java - I want to make a treemap structure. And I want to find a previouse tree -

class fran {   string name;   int size;    fran(string name,int size)   {  this.name=name;  this.size=size;  }    string getname()   {  return this.name;  }    int getsize()   {  return this.size;  } }  class node {     fran fran;     node prenode;      node[] children=new node[10];     int childcount=0;     node child;      /*  there child?  */     int ischild=0;      node(fran fran)     {        this.fran=fran;     }      node(node prenode,fran fran)     {       this.fran=fran;       this.prenode=prenode;     }      void setchild(node prenode,node child)     {         this.prenode=prenode;         this.child=child;         this.children[childcount]=child;         this.ischild=1;         this.childcount++;     }      int getchildcount()     {  return childcount;  }      node prenode()     {  return this.prenode;  }      string[] getname()     {         string[] t=new string[childcount];          if(childcount==0)  {             t[0]=this.fran.getname();             return t;         }         else          {               for(int i=0;i<=childcount-1;i++)             {                t[i]=children[i].fran.getname();             }             return t;         }     }      int[] getsize()     {       int[] t=new int[childcount];        if(childcount==0)  {         t[0]=this.fran.getsize();         return t;       }       else        {         for(int i=0;i<=childcount-1;i++)         {            t[i]=children[i].fran.getsize();         }       return t;       }     }      string getn()     {        return this.fran.getname();     } }  void setup()  {   fran aa=new fran("apt",36);   fran bb=new fran("bpt",26);   fran cc=new fran("cpt",16);   /*  fran dd=new fran("dpt",56);  */    node a=new node(aa);   node b=new node(bb);   node c=new node(cc);    a.setchild(a,b);   a.setchild(a,c);    print(a.getn());    node f=b.prenode();   print(f.getn()); } 

• i want make tree structure. , want return previouse node.
• node a->b, a->c . , want make b.prenode() -> node a.
• but have nullpointerexception, how can solve problem?

thank you

first, have decide want do. have not made clear @ all. root, , b , c children? or root, b child, , c child of b?

you need add method add child node:

node = new node (...); node b = new node(...); a.add(b); // should hook them 

the add method should set parent of b a, , set child of b.

in addition, note have stylistic mistakes. should remove code setting variables , put in constructor. hard @ moment see happens when create node because have code spread on whole class.

you should consider removing fran class , put data in node. why have 2 classes? adds overhead, complexity, , not seem achieve anything. that's style, clean code lot.