函數調用期間的隱式對象轉換

Question

參考下面提供的代碼，在main（）方法中，傳遞給靜態sort（）函數的實際參數的類型爲String []（變量a），但是類型的形式參數（）函數的類型爲Comparable []。由於這兩種類型似乎不匹配，這怎麼可能？是否有某種隱式對象轉換適用於我不知道的函數調用？任何幫助，將不勝感激。

Merge.java

下面是語法從§2.2歸併強調Merge.java的版本。

/****************************************************************************** 
* Compilation: javac Merge.java 
* Execution: java Merge < input.txt 
* Dependencies: StdOut.java StdIn.java 
* 
* 
* Sorts a sequence of strings from standard input using mergesort. 
* 
* % more tiny.txt 
* S O R T E X A M P L E 
* 
* % java Merge < tiny.txt 
* A E E L M O P R S T X     [ one string per line ] 
*  
* % more words3.txt 
* bed bug dad yes zoo ... all bad yet 
* 
* % java Merge < words3.txt 
* all bad bed bug dad ... yes yet zoo [ one string per line ] 
* 
******************************************************************************/ 

/** 
* The {@code Merge} class provides static methods for sorting an 
* array using mergesort. 
* <p> 
* For additional documentation, see <a href="http://algs4.cs.princeton.edu/22mergesort">Section 2.2</a> of 
* <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne. 
* For an optimized version, see {@link MergeX}. 
* 
* @author Robert Sedgewick 
* @author Kevin Wayne 
*/ 
public class Merge { 

    // This class should not be instantiated. 
    private Merge() { } 

    // stably merge a[lo .. mid] with a[mid+1 ..hi] using aux[lo .. hi] 
    private static void merge(Comparable[] a, Comparable[] aux, int lo, int mid, int hi) { 
     // precondition: a[lo .. mid] and a[mid+1 .. hi] are sorted subarrays 
     assert isSorted(a, lo, mid); 
     assert isSorted(a, mid+1, hi); 

     // copy to aux[] 
     for (int k = lo; k <= hi; k++) { 
      aux[k] = a[k]; 
     } 

     // merge back to a[] 
     int i = lo, j = mid+1; 
     for (int k = lo; k <= hi; k++) { 
      if  (i > mid)    a[k] = aux[j++]; 
      else if (j > hi)    a[k] = aux[i++]; 
      else if (less(aux[j], aux[i])) a[k] = aux[j++]; 
      else       a[k] = aux[i++]; 
     } 

     // postcondition: a[lo .. hi] is sorted 
     assert isSorted(a, lo, hi); 
    } 

    // mergesort a[lo..hi] using auxiliary array aux[lo..hi] 
    private static void sort(Comparable[] a, Comparable[] aux, int lo, int hi) { 
     if (hi <= lo) return; 
     int mid = lo + (hi - lo)/2; 
     sort(a, aux, lo, mid); 
     sort(a, aux, mid + 1, hi); 
     merge(a, aux, lo, mid, hi); 
    } 

    /** 
    * Rearranges the array in ascending order, using the natural order. 
    * @param a the array to be sorted 
    */ 
    public static void sort(Comparable[] a) { 
     Comparable[] aux = new Comparable[a.length]; 
     sort(a, aux, 0, a.length-1); 
     assert isSorted(a); 
    } 


    /*************************************************************************** 
    * Helper sorting function. 
    ***************************************************************************/ 

    // is v < w ? 
    private static boolean less(Comparable v, Comparable w) { 
     return v.compareTo(w) < 0; 
    } 

    /*************************************************************************** 
    * Check if array is sorted - useful for debugging. 
    ***************************************************************************/ 
    private static boolean isSorted(Comparable[] a) { 
     return isSorted(a, 0, a.length - 1); 
    } 

    private static boolean isSorted(Comparable[] a, int lo, int hi) { 
     for (int i = lo + 1; i <= hi; i++) 
      if (less(a[i], a[i-1])) return false; 
     return true; 
    } 


    /*************************************************************************** 
    * Index mergesort. 
    ***************************************************************************/ 
    // stably merge a[lo .. mid] with a[mid+1 .. hi] using aux[lo .. hi] 
    private static void merge(Comparable[] a, int[] index, int[] aux, int lo, int mid, int hi) { 

     // copy to aux[] 
     for (int k = lo; k <= hi; k++) { 
      aux[k] = index[k]; 
     } 

     // merge back to a[] 
     int i = lo, j = mid+1; 
     for (int k = lo; k <= hi; k++) { 
      if  (i > mid)     index[k] = aux[j++]; 
      else if (j > hi)      index[k] = aux[i++]; 
      else if (less(a[aux[j]], a[aux[i]])) index[k] = aux[j++]; 
      else         index[k] = aux[i++]; 
     } 
    } 

    /** 
    * Returns a permutation that gives the elements in the array in ascending order. 
    * @param a the array 
    * @return a permutation {@code p[]} such that {@code a[p[0]]}, {@code a[p[1]]}, 
    * ..., {@code a[p[N-1]]} are in ascending order 
    */ 
    public static int[] indexSort(Comparable[] a) { 
     int n = a.length; 
     int[] index = new int[n]; 
     for (int i = 0; i < n; i++) 
      index[i] = i; 

     int[] aux = new int[n]; 
     sort(a, index, aux, 0, n-1); 
     return index; 
    } 

    { 
     if (hi <= lo) return; 
     int mid = lo + (hi - lo)/2; 
     sort(a, index, aux, lo, mid); 
     sort(a, index, aux, mid + 1, hi); 
     merge(a, index, aux, lo, mid, hi); 
    } 

    // print array to standard output 
    private static void show(Comparable[] a) { 
     for (int i = 0; i < a.length; i++) { 
      StdOut.println(a[i]); 
     } 
    } 

    /** 
    * Reads in a sequence of strings from standard input; mergesorts them; 
    * and prints them to standard output in ascending order. 
    * 
    * @param args the command-line arguments 
    */ 
    public static void main(String[] args) { 
     String[] a = StdIn.readAllStrings(); 
     Merge.sort(a); 
     show(a); 
    } 
} 

Answer 1

是否有某種隱含的對象鑄造的適用

鑄造是不是你的對象做一些事情，這是你在值做一些（如對象參考）。鑄造不會影響對象。但這裏沒有演員。

撇開原始類型，變量或X類型的放慢參數可以如果被分配Y類型的值類型Y：

• 是對相同類型X，或
• 是一個class型實施X，或
• 是一個子或子接口的X，直接或間接的影響。

類似地，陣列型X[]可分配Y[]類型的值，如果X可根據上述規則被分配Y。

String implements Comparable，因此您可以將String分配給Comparable類型的變量或參數。由於陣列類型可根據元素類型進行分配，因此可將String[]分配給Comparable[]類型的變量或參數。

Answer 2

這是不特定於函數調用：

Comparable comparable = "hello"; 

String[] strings = {"hello", "world"}; 
Comparable[] comparables = strings; 

Java數組是協變的，這意味着數組分配，如果他們的元素類型。並且String可分配給Comparable，因爲String類實現了Comparable接口。