Java 函数调用期间的隐式对象转换
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sort()函数的形式参数的类型为compariable[]。既然这两种类型在我看来不匹配,这怎么可能呢?是否有某种隐式对象转换在我不知道的函数调用期间应用?任何帮助都将不胜感激 Merge.java 下面是§2.2 Mergesort中Merge.java的语法突出显示版本Java 函数调用期间的隐式对象转换,java,arrays,sorting,merge,Java,Arrays,Sorting,Merge,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sort()函数的形式参数的类型为compariable[]。既然这两种类型在我看来不匹配,这怎么可能呢?是否有某种隐式对象转换在我不知道的函数调用期间应用?任何帮助都将不胜感激 Merge.java 下面是§2.2 Mergesort中Merge.java的语法突出显示版本 /****************************************************
/******************************************************************************
* 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);
}
}
/******************************************************************************
*编译:javacmerge.java
*执行:java Merge
强制转换不是对对象执行的操作,而是对值(例如对象引用)执行的操作。对象不受强制转换的影响。但这里没有强制转换
撇开基本类型不谈,如果类型Y
,则类型X
的变量或参数可以指定类型Y
的值:
- 与
X
类型相同,或
- 是实现
X
的类
类型,或
- 是直接或间接的
X
的子类或子接口
类似地,如果可以根据上述规则为X
分配Y[]
类型的值,则可以为数组类型X[]
分配Y
String
实现了compariable
,因此您可以将String
分配给类型为compariable
的变量或参数。由于数组类型可以根据其元素类型进行分配,String[]
可以分配给类型为compariable[]的变量或参数
这不是特定于函数调用的:
Comparable comparable = "hello";
String[] strings = {"hello", "world"};
Comparable[] comparables = strings;
Java数组是协变的,这意味着如果数组的元素类型是可分配的,String
可以分配给Comparable
,因为类String
实现了Comparable
接口。无需强制转换,String
实现了Comparable
,因此它已经是req的一部分uired类型。为便于将来参考,代码示例应尽量少,即只包含与您的问题相关的代码。因此,如果您对两行代码有疑问,请展示这两行代码,而不是它们来自的整个程序。如果您在编写代码之前没有如此简洁地总结您的问题,我就不会费心去理解问题了n、 另外,在stackoverflow上,您可以通过在编辑器中选择代码并按下相应的工具栏按钮来格式化代码。