Arrays类

Arrays常见方法案例与解析

• toString返回数组的字符串形式

toString源码

public static String toString(int[] a) {
        if (a == null)
            return "null";
        int iMax = a.length - 1;
        if (iMax == -1)
            return "[]";

        StringBuilder b = new StringBuilder();
        b.append('[');
        for (int i = 0; ; i++) {
            b.append(a[i]);
            if (i == iMax)
                return b.append(']').toString();
            b.append(", ");
        }
    }

使用tostring与不使用的区别

package Learn;

import java.util.Arrays;

public class Test {
    public static void main(String[] args) {
       int []array={1,2,3,4,5};
       System.out.println(array);
       System.out.println(Arrays.toString(array));
    }
}

• sort排序

首先我们排序的对象为数组，数组在堆中存储，我们排序之后就会影响到其中的实参，位置不变，储存的顺序改变

package Learn;

import java.util.Arrays;

public class Test {
    public static void main(String[] args) {
       int []array={1,2,3,5,6,9,2,5};
       System.out.println(array);
       Arrays.sort(array);
       System.out.println(Arrays.toString(array));
        System.out.println(array);
    }
}

sort方法是重载的，也可以传入一个接口Comparadetor，要求实现compare方法，从而改变排序的规则，在这里debug中，我们可以发现其中会进入到二分排序方法中（如下），而二分排序中0与1会影响到最终的排序结果。

private static <T> void binarySort(T[] a, int lo, int hi, int start,
                                       Comparator<? super T> c) {
        assert lo <= start && start <= hi;
        if (start == lo)
            start++;
        for ( ; start < hi; start++) {
            T pivot = a[start];

            // Set left (and right) to the index where a[start] (pivot) belongs
            int left = lo;
            int right = start;
            assert left <= right;
            /*
             * Invariants:
             *   pivot >= all in [lo, left).
             *   pivot <  all in [right, start).
             */
            while (left < right) {
                int mid = (left + right) >>> 1;
                if (c.compare(pivot, a[mid]) < 0)
                    right = mid;
                else
                    left = mid + 1;
            }
            assert left == right;

package Learn;

import java.util.Arrays;
import java.util.Comparator;

public class Test {
    public static void main(String[] args) {
       Integer []array={5,2,3,5,6,9,2,5};
        //将匿名类当作参数传入
       Arrays.sort(array, new Comparator() {
           @Override
           public int compare(Object o1, Object o2) {
               Integer n1=(Integer) o1;
               Integer n2=(Integer) o2;
               //该处返回的结果会影响排序的结果
               return n2-n1;
           }

       });
       System.out.println(Arrays.toString(array));
    }
}

• 二分搜索查找

通过二分搜索查找，要求必须排好序（而且是正序），如果没有找到，则返回-（low+1），low表示查找的目标应该在的位置。注意有时候二分查找法不一定找出第一个数。

package Learn;

import java.util.Arrays;
import java.util.Comparator;

public class Test {
    public static void main(String[] args) {
       Integer []array={5,2,3,5,6,9,2,5};
       Arrays.sort(array);
       System.out.println(Arrays.toString(array));
       System.out.println(Arrays.binarySearch(array,2));
       System.out.println(Arrays.binarySearch(array,7));
    }
}

• 数组元素的赋值copyOf

当范围不超出时，进行正常的赋值，当大于时，则用null来代替

package Learn;

import java.util.Arrays;

public class Test {
    public static void main(String[] args) {
       Integer []array={5,2,3,5,6,9,2,5};
       Integer []array2=Arrays.copyOf(array,5);
       System.out.println(Arrays.toString(array2));

        Integer []array3=Arrays.copyOf(array,11);
        System.out.println(Arrays.toString(array));
        System.out.println(Arrays.toString(array3));
    }
}

• 数组填充

package Learn;

import java.util.Arrays;

public class Test {
    public static void main(String[] args) {
       Integer []array={5,2,3,5,6,9,2,5};
       Arrays.fill(array,99);
       System.out.println(Arrays.toString(array));
    }
}

• 比较两个数组是否相等

也是包含顺序的

package Learn;

public class Test {
    public static void main(String[] args) {
       Integer []array={5,2,3,5,6,9,2,5};
       Integer []array1={5,2,3,5,6,9,5,2};
       System.out.println(array.equals(array1));
    }
}

• 将一组数据转换成list集合

package Learn;

import java.util.Arrays;
import java.util.List;

public class Test {
    public static void main(String[] args) {
       Integer []array={5,2,3,5,6,9,2,5};
        List list= Arrays.asList(array);
        System.out.println(list);
        //他的运行类型为
        System.out.println(list.getClass());
    }
}

练习：按照书的价格大小顺序输出

package Learn;

import java.util.Arrays;
import java.util.Comparator;

public class Exercise {
    public static void main(String[] args) {
        Book []books=new Book[4];
        books[0]=new Book("ho",45);
        books[1]=new Book("h7",450);
        books[2]=new Book("h4737",272);
        books[3]=new Book("737",753);
        Arrays.sort(books, new Comparator() {
            @Override
            public int compare(Object o1, Object o2) {
                Book book1=(Book) o1;
                Book book2=(Book) o2;
                return book1.price-book2.price;
            }
        });
        for (int i=0;i< books.length;i++){
            System.out.println(books[i].name+books[i].price);
        }
        System.out.println(Arrays.toString(books));
    }
}
class Book{
    String name;
    int price;
    public Book(String name,int price){
        this.name=name;
        this.price=price;
    }

    @Override
    public String toString() {
        return "Book{" +
                "name='" + name + '\'' +
                ", price=" + price +
                '}';
    }
}