函数调用期间的隐式对象转换

Question

参考下面提供的代码，在main（）方法中，传递给静态sort（）函数的实际参数的类型为String []（变量a），但是类型的形式参数（）函数的类型为Comparable []。由于这两种类型似乎不匹配，这怎么可能？是否有某种隐式对象转换适用于我不知道的函数调用？任何帮助，将不胜感激。

Merge.java

下面是语法从§2.2归并强调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

是否有某种隐含的对象铸造的适用

铸造是不是你的对象做一些事情，这是你在值做一些（如对象参考）。铸造不会影响对象。但这里没有演员。

撇开原始类型，变量或X类型的放慢参数可以如果被分配Y类型的值类型Y：

• 是对相同类型X，或
• 是一个class型实施X，或
• 是一个子或子接口的X，直接或间接的影响。

类似地，阵列型X[]可分配Y[]类型的值，如果X可根据上述规则被分配Y。

String implements Comparable，因此您可以将String分配给Comparable类型的变量或参数。由于阵列类型可根据元素类型进行分配，因此可将String[]分配给Comparable[]类型的变量或参数。

Answer 2

这是不特定于函数调用：

Comparable comparable = "hello"; 

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

Java数组是协变的，这意味着数组分配，如果他们的元素类型。并且String可分配给Comparable，因为String类实现了Comparable接口。