报错java.lang.NullPointerException，求指点

[qq_36567914]

在输出单个建站性价比时OK，但后面就报错了，应该是134,135行有错，但不知道怎么改，请教大神

import java.util.*;

class site implements Cloneable{
	
	char name;
	int cost ;
	int sale ;
        site[] relations;
  
	
	site(char a,int c,int b){
		
		this.name = a;
		this.cost = c;
		this.sale = b;
		
	}
        site(site s){
                this.name = s.name;
		this.cost = s.cost;
		this.sale = s.sale;
                this.relations = s.relations;


                  }
      
        void set_relations(site[] s){
                this.relations = s;
            }
	
	int saleadditor(){
		int sum = this.sale;
		for(int i = 0;i<relations.length;i++){
			
			sum +=relations[i].sale;
		}
		return sum;
	} 
	
	double caculator(){
		int sum = saleadditor();
		return (double)sum*2/this.cost;
	}
	
        protected Object clone(){  
        site s = null;  
        try {  
            s = (site)super.clone();  
        } catch (CloneNotSupportedException e) {  
            e.printStackTrace();  
        }  
        return s;  
    }  


}

public class cost_performance {
	
	      static int check(site[] s, site a,int n){  //n表示现有数组中含有的非空元素个数
	    	   boolean  check = false;
	    	  for(int i = 0;i<n;i++){ 		  
	    		if(a.name == s[i].name)
	    	        check = true;
	    	  }
	    	  if(check ==false){
	    		  s[n] = new site(a);
	    		  n++;
	    	  } 
                   return n;
	      }
	
	
	
         public static void main(String args[]){
        	 
        	 site A,B,C,D,E,F,G;
        	 
        	 A = new site('A',240,700);
        	 B = new site('B',280,500);
        	 C = new site('C',220,1100);
        	 D = new site('D',260,800);   
        	 E = new site('E',380,600); 
        	 F = new site('F',250,1400); 
        	 G = new site('G',200,1600); 
                 site[] r1 = {B,D};
        	 site[] r2 = {A,C,E};
        	 site[] r3 = {B,E,F};
        	 site[] r4 = {A,E};
        	 site[] r5 = {B,C,D,F,G};
        	 site[] r6 = {C,E};
        	 site[] r7 = {E}; 
        	 A.set_relations(r1);
        	 B.set_relations(r2);
        	 C.set_relations(r3);
        	 D.set_relations(r4);
        	 E.set_relations(r5);
        	 F.set_relations(r6);
        	 G.set_relations(r7);
                 site[] all = {A,B,C,D,E,F,G};
        	 
        	System.out.println("A单独建站的性价比为："+A.caculator()); 
        	System.out.println("B单独建站的性价比为："+B.caculator()); 
        	System.out.println("C单独建站的性价比为："+C.caculator()); 
        	System.out.println("D单独建站的性价比为："+D.caculator()); 
        	System.out.println("E单独建站的性价比为："+E.caculator()); 
        	System.out.println("F单独建站的性价比为："+F.caculator()); 
        	System.out.println("G单独建站的性价比为："+G.caculator()); 
        	
        	for(int i = 0; i<7;i++){
        		

        		for(int j = 1+i;j<7;j++){
        			    site[] box = new site[7];	
        			    box[0] =new site(all[i]);
        			    int num = 1;
        			    for(int a = 0;a<all[i].relations.length;a++){
        			    	box[1+i] = new site(all[i].relations[i]);
        			    	num++;
        			    }
        			    
        				num = check(box,all[j],num);
                                          
        				
        				for(int b=0;b<all[j].relations.length;b++){
        					
        					num = check(box,all[j].relations[b],num);
        					
        				}
        				
        				int salenum = 0;
        				int totalcost = 0;
        				for(int y = 0;y<num;y++){	
        					salenum = salenum+ box[y].sale;
        					totalcost = totalcost+ box[y].cost;
        				}
        				double costperformance = 2*salenum/totalcost;
        				System.out.println("在" + all[i].name +"和"+all[j].name+"处建站的性价比为"+costperformance);
        		}
	
        	}
 
         }
}

[tom_66]

box[y] 要判断是否为空，后面不为空，属性做加减运算的时候也要判断空 box[y].sale box[y].cost

[墨笙弘一]

楼上+1，很容易忽略这种情况，如果它是null,然后你对null调用其他的方法或者进行运算，当然是会报空指针

[作现]

一楼正解，你要保证你的参数不能为null

[半城煙沙]

楼上+1,做任何运算之前要对运算的参数做判断，保证不能为 空