplicate]

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Possible Duplicate: Java: Instanceof and Generics I am trying to write a function which cast a generic List to specific type List. Find the code below public <T>List<T> castCollection(List srcList, Class<T> clas){ List<T> list =new ArrayList<T>(); for (Object obj : srcList) { if(obj instanceof T){ ... } } return list; } But obj instanceof T showing a compilation error - Cannot perform instanceof check against type parameter T. Use instead its erasure Object >instead since further generic type information will be erased at runtime. any clarification or way to get the desired result? Thanks in

Does “instanceof Void” always return false?

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Can this method return true somehow? public static <T> boolean isVoid(T t) { return t instanceof Void; } Sanjay T. Sharma Yes, but I'm sure that isn't really useful: public static void main(final String[] args) throws Exception { final Constructor c = Void.class.getDeclaredConstructors()[0]; c.setAccessible(true); System.out.println(c.newInstance(null) instanceof Void); } A Void class can't be instantiated so normally your code wouldn't require to deal with Void instances. The above code snippet is just an example of what havoc you can unleash when using reflection... ;-) I fail to see why you

How to efficiently check if variable is Array or Object (in NodeJS & V8)?

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问题 Is there any way to efficiently check if the variable is Object or Array, in NodeJS & V8? I'm writing a Model for MongoDB and NodeJS, and to traverse the object tree I need to know if the object is simple (Number, String, ...) or composite (Hash, Array). It seems that V8 has fast built-in Array.isArray , but how to check if object is an Object? I mean complex object like hash {} or instance of class, not something like new String() ? Usually it may be done as this: Object.prototype.toString

MyCAT SQL ON MongoDB

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1. 概述可能你在看到这个标题会小小的吃惊，MyCAT 能使用 MongoDB 做数据节点。是的，没错，确实可以。吼吼吼，让我们开启这段神奇的“旅途”。本文主要分成四部分：总体流程，让你有个整体的认识查询操作插入操作彩蛋，😈彩蛋，🙂彩蛋建议你看过这两篇文章（非必须）：《MyCAT 单库单表插入》《 MyCAT 单库单表查询》 2. 主流程 MyCAT Server 接收 MySQL Client 基于 MySQL协议的请求，翻译 SQL 成 MongoDB操作发送给 MongoDB Server 。 MyCAT Server 接收 MongoDB Server 返回的 MongoDB数据，翻译成 MySQL数据结果返回给 MySQL Client 。这样一看，MyCAT 连接 MongoDB 是不是少神奇一点列。 Java数据库连接，（Java Database Connectivity，简称JDBC）是Java语言中用来规范客户端程序如何来访问数据库的应用程序接口，提供了诸如查询和更新数据库中数据的方法。JDBC也是Sun Microsystems的商标。JDBC是面向关系型数据库的。 MyCAT 使用 JDBC 规范，抽象了对 MongoDB 的访问。通过这样的方式，MyCAT 也抽象了 SequoiaDB 的访问。可能这样说法有些抽象

How to determine if an object is an instance of certain derived C++ class from a pointer to a base class in GDB?

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问题 I'm debugging a C++ program with GDB. I have a pointer to an object of certain class. The pointer is declared to be of some super class which is extended by several sub-classes. There is no fields in the object to specify the precise class type of this object but some virtual functions (e.g. bool is_xxx()) are defined to tell the class type at runtime. Is there some way to tell the precise class type of an object in GDB without calling these virtual functions. Calling such functions in GDB

Why does this instanceof code work and does not cause a compile time error?

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问题 In the following code, the type of x is I (although x also implements J but thats not known at compile time) so why is it that the code at (1) doesn't result in a compile time error. Because at compile time only the type of the reference is considered. public class MyClass { public static void main(String[] args) { I x = new D(); if (x instanceof J) //(1) System.out.println("J"); } } interface I {} interface J {} class C implements I {} class D extends C implements J {} 回答1: instanceof is

Check ArrayList for instance of object

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问题 I have a java method that should check through an ArrayList and check if it contains an instance of a given class. I need to pass the method the type of class to check for as a parameter, and if the List contains an object of the given type, then return it. Is this achievable? 回答1: public static <T> T find(Collection<?> arrayList, Class<T> clazz) { for(Object o : arrayList) { if (o != null && o.getClass() == clazz) { return clazz.cast(o); } } return null; } and call String match = find

Is it possible to use instanceof when passing objects between Threads?

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I've run into an issue where instanceof works, and then it doesn't. Going into details is difficult, but I think this might be the problem: Reading this: http://www.theserverside.com/news/thread.tss?thread_id=40229 (search for Thread.currentThread), it seems to imply that, even if the two objects are the same class, if you pass them between threads with different class loaders, instanceof (and isAssignableFrom) might still fail. This certainly would explain the behavior I'm having, but I was wondering if anyone could verify it? (I wish the article linked at the beginning of the discussion was

Example of ´instanceof´

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public class TableModel2 extends TableModel1 { ... } TableModel2 tableModel = new TableModel2(); boolean t1 = tableModel instanceof TableModel1; boolean t2 = tableModel instanceof TableModel2; In the above example, t1 and t2 are true . So, how could I differentiate between TableModel1 and TableModel2 using instanceof ? boolean t2 = tableModel.getClass().equals(TableModel1.class); //False boolean t2 = tableModel.getClass().equals(TableModel2.class); //True You cannot do it with instanceof , but you can do it with getClass : boolean t1 = tableModel.getClass().equals(TableModel1.class); boolean