用單個null參數調用Java可變參數方法？

Question

如果我有一個可變參數Java方法foo(Object ...arg)，我呼叫foo(null, null)，我同時使用arg[0]和arg[1]作爲null s。但如果我撥打foo(null)，arg本身爲空。這是爲什麼發生？

如何撥打foo使foo.length == 1 && foo[0] == null爲true？

Answer 1

問題是，當您使用字面null時，Java不知道它應該是什麼類型。它可能是一個空對象，或者它可能是一個空對象數組。對於單個論點，它假定後者。

你有兩種選擇。將null顯式轉換爲Object或使用強類型變量調用該方法。請參見下面的示例：

public class Temp{ 
    public static void main(String[] args){ 
     foo("a", "b", "c"); 
     foo(null, null); 
     foo((Object)null); 
     Object bar = null; 
     foo(bar); 
    } 

    private static void foo(Object...args) { 
     System.out.println("foo called, args: " + asList(args)); 
    } 
} 

輸出：

foo called, args: [a, b, c] 
foo called, args: [null, null] 
foo called, args: [null] 
foo called, args: [null] 

Answer 2

你需要一個顯式的Object：

foo((Object) null); 

否則該參數，則是整個陣列的可變參數代表。

Answer 3

這是因爲varargs方法可以用實際數組而不是一系列數組元素來調用。當你自己提供模糊的null時，它假定null是Object[]。將null投射到Object將解決此問題。

Answer 4

測試用例來說明這一點：

的Java代碼與可變參數回吐方法聲明（這恰好是靜態的）：

public class JavaReceiver { 
    public static String receive(String... x) { 
     String res = ((x == null) ? "null" : ("an array of size " + x.length)); 
     return "received 'x' is " + res; 
    } 
} 

這個Java代碼（一個JUnit4測試用例）調用上述（我們使用的測試用例不是測試任何的事情，只是產生一些輸出）：

import org.junit.Test; 

public class JavaSender { 

    @Test 
    public void sendNothing() { 
     System.out.println("sendNothing(): " + JavaReceiver.receive()); 
    } 

    @Test 
    public void sendNullWithNoCast() { 
     System.out.println("sendNullWithNoCast(): " + JavaReceiver.receive(null)); 
    } 

    @Test 
    public void sendNullWithCastToString() { 
     System.out.println("sendNullWithCastToString(): " + JavaReceiver.receive((String)null)); 
    } 

    @Test 
    public void sendNullWithCastToArray() { 
     System.out.println("sendNullWithCastToArray(): " + JavaReceiver.receive((String[])null)); 
    } 

    @Test 
    public void sendOneValue() { 
     System.out.println("sendOneValue(): " + JavaReceiver.receive("a")); 
    } 

    @Test 
    public void sendThreeValues() { 
     System.out.println("sendThreeValues(): " + JavaReceiver.receive("a", "b", "c")); 
    } 

    @Test 
    public void sendArray() { 
     System.out.println("sendArray(): " + JavaReceiver.receive(new String[]{"a", "b", "c"})); 
    } 
} 

運行此作爲JUnit測試得出：

 
sendNothing(): received 'x' is an array of size 0 
sendNullWithNoCast(): received 'x' is null 
sendNullWithCastToString(): received 'x' is an array of size 1 
sendNullWithCastToArray(): received 'x' is null 
sendOneValue(): received 'x' is an array of size 1 
sendThreeValues(): received 'x' is an array of size 3 
sendArray(): received 'x' is an array of size 3 

爲了使這更有趣，讓我們調用功能在Groovy 2.1.2和看什麼發生。事實證明，結果是不一樣的！這可能是一個錯誤。

import org.junit.Test 

class GroovySender { 

    @Test 
    void sendNothing() { 
     System.out << "sendNothing(): " << JavaReceiver.receive() << "\n" 
    } 

    @Test 
    void sendNullWithNoCast() { 
     System.out << "sendNullWithNoCast(): " << JavaReceiver.receive(null) << "\n" 
    } 

    @Test 
    void sendNullWithCastToString() { 
     System.out << "sendNullWithCastToString(): " << JavaReceiver.receive((String)null) << "\n" 
    } 

    @Test 
    void sendNullWithCastToArray() { 
     System.out << "sendNullWithCastToArray(): " << JavaReceiver.receive((String[])null) << "\n" 
    } 

    @Test 
    void sendOneValue() { 
     System.out << "sendOneValue(): " + JavaReceiver.receive("a") << "\n" 
    } 

    @Test 
    void sendThreeValues() { 
     System.out << "sendThreeValues(): " + JavaReceiver.receive("a", "b", "c") << "\n" 
    } 

    @Test 
    void sendArray() { 
     System.out << "sendArray(): " + JavaReceiver.receive(["a", "b", "c"] as String[]) << "\n" 
    } 

} 

將此作爲JUnit測試運行會產生以下結果，其中Java以粗體突出顯示。

 
sendNothing(): received 'x' is an array of size 0 
sendNullWithNoCast(): received 'x' is null 
sendNullWithCastToString(): received 'x' is null 
sendNullWithCastToArray(): received 'x' is null 
sendOneValue(): received 'x' is an array of size 1 
sendThreeValues(): received 'x' is an array of size 3 
sendArray(): received 'x' is an array of size 3 

Answer 5

我喜歡

foo(new Object[0]); 

避免空指針異常。

希望它有幫助。