implements Comparable接口并重写compareTo(T o)方法实现比较与排序

实现Comparable接口并重写compareTo(T o)方法后实现比较排序：

例：

插入排序（它对实现Comparable接口的任何类型的数据进行排序，时间复杂度n^2）：

public class Insertion {
   

    @SuppressWarnings({
   "rawtypes", "unchecked"})
    public static void sort(Comparable[] a) {
   
        int n = a.length;
        for (int i = 1; i < n; i++) {
   
            for (int j = i; j > 0; j--) {
   
                if (a[j-1].compareTo(a[j]) > 0) {
   
                    exch(a, j-1, j);
                }
                else break;
            }
        }
    }

    // exchange a[i] and a[j]
    @SuppressWarnings("rawtypes")
    private static void exch(Comparable[] a, int i, int j) {
   
        Comparable swap = a[i];
        a[i] = a[j];
        a[j] = swap;
    }

    // read in a sequence of words from standard input and print
    // them out in sorted order
    public static void main(String[] args) {
   
        String[] a = StdIn.readAllStrings();
        sort(a);
        for (int i = 0; i < a.length; i++) {
   
            StdOut.print(a[i] + " ");
        }
        StdOut.println();
    }
}

例如我们在counter类中实现comparable接口并实现了CompareTo()方法以期进行比较排序：

public class Counter implements Comparable<Counter> {
   

    private final String name;     // counter name
    private final int maxCount;    // maximum value
    private int count;             // current value

    // create a new counter with the given parameters
    public Counter(String id, int max) {
   
        name = id;
        maxCount = max;
        count = 0;
    } 
    
    // compare two Counter objects based on their count
    public int compareTo(Counter that) {
   
        if      (this.count < that.count) return -1;
        else if (this.count > that.count) return +1;
        else                              return  0;
    }
 }

归并排序（它对实现Comparable接口的任何类型的数据进行排序，时间复杂度nlgn，但空间上需要Comparable[] aux这样一个与原数组等大小的数组空间作为开销）（jdk排序中采用了归并排序方法进行排序实现）：

public class Merge {
   

   /*************************************************************************** * Merge the subarrays a[lo] .. a[mid-1] and a[mid] .. a[hi-1] into * a[lo] .. a[hi-1] using the auxilliary array aux[] as scratch space. * * Precondition: the two subarrays are in ascending order * Postcondition: a[lo] .. a[hi-1] is in ascending order * ***************************************************************************/
    @SuppressWarnings({
   "rawtypes", "unchecked"})
    private static void merge(Comparable[] a, Comparable[] aux, int lo, int mid, int hi) {
   
        int i = lo, j = mid;
        for (int k = lo; k < hi; k++) {
   
            if      (i == mid)                 aux[k] = a[j++];
            else if (j == hi)                  aux[k] = a[i++];
            else if (a[j].compareTo(a[i]) < 0) aux[k] = a[j++];
            else                               aux[k] = a[i++];
        }

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


   /*************************************************************************** * Mergesort the subarray a[lo] .. a[hi-1], using the * auxilliary array aux[] as scratch space. ***************************************************************************/
    @SuppressWarnings("rawtypes")
    public static void sort(Comparable[] a, Comparable[] aux, int lo, int hi) {
   

        // base case
        if (hi - lo <= 1) return;

        // sort each half, recursively
        int mid = lo + (hi - lo) / 2;
        sort(a, aux, lo, mid);
        sort(a, aux, mid, hi);

        // merge back together
        merge(a, aux, lo, mid, hi);
    }


   /*************************************************************************** * Sort the array a using mergesort. ***************************************************************************/
    @SuppressWarnings("rawtypes")
    public static void sort(Comparable[] a) {
   
        int n = a.length;
        Comparable[] aux = new Comparable[n];
        sort(a, aux, 0, n);
    }

   /*************************************************************************** * Sort the subarray a[lo..hi] using mergesort. ***************************************************************************/
    @SuppressWarnings("rawtypes")
    public static void sort(Comparable[] a, int lo, int hi) {
   
        int n = hi - lo + 1;
        Comparable[] aux = new Comparable[n];
        sort(a, aux, lo, hi);
    }


   /*************************************************************************** * Check if array is sorted - useful for debugging. ***************************************************************************/
    @SuppressWarnings({
   "rawtypes", "unchecked"})
    private static boolean isSorted(Comparable[] a) {
   
        for (int i = 1; i < a.length; i++)
            if (a[i].compareTo(a[i-1]) < 0) return false;
        return true;
    }

   /*************************************************************************** * Show results. ***************************************************************************/
    @SuppressWarnings("rawtypes")
    public static void show(Comparable[] a) {
   
        for (int i = 0; i < a.length; i++)
            StdOut.println(a[i]);
    }


   /*************************************************************************** * Test client. ***************************************************************************/
    public static void main(String[] args) {
   
        String[] a = StdIn.readAllStrings();
        Merge.sort(a);
        assert isSorted(a);
        for (int i = 0; i < a.length; i++) {
   
            StdOut.print(a[i] + " ");
        }
        StdOut.println();
    }
}