Java 中关键字的实例
原文:https://www.geeksforgeeks.org/instanceof-keyword-in-java/
instanceof 是一个关键字，用于检查引用变量是否包含给定类型的对象引用。下面是一个展示 instanceof 不同行为的 Java 程序。此后，它被称为比较运算符，其中实例与返回布尔真或假的类型进行比较，因为在 java 中，我们没有 0 和 1 布尔返回类型。
示例:
Java
// Java Program to Illustrate instanceof Keyword
// Importing required I/O classes
import java.io.*;
// Main class
class GFG {
public static void main(String[] args)
{
// Creating object of class inside main()
GFG object = new GFG();
// Returning instanceof
System.out.println(object instanceof GFG);
}
}
输出
true
实现:这里我们将创建具有父子关系的示例类。
例 1:
Java
// Java program to demonstrate working of instanceof Keyword
// Class 1
// Parent class
class Parent {}
// Class 2
// Child class
class Child extends Parent {}
// Class 3
// Main class
class GFG {
// Main driver method
public static void main(String[] args)
{
// Creating object of child class
Child cobj = new Child();
// A simple case
if (cobj instanceof Child)
System.out.println("cobj is instance of Child");
else
System.out.println(
"cobj is NOT instance of Child");
// instanceof returning true for Parent class also
if (cobj instanceof Parent)
System.out.println(
"cobj is instance of Parent");
else
System.out.println(
"cobj is NOT instance of Parent");
// instanceof returns true for all ancestors
// Note : Object is ancestor of all classes in Java
if (cobj instanceof Object)
System.out.println(
"cobj is instance of Object");
else
System.out.println(
"cobj is NOT instance of Object");
}
}
输出
cobj is instance of Child
cobj is instance of Parent
cobj is instance of Object
示例 2: 为空返回假的实例
Java
// Java program to demonstrate that instanceof
// returns false for null
class Test { }
class Main
{
public static void main(String[] args)
{
Test tobj = null;
// A simple case
if (tobj instanceof Test)
System.out.println("tobj is instance of Test");
else
System.out.println("tobj is NOT instance of Test");
}
}
输出:
tobj is NOT instance of Test
示例 3: 父对象不是子对象
Java
// A Java program to show that a parent object is
// not an instance of Child
class Parent { }
class Child extends Parent { }
class Test
{
public static void main(String[] args)
{
Parent pobj = new Parent();
if (pobj instanceof Child)
System.out.println("pobj is instance of Child");
else
System.out.println("pobj is NOT instance of Child");
}
}
的实例
输出:
pobj is NOT instance of Child
示例 4: 引用子代的父代引用是子代
Java
// A Java program to show that a parent reference
// referring to a Child is an instance of Child
class Parent { }
class Child extends Parent { }
class Test
{
public static void main(String[] args)
{
// Reference is Parent type but object is
// of child type.
Parent cobj = new Child();
if (cobj instanceof Child)
System.out.println("cobj is instance of Child");
else
System.out.println("cobj is NOT instance of Child");
}
}
的实例
输出:
cobj is an instance of Child
现在关键词的应用实例如下:
我们在这里看到当父类型的引用引用子对象时，父类数据成员被访问。我们可以使用类型转换访问子数据成员。
语法:
((child_class_name) Parent_Reference_variable).func.name()
当我们进行类型转换时，检查类型转换是否有效总是一个好主意。instanceof 在这里帮助我们。我们总是可以首先使用 instancef 检查有效性，然后进行类型转换。
例
爪哇
// Java program to demonstrate that non-method
// members are accessed according to reference
// type (Unlike methods which are accessed according
// to the referred object)
class Parent
{
int value = 1000;
}
class Child extends Parent
{
int value = 10;
}
// Driver class
class Test
{
public static void main(String[] args)
{
Parent cobj = new Child();
Parent par = cobj;
// Using instanceof to make sure that par
// is a valid reference before typecasting
if (par instanceof Child)
{
System.out.println("Value accessed through " +
"parent reference with typecasting is " +
((Child)par).value);
}
}
}
输出:
Value accessed through parent reference with typecasting is 10
本文由闪烁泰亚吉供稿。如果你喜欢 GeeksforGeeks 并想投稿，你也可以使用write.geeksforgeeks.org写一篇文章或者把你的文章邮寄到 review-team@geeksforgeeks.org。看到你的文章出现在极客博客主页上，帮助其他极客。如果你发现任何不正确的地方，或者你想分享更多关于上面讨论的话题的信息，请写评论。
版权属于：月萌API www.moonapi.com，转载请注明出处
