初学Java：计算数组中最大值 ---计算数组中最小值----计算数组之和----实现两个数组----拼接数组截取

public class ArrayUtils{    //创建类（数组工具类）

    //1.计算数组中最大值
    public static int arrayMaxElement(int [] data){  //创建方法
        if(data == null){     //如果数组为空
            //IllegalArgumentException 参数异常
            throw new IllegalArgumentException("data must be not null.");
        }                             //抛出异常
        int max = Integer.MIN_VALUE;        //初始max为默认最小值
        for(int i=0; i<data.length; i++){ 
            if(data[i]>max){           //for循环遍历数组  如果大于max将这个数赋给max
                max=data[i];
            }
        }
        return max;
    }
    
    //2.计算数组中最小值
    public static int arrayMinElement(int [] data){
        if(data == null){
            throw new IllegalArgumentException("data must be not null.");
        }
        int min = Integer.MAX_VALUE;
        for(int i=0; i<data.length; i++){
            if(data[i]<min){
                min=data[i];
            }
        }
        return min;
    }
    
    
    //3.计算数组值之和
    public static int arrayElementSum(int [] data){
        if(data == null){
            throw new IllegalArgumentException("data must be not null.");
        }
        int sum = 0;
        for(int i=0; i<data.length; i++){
            sum=sum+data[i];
        }
        return sum;
    }
    
    //4.数组拼接
    public static int[] arrayJoin(int[] a, int[] b){
        if(a ==null){
            throw new IllegalArgumentException("a must be not null.");
        }                                     //数组ab为空时抛出异常
        if(b == null){
            throw new IllegalArgumentException("b must be not null.");
        }
             
        int len = a.length+b.length;    //计算拼接后的数组c的长度
        int[] c= new int[len];          //初始化c
        for(int i=0; i<a.length; i++){
            c[i]=a[i];                //先将a数组全部放在c中
        }
        for(int i=a.length; i<c.length;i++){
            c[i]=b[i-a.length];       //将b数组从第零个放在c后面
        }
        return c;
    }

    //5.数组截取
    //[start, end)
    public static int[] arraySub(int[] a, int start , int end){
        if(a==null){
           throw new IllegalArgumentException("a must be not null.");
        }
        if(start < 0 || end < 0){
            throw new IllegalArgumentException("start or end  must be > 0.");
        }
        if(end < start){
            throw new IllegalArgumentException("end > start.");
        }
        if(start>=a.length){
            throw new IllegalArgumentException("start must be < a.length .");
        }
        //各种异常下  抛出异常  使代码比较健壮 不容易有bug
        int count = end-start;      //计算截取后数组的长度
        if(count>a.length){
            throw new IllegalArgumentException("sub length must be <= a.length .");
        }
        
        int[] b = new int[count];  //初始化b
        for(int i=start ; i< end ;i++){
            b[i-start]=a[i];        //从a的start到end   赋值给新数组b
        }
        return b;
    }
    
    public static void printArray(int [] a){
        if(a  == null){
            return;
        }
        for(int i=0; i< a.length; i++){
             System.out.print(a[i]+" ");
        }
        System.out.println();
    }
    
    
    public static void main(String[] args){
        
        int[] a = new int[]{23,12,43,51,21};
        int max = arrayMaxElement(a);
        System.out.println(max);//51
        
        int[] sub = arraySub(a,0,5);
        
        printArray(sub);
       
        
    }
}