Java中Arrays工具类概述

青旅半醒 2022-02-09 10:29 249阅读 0赞

**Java中Arrays工具类是针对数组进行操作的工具类，包括排序和查找功能。**  
三个主要方法：

*  public static String toString(int\[\] a):把数组转成字符串
 *  public static void sort(int\[\] a):对数组进行排序
 *  public static int binarySearch(int\[\] a,int key):二分查找

代码演示：

public class ArraysDemo {
    	public static void main(String[] args) {
    		//定义一个数组
    		int[] arr = {24,69,80,57,13};
    		
    		//public static String toString(int[] a):把数组转成字符串
    		System.out.println("排序前："+Arrays.toString(arr));
    		
    		//public static void sort(int[] a):对数组进行排序
    		Arrays.sort(arr);
    		System.out.println("排序后："+Arrays.toString(arr));
    		
    		//public static int binarySearch(int[] a,int key):二分查找
    		System.out.println("binarySearch:"+Arrays.binarySearch(arr, 57));
    		System.out.println("binarySearch:"+Arrays.binarySearch(arr, 577));
    	}
    }

运行结果：  
排序前：\[24, 69, 80, 57, 13\]  
排序后：\[13, 24, 57, 69, 80\]  
binarySearch:2  
binarySearch:-6

对Arrays工具类的方法的源码进行解析：

int[] arr = {24,69,80,57,13};
    System.out.println("排序前:"+Arrays.toString(arr));
    
    toString源码：
    	public static String toString(int[] a) {
    		if(a == null) {
    			return "null";//说明数组对象不存在
    		}
    		int iMax = a.length - 1;//a.length=0;说明没有元素,所以iMax==-1
    		if(iMax == -1) {
    			return "[]";//说明数组存在，但是没有元素
    		}
    		
    		StringBuilder b = new StringBuilder();
    		b.append('[');//"["
    		for(int i = 0;;i++) {//此处第二个分号处为空，默认永远为true
    			b.append(a[i]);//"[24, 69, 80, 57, 13"
    			if(i == iMax)
    				//"[24 ,69 ,80 ,57, 13]",然后转换成字符串
    				return b.append(']').toString();
    			b.append(", ");//"[24, 69, 80, 57, "
    		}
    	}
    	
    --------------------------------------------------------------------
    binarySearch源码:
    public static int binarySearch(int[] a,int key)
    int[] arr = {13, 24, 57, 69, 80};
    System.out.println("binarySearch:"+Arrays.binarySearch(arr,577));
    
    public static int binarySearch(int[] a,int key) {
    	//a	---	arr	---	{13, 24, 57, 69, 80}
    	//key	---	577
    	return binarySearch0(a,0,a.length,key);
    }
    
    private static int binarySearch0(int[] a,int fromIndex,int toIndex,int key) {
    	//a	---	arr	---	{13, 24, 57, 69, 80}
    	//fromIndex	---	0
    	//toIndex	--	5
    	//key	---	577
    	
    	int low = fromIndex;//low = 0
    	int high = toIndex - 1;//high = 4
    	
    	while(low <= high) {
    		int mid = (low + high) >>> 1;//无符号右移//mid = 2,mid=3,mid=4
    		int midVal = a[mid];//midVal = 57 midVal = 69,midVal=80
    		
    		if(midVal < key) {
    			low = mid + 1;//low = 3	low=4 low = 5
    		else if(midVal > key) {
    			high = mid - 1;
    		}else{
    			return mid;//key found
    	}
    		return - (low + 1);//key not found
    }
    	}
    }

从以上源码分析我们可以得到一个开发原则：**只要是对象，我们就要判断对象是否为null！**