模版方法模式--java数组排序及Comparable接口用法-- 转

1、概述

     Arrays类有一个静态方法sort,利用这个方法我们可以传入我们要排序的数组进去排序，因为我们传入的是一个数组的引用，所以排序完成的结果也通过这个引用的来更改数组。对于整数、字符串排序，jdk提供了默认的实现，如果要对一个对象数组排序，则要自己实现java.util.Comparator接口。

 

java.util.Arrays中代码

public static void sort(Object[] a) {
        Object[] aux = (Object[])a.clone();
        mergeSort(aux, a, 0, a.length, 0);
}

 

private static void mergeSort(Object[] src,
                  Object[] dest,
                  int low,
                  int high,
                  int off) {
    int length = high - low;

    // Insertion sort on smallest arrays
        if (length < INSERTIONSORT_THRESHOLD) {
            for (int i=low; i<high; i++)
                for (int j=i; j>low &&
             ((Comparable) dest[j-1]).compareTo(dest[j])>0; j--)
                    swap(dest, j, j-1);
            return;
        }

        // Recursively sort halves of dest into src
        int destLow  = low;
        int destHigh = high;
        low  += off;
        high += off;
        int mid = (low + high) >>> 1;
        mergeSort(dest, src, low, mid, -off);
        mergeSort(dest, src, mid, high, -off);

        // If list is already sorted, just copy from src to dest.  This is an
        // optimization that results in faster sorts for nearly ordered lists.
        if (((Comparable)src[mid-1]).compareTo(src[mid]) <= 0) {
            System.arraycopy(src, low, dest, destLow, length);
            return;
        }

        // Merge sorted halves (now in src) into dest
        for(int i = destLow, p = low, q = mid; i < destHigh; i++) {
            if (q >= high || p < mid && ((Comparable)src[p]).compareTo(src[q])<=0)
                dest[i] = src[p++];
            else
                dest[i] = src[q++];
        }
    }

 

private static void swap(Object[] x, int a, int b) {
    Object t = x[a];
    x[a] = x[b];
    x[b] = t;
    }

 冒泡排序法

 

2、例子（鸭子大小比较）

 

Duck需要实现Comparable

public class Duck implements Comparable {

    String name;

    int weight;

    public Duck(String name, int weight) {
        this.name = name;
        this.weight = weight;
    }

    public String toString() {
        return name + " weights " + weight;
    }

    @Override
    public int compareTo(Object o) {
        Duck otherDuck = (Duck) o;
        if (this.weight < otherDuck.weight)
            return -1;
        else if (this.weight == otherDuck.weight)
            return 0;
        else
            return 1;
    }
}

 

public class DackSortTestDrive {

    public static void main(String[] args) {
        Duck[] ducks = {
                new Duck("Daffy", 8),
                new Duck("Dewey", 2),
                new Duck("Howard", 7),
                new Duck("Louie", 2),
                new Duck("Donald", 10),
                new Duck("Huey", 3)
        };

        System.out.println("Before sorting:");
        display(ducks);

        Arrays.sort(ducks);

        System.out.println("\nAfter sorting:");
        display(ducks);
    }

    public static void display(Duck[] ducks){
        for (int i = 0; i < ducks.length; i++) {
            System.out.println(ducks[i]);
        }
    }
}

 结果：

Before sorting:
Daffy weights 8
Dewey weights 2
Howard weights 7
Louie weights 2
Donald weights 10
Huey weights 3

After sorting:
Dewey weights 2
Louie weights 2
Huey weights 3
Howard weights 7
Daffy weights 8
Donald weights 10

 

3、总结

 

Arrays.sort方法控制算法，sort()依赖一个Comparable类提供compareTo()的实现。

 

for(int i=low;i<high;i++){
        ...compareTo()...
        ...swap()...
}