java - Why am I getting a NullPointerException on my clear() method? -

on junittest getting java.lang.nullpointerexception error, mean? clear() method wrong?

here fields:

private static class node<e> {     /** data value. */     private e data;     /** link */     private node<e> next = null;      /**      * construct node given data value , link      * @param data - data value       * @param next - link      */     public node(e data, node<e> next) {         this.data = data;         this.next = next;     }      /**      * construct node given data value      * @param data - data value       */     public node(e data) {         this(data, null);     } } /*</listing>*/ // data fields /** reference head of list */ private node<e> head = null; /** size of list */ private int size = 0;     

this clear() method:

/**  * removes of elements list (optional operation).   * list empty after call returns  * @throws unsupportedoperationexception if clear operation not supported list  */ public void clear() {      node<e> temp = head;     for(int i=0; < size; i++){         if(temp != null){             temp.data = null;             temp = temp.next;         }     }    } 

here test method clear():

public void testclear() {     multipleitems.clear();     assertequals("", multipleitems.tostring()); } 

stack trace?

java.lang.nullpointerexception @ singlelinkedlist.tostring(singlelinkedlist.java:196) @ singlelinkedlisttest.testclear(singlelinkedlisttest.java:93) @ sun.reflect.nativemethodaccessorimpl.invoke0(native method) @ sun.reflect.nativemethodaccessorimpl.invoke(unknown source) @ sun.reflect.delegatingmethodaccessorimpl.invoke(unknown source) @ java.lang.reflect.method.invoke(unknown source) @ org.junit.runners.model.frameworkmethod$1.runreflectivecall(frameworkmethod.java:47) @ org.junit.internal.runners.model.reflectivecallable.run(reflectivecallable.java:12) @ org.junit.runners.model.frameworkmethod.invokeexplosively(frameworkmethod.java:44) @ org.junit.internal.runners.statements.invokemethod.evaluate(invokemethod.java:17) @ org.junit.internal.runners.statements.runbefores.evaluate(runbefores.java:26) @ org.junit.runners.parentrunner.runleaf(parentrunner.java:271) @ org.junit.runners.blockjunit4classrunner.runchild(blockjunit4classrunner.java:70) @ org.junit.runners.blockjunit4classrunner.runchild(blockjunit4classrunner.java:50) @ org.junit.runners.parentrunner$3.run(parentrunner.java:238) @ org.junit.runners.parentrunner$1.schedule(parentrunner.java:63) @ org.junit.runners.parentrunner.runchildren(parentrunner.java:236) @ org.junit.runners.parentrunner.access$000(parentrunner.java:53) @ org.junit.runners.parentrunner$2.evaluate(parentrunner.java:229) @ org.junit.runners.parentrunner.run(parentrunner.java:309) @ org.eclipse.jdt.internal.junit4.runner.junit4testreference.run(junit4testreference.java:50) @ org.eclipse.jdt.internal.junit.runner.testexecution.run(testexecution.java:38) @ org.eclipse.jdt.internal.junit.runner.remotetestrunner.runtests(remotetestrunner.java:467) @ org.eclipse.jdt.internal.junit.runner.remotetestrunner.runtests(remotetestrunner.java:683) @ org.eclipse.jdt.internal.junit.runner.remotetestrunner.run(remotetestrunner.java:390) @ org.eclipse.jdt.internal.junit.runner.remotetestrunner.main(remotetestrunner.java:197)  java.lang.nullpointerexception @ singlelinkedlist.tostring(singlelinkedlist.java:196) @ singlelinkedlisttest.testclear(singlelinkedlisttest.java:93) @ sun.reflect.nativemethodaccessorimpl.invoke0(native method) @ sun.reflect.nativemethodaccessorimpl.invoke(unknown source) @ sun.reflect.delegatingmethodaccessorimpl.invoke(unknown source) @ java.lang.reflect.method.invoke(unknown source) @ org.junit.runners.model.frameworkmethod$1.runreflectivecall(frameworkmethod.java:47) @ org.junit.internal.runners.model.reflectivecallable.run(reflectivecallable.java:12) @ org.junit.runners.model.frameworkmethod.invokeexplosively(frameworkmethod.java:44) @ org.junit.internal.runners.statements.invokemethod.evaluate(invokemethod.java:17) @ org.junit.internal.runners.statements.runbefores.evaluate(runbefores.java:26) @ org.junit.runners.parentrunner.runleaf(parentrunner.java:271) @ org.junit.runners.blockjunit4classrunner.runchild(blockjunit4classrunner.java:70) @ org.junit.runners.blockjunit4classrunner.runchild(blockjunit4classrunner.java:50) @ org.junit.runners.parentrunner$3.run(parentrunner.java:238) @ org.junit.runners.parentrunner$1.schedule(parentrunner.java:63) @ org.junit.runners.parentrunner.runchildren(parentrunner.java:236) @ org.junit.runners.parentrunner.access$000(parentrunner.java:53) @ org.junit.runners.parentrunner$2.evaluate(parentrunner.java:229) @ org.junit.runners.parentrunner.run(parentrunner.java:309) @ org.eclipse.jdt.internal.junit4.runner.junit4testreference.run(junit4testreference.java:50) @ org.eclipse.jdt.internal.junit.runner.testexecution.run(testexecution.java:38) @ org.eclipse.jdt.internal.junit.runner.remotetestrunner.runtests(remotetestrunner.java:467) @ org.eclipse.jdt.internal.junit.runner.remotetestrunner.runtests(remotetestrunner.java:683) @ org.eclipse.jdt.internal.junit.runner.remotetestrunner.run(remotetestrunner.java:390) @ org.eclipse.jdt.internal.junit.runner.remotetestrunner.main(remotetestrunner.java:197) 

tostring method: error on line "sb.append(p.data.tostring());"

public string tostring() {     stringbuilder sb = new stringbuilder("[");     node p = head;     if (p != null) {         while (p.next != null) {             sb.append(p.data.tostring());             //sb.append(" ==> ");             sb.append(", ");             p = p.next;         }         sb.append(p.data.tostring());     }     sb.append("]");     return sb.tostring(); } 

nullpointerexceptions hint null objects, multipleitems possible null when call clear() on it.