Java中的instanceof运算符与isInstance()方法

instanceof运算符和isInstance()方法都用于检查对象的类。但主要区别在于，当我们想要动态检查对象的类时，isInstance() 方法将起作用。我们无法通过 instanceof运算符来做到这一点。

isInstance 方法等价于 instanceof运算符。该方法用于在运行时使用反射创建对象的情况。一般做法是，如果要在运行时检查类型，则使用 isInstance 方法，否则可以使用 instanceof运算符。

instanceof运算符和 isInstance() 方法都返回一个布尔值。 isInstance() 方法是Java中 Class 类的方法，而 instanceof 是运算符。

考虑一个例子：

Java

// Java program to demonstrate working of
// instanceof operator
  
public class Test
{
    public static void main(String[] args)
    {
        Integer i = new Integer(5);
  
        // prints true as i is instance of class
        // Integer
        System.out.println(i instanceof Integer);
    }
}

输出：

true

现在如果我们想在运行时检查对象的类，那么我们必须使用isInstance()方法。

Java

// Java program to demonstrate working of isInstance()
// method
public class Test {
    // This method tells us whether the object is an
    // instance of class whose name is passed as a
    // string 'c'.
    public static boolean fun(Object obj, String c)
        throws ClassNotFoundException
    {
        return Class.forName(c).isInstance(obj);
    }
  
    // Driver code that calls fun()
    public static void main(String[] args)
        throws ClassNotFoundException
    {
        Integer i = new Integer(5);
  
        // print true as i is instance of class
        // Integer
        boolean b = fun(i, "java.lang.Integer");
  
        // print false as i is not instance of class
        // String
        boolean b1 = fun(i, "java.lang.String");
  
        // print true as i is also instance of class
        // Number as Integer class extends Number
        // class
        boolean b2 = fun(i, "java.lang.Number");
  
        System.out.println(b);
        System.out.println(b1);
        System.out.println(b2);
    }
}

输出：

true
false
true

Note: instanceof operator throws compile-time error(Incompatible conditional operand types) if we check object with other classes which it doesn’t instantiate.

Java

public class Test {
    public static void main(String[] args)
    {
        Integer i = new Integer(5);
  
        // Below line causes compile time error:-
        // Incompatible conditional operand types
        // Integer and String
        System.out.println(i instanceof String);
    }
}

输出 ：

9: error: incompatible types: Integer cannot be converted to String
        System.out.println(i instanceof String);
                           ^

必读：

• Java中的 new运算符与 newInstance() 方法
• Java中的反射