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- what nullpointerexception, , how fix it? 12 answers
ok, rewrote class,i rushed code not clean yet anyway comipiles, , running mainmethod problem still there.
import java.util.*; public class myclass { public random myrandom; public hashmap<string, arraylist<string>> mymap; public arraylist<string> ar; public arraylist<string> nexts; // arraylist<string> follows; public myclass(){ myrandom = new random(); } public void setrandom(int seed){ myrandom = new random(seed); } public hashmap<string, arraylist<string>> buildhashmap(){ hashmap<string, arraylist<string>> mymap = new hashmap<string, arraylist<string>>(); arraylist<string> ar = new arraylist<string>(); ar.add("2"); ar.add("2"); string test = "test"; string anothertest = "anothertest"; mymap.put(test, ar); mymap.put(anothertest, ar); return mymap; } public arraylist<string> arraylistgetter(string st){ system.out.println("++++++++++++++++++++++++++++++++++"); system.out.println(mymap.size()); arraylist ar = mymap.get(st); return ar; } public void mainmethod(){ hashmap<string, arraylist<string>> mymap = new hashmap<string, arraylist<string>>(); mymap = buildhashmap(); system.out.println("\n\nnumber of keys found: " + mymap.size()); (string st : mymap.keyset()){ system.out.println(st + ": "); arraylist<string> al = mymap.get(st); system.out.println(al.size()); } stringbuilder sb = new stringbuilder(); string test = "test"; system.out.println(mymap.get(test)); system.out.println(sb); system.out.println(mymap.get(test).size()); system.out.println(mymap.get(test).size()); // arraylist<string> follows = getfollows(key); system.out.println("something"); int index = myrandom.nextint(mymap.get(test).size()); system.out.println(index); // index = myrandom.nextint(follows.size()); string next = mymap.get("test").get(index); sb.append(next); system.out.println(sb); system.out.println(mymap.get("test").getclass()); arraylist<string> follows = new arraylist<string>();<------new empty arraylist system.out.println(follows.size() + " **********"); arraylist<string> nexts = new arraylist<string>(); nexts = arraylistgetter ("test"); } } am missing obvious? in advance time , patience.
you're shadowing mymap variable re-declaring in method , leaving field null.
import java.util.*; public class myclass2 { public hashmap<string, arraylist<string>> mymap; // guy null // arraylist<string> follows; public hashmap<string, arraylist<string>> buildhashmap() { hashmap<string, arraylist<string>> mymap = new hashmap<string, arraylist<string>>(); return mymap; } public arraylist<string> arraylistgetter(string st) { system.out.println("++++++++++++++++++++++++++++++++++"); system.out.println(mymap.size()); arraylist ar = mymap.get(st); return ar; } public void mainmethod() { // mymap local variable, , assigning // leave class field null hashmap<string, arraylist<string>> mymap = new hashmap<string, arraylist<string>>(); mymap = buildhashmap(); // field in class still null!!! system.out.println("\n\nnumber of keys found: " + mymap.size()); (string st : mymap.keyset()) { system.out.println(st + ": "); arraylist<string> al = mymap.get(st); system.out.println(al.size()); } // throws npe arraylist<string> somemap = arraylistgetter("test"); } } meaning you're initializing hashmap you've declared local method. yes you've given same name, mymap, field in class, declaring local method, variable visible in method only, , field in class remains null.
solution: don't this, don't give local fields same name fields, , don't re-declare variables in local scope if mean assign field in class.
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