java - 使用instanceof检查数组元素是否属于某个类

Question

所以我做了两个类。

1. 矩形
2. coloredRectangle 扩展了 Rectangle

然后我制作了一个矩形数组并插入了此检查:

if(tabel[i] instanceof coloredRectangle){}

Netbeans 告诉我“不可转换的类型，需要有色矩形，找到矩形”我认为通过多态性它可以工作。我做错了什么？

代码矩形:

public class Rechthoek implements Printbaar {
protected double lengte,breedte;
private Rechthoek[] tabel;

public Rechthoek(int lengte,int breedte){
    this.lengte=lengte;
    this.breedte=breedte;
}

public String getInfo(){
    return ("De lengte van de rechthoek is "+lengte+" en de breedte is "+breedte+" .");
}

public void schrijfTabel(Rechthoek[] tabel){
    for (int i = 0; i < tabel.length; i++) {
        tabel[i].getInfo();
    }
}

//错误代码在以下方法中 - Rechthoek = Rectangle 和 kleurRechthoek = ColoredRectangle

public boolean bevatKleur(Rechthoek[] tabel,String kleur){
    for (int i = 0; i < tabel.length; i++) {
        if(tabel[i] instanceof kleurRechthoek ){
            return true;
        }
    }
}

Answer 1

那是因为您(可能)无法从 tabel[i] 转换为 coloredRectangle。

JLS 中有说明如果发生上述情况，instanceof 将导致编译问题:

If a cast of the RelationalExpression to the ReferenceType would be rejected as a compile-time error, then the instanceof relational expression likewise produces a compile-time error. In such a situation, the result of the instanceof expression could never be true.