Java 比较两个字符串时出现空指针异常
在比较两个字符串时,我总是得到一个空指针选项。我知道这两个字符串都不是空的,所以我不确定发生了什么Java 比较两个字符串时出现空指针异常,java,nullpointerexception,equals,Java,Nullpointerexception,Equals,在比较两个字符串时,我总是得到一个空指针选项。我知道这两个字符串都不是空的,所以我不确定发生了什么 public void search() { while (!openList.isEmpty()) { currState = openList.removeFirst(); if (currState.equals(goal)) { //this line produces NullPointerException solut
public void search() {
while (!openList.isEmpty()) {
currState = openList.removeFirst();
if (currState.equals(goal)) { //this line produces NullPointerException
solution = true;
printSolution(currState);
break;
目标是我从文件中读入的字符串。
Openlist是一个链接列表
字符串开头是:120345678
目标是:012345678
public class BFS {
public String start;
public String goal;
public String startFinal;
LinkedList<String> openList;
Map<String, Integer> levelDepth;
Map<String, String> stateHistory;
int nodes = 0;
int limit = 100;
int unique = -1;
int newValue;
int a;
public String currState;
boolean solution = false;
public BFS() {
openList = new LinkedList<String>();
levelDepth = new HashMap<String, Integer>();
stateHistory = new HashMap<String, String>();
this.start = start;
this.goal = goal;
addToOpenList(start, null);// add root
}
public void loadStartState(String filename) throws IOException {
BufferedReader reader = new BufferedReader(new FileReader(filename));
try {
StringBuilder sb = new StringBuilder();
String line = reader.readLine();
StringBuilder currentLine = new StringBuilder();
while (line != null) {
currentLine.delete(0, currentLine.capacity());
currentLine.append(line);
currentLine.deleteCharAt(1);
currentLine.deleteCharAt(2);
sb.append(currentLine.toString());
sb.append("\n");
line = reader.readLine();
}
start = sb.toString();
System.out.println(start);
} finally {
reader.close();
}
}
public void loadGoalState(String filename)throws IOException{
BufferedReader reader = new BufferedReader(new FileReader(filename));
try {
StringBuilder sb = new StringBuilder();
String line = reader.readLine();
StringBuilder currentLine = new StringBuilder();
while (line != null) {
currentLine.delete(0, currentLine.capacity());
currentLine.append(line);
currentLine.deleteCharAt(1);
currentLine.deleteCharAt(2);
sb.append(currentLine.toString());
sb.append("\n");
line = reader.readLine();
}
goal = sb.toString();
System.out.println(goal);
} finally {
reader.close();
}
}
public void search() {
while (!openList.isEmpty()) {
currState = openList.removeFirst();
if (currState != null && currState.equals(goal)) {
solution = true;
printSolution(currState);
break;
} else {
a = currState.indexOf("0");
// left
while (a != 0 && a != 3 && a != 6) {
String nextState = currState.substring(0, a - 1) + "0"
+ currState.charAt(a - 1)
+ currState.substring(a + 1);
addToOpenList(nextState, currState);
nodes++;
break;
}
// up
while (a != 0 && a != 1 && a != 2) {
String nextState = currState.substring(0, a - 3) + "0"
+ currState.substring(a - 2, a)
+ currState.charAt(a - 3)
+ currState.substring(a + 1);
addToOpenList(nextState, currState);
nodes++;
break;
}
// right
while (a != 2 && a != 5 && a != 8) {
String nextState = currState.substring(0, a)
+ currState.charAt(a + 1) + "0"
+ currState.substring(a + 2)
+ currState.substring(a + 1);
addToOpenList(nextState, currState);
nodes++;
break;
}
// down
while (a != 6 && a != 7 && a != 8) {
String nextState = currState.substring(0, a)
+ currState.substring(a + 3, a + 4)
+ currState.substring(a + 1, a + 3) + "0"
+ currState.substring(a + 4);
addToOpenList(nextState, currState);
nodes++;
break;
}
}
}
}
private void addToOpenList(String newState, String oldState) {
if (!levelDepth.containsKey(newState)) {
newValue = oldState == null ? 0 : levelDepth.get(oldState) + 1;
unique++;
levelDepth.put(newState, newValue);
openList.add(newState);
stateHistory.put(newState, oldState);
}
}
公共类BFS{
公共字符串开始;
公共目标;
公共字符串开始结束;
LinkedList开放列表;
地图水平深度;
地图状态历史;
int节点=0;
整数极限=100;
int unique=-1;
int新值;
INTA;
公共字符串状态;
布尔解=假;
公共BFS(){
openList=newlinkedlist();
levelDepth=新的HashMap();
stateHistory=新HashMap();
this.start=start;
这个。目标=目标;
addToOpenList(开始,null);//添加根
}
公共void loadStartState(字符串文件名)引发IOException{
BufferedReader reader=新的BufferedReader(新文件读取器(文件名));
试一试{
StringBuilder sb=新的StringBuilder();
字符串行=reader.readLine();
StringBuilder currentLine=新的StringBuilder();
while(行!=null){
currentLine.delete(0,currentLine.capacity());
currentLine.append(行);
currentLine.deleteCharAt(1);
currentLine.deleteCharAt(2);
sb.append(currentLine.toString());
某人附加(“\n”);
line=reader.readLine();
}
start=sb.toString();
系统输出打印项次(开始);
}最后{
reader.close();
}
}
public void loadGoalState(字符串文件名)引发IOException{
BufferedReader reader=新的BufferedReader(新文件读取器(文件名));
试一试{
StringBuilder sb=新的StringBuilder();
字符串行=reader.readLine();
StringBuilder currentLine=新的StringBuilder();
while(行!=null){
currentLine.delete(0,currentLine.capacity());
currentLine.append(行);
currentLine.deleteCharAt(1);
currentLine.deleteCharAt(2);
sb.append(currentLine.toString());
某人附加(“\n”);
line=reader.readLine();
}
goal=sb.toString();
System.out.println(目标);
}最后{
reader.close();
}
}
公开无效搜索(){
而(!openList.isEmpty()){
currState=openList.removeFirst();
如果(currState!=null&&currState.equals(goal)){
解=真;
打印解决方案(currState);
打破
}否则{
a=当前状态的索引(“0”);
//左
而(a!=0&&a!=3&&a!=6){
字符串nextState=currState.substring(0,a-1)+“0”
+currState.charAt(a-1)
+当前状态子串(a+1);
addToOpenList(下一个州、当前州);
节点++;
打破
}
//向上
而(a!=0&&a!=1&&a!=2){
字符串nextState=currState.substring(0,a-3)+“0”
+当前状态子字符串(a-2,a)
+currState.charAt(a-3)
+当前状态子串(a+1);
addToOpenList(下一个州、当前州);
节点++;
打破
}
//对
而(a!=2&&a!=5&&a!=8){
字符串nextState=currState.substring(0,a)
+当前状态字符(a+1)+“0”
+当前状态子字符串(a+2)
+当前状态子串(a+1);
addToOpenList(下一个州、当前州);
节点++;
打破
}
//向下
而(a!=6&&a!=7&&a!=8){
字符串nextState=currState.substring(0,a)
+当前状态子字符串(a+3,a+4)
+当前状态。子字符串(a+1,a+3)+“0”
+当前状态子串(a+4);
addToOpenList(下一个州、当前州);
节点++;
打破
}
}
}
}
私有void addToOpenList(字符串newState、字符串oldState){
如果(!levelDepth.containsKey(newState)){
newValue=oldState==null?0:levelDepth.get(oldState)+1;
唯一++;
levelDepth.put(newState,newValue);
添加(newState);
stateHistory.put(newState、oldState);
}
}
解决方案
尝试此操作,在加载goal
和start
的值之前,删除对addToOpenList(start,null)
的调用
老东西
这里是空的
addToOpenList(start, null);
String currState = openList.removeFirst();
currState == null
补充资料
public BFS() {
this.start = start; // Variable 'start' is assigned to itself
this.goal = goal; // Variable 'goal' is assigned to itself
addToOpenList(start, null); // start is null
}
甚至我的IntelliJ
也看到了这一点:)
第115行的方法调用currState.indexOf(“0”)可能会产生
java.lang.NullPointerException
或许您可以尝试以下方式:
public void search(){
currState = openList.removeFirst();
while(currState != null){
if(currState.equals(goal)){
solution = true;
printSolution(currState);
break;
}
currState = openList.removeFirst();
}
}
100%
currState
是null
,为什么?我从你发布的代码中不知道。我知道这两个字符串都不是null,所以我不确定发生了什么。我不相信你。currState
是null
,可能是因为openList
的第一个元素是null
。一定要检查currState
;)Open list第一个元素不应该为null,因为它包含字符串start?@MarounMaroun我添加了其余的代码。也许我说“我知道它不是null”时有点鲁莽。或者,假设goal
不是null(可能不是),goal.equals(currState)
,甚至Objects.equals(currState,goal)
我仍然不明白为什么addToOpenList(start,null);使currState为null。addToOpenList的第二个参数是oldState,它在开始时为null。为什么它也使start为null?为什么while语句不循环?这是另一个问题:)关注这个问题:)@MariuszS我不明白为什么你要这样做
public void search(){
currState = openList.removeFirst();
while(currState != null){
if(currState.equals(goal)){
solution = true;
printSolution(currState);
break;
}
currState = openList.removeFirst();
}
}