Java 程式設計/關鍵字/instanceof

instanceof 是一個關鍵字。

它檢查一個物件引用是否是一個型別的例項，並返回一個布林值；

語法

<object-reference> instanceof TypeName

例如

class Fruit
 {
  //...	
 } 
 class Apple extends Fruit
 {
  //...
 }
 class Orange extends Fruit
 {
  //...
 }
 public class Test 
 {
    public static void main(String[] args) 
    {
       Collection<Object> coll = new ArrayList<Object>();
 
       Apple app1 = new Apple();
       Apple app2 = new Apple();
       coll.add(app1);
       coll.add(app2);
 
       Orange or1 = new Orange();
       Orange or2 = new Orange();
       coll.add(or1);
       coll.add(or2);
 
       printColl(coll);
    }
 
    private static String printColl( Collection<?> coll )
    {
       for (Object obj : coll)
       {
          if ( obj instanceof Object )
          {
             System.out.print("It is a Java Object and");
          }
          if ( obj instanceof Fruit )
          {
             System.out.print("It is a Fruit and");
          }
          if ( obj instanceof Apple )
          {
             System.out.println("it is an Apple");
          } 
          if ( obj instanceof Orange )
          {
             System.out.println("it is an Orange");
          }
       }
    }
 }

執行程式

java Test

輸出

"It is a Java Object and It is a Fruit and it is an Apple"
"It is a Java Object and It is a Fruit and it is an Apple"
"It is a Java Object and It is a Fruit and it is an Orange"
"It is a Java Object and It is a Fruit and it is an Orange"

請注意，instanceof 運算子也可以應用於介面。例如，如果上面的示例用介面增強了

interface Edible 
{
 //...
}

以及修改後的類以實現此介面

class Orange extends Fruit implements Edible
 {
  ...
 }

我們可以詢問我們的物件是否可食用。

if ( obj instanceof Edible )
 {
   System.out.println("it is edible");
 }