在我的android应用程序的EditText字段中引入输入（字符串，例如：A，B等）后，我按下按钮，我的应用程序停止。我是Android应用程序的新手。当我尝试在空对象引用＆＃34; 上调用方法＆＃34;但是我不明白为什么我的对象为null时，我得到空指针异常... 如果我在变量
中写入值weight = Main.callMethod("A", "B");
它完美地工作，但是如果我让应用程序中的值像行
那样weight = Main.callMethod(source, destination);
申请停止。
以下是我使用的两个班级：
public class MainActivity extends Activity {
private Button b;
public static EditText currentLoc, selectDestination;
TextView tv;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
currentLoc = (EditText)findViewById(R.id.editText);
selectDestination = (EditText)findViewById(R.id.editText2);
tv = (TextView) findViewById(R.id.textView);
// this.tv.setMovementMethod(new ScrollingMovementMethod());
currentLoc.addTextChangedListener(new TextWatcher() {
@Override
public void beforeTextChanged(CharSequence s, int start, int count, int after) {
}
@Override
public void onTextChanged(CharSequence s, int start, int before, int count) {
//tv.setText(s);
source = s.toString();
}
@Override
public void afterTextChanged(Editable s) {
}
});
selectDestination.addTextChangedListener(new TextWatcher() {
@Override
public void beforeTextChanged(CharSequence s, int start, int count, int after) {
}
@Override
public void onTextChanged(CharSequence s, int start, int before, int count) {
//tv.setText(s);
destination = s.toString();
}
@Override
public void afterTextChanged(Editable s) {
}
});
//source = currentLoc.getText().toString(); //getHint() instead og getText()
//destination = selectDestination.getText().toString();
final Button button = findViewById(R.id.button);
button.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
// Code here executes on main thread after user presses button
int weight; //= 0;
Log.d("Source", "Current loc:" + source + " END");
Log.d("Destination", "Select destination:" + destination + " END");
Log.d("Compare", "2 strings " + source.equals("A"));
Log.d("Source length" ,"is: " + source.length());
Log.d("Destination length", "is: " + destination.length());
//weight = Main.callMethod("A", "B");
weight = Main.callMethod(source, destination); //not working !!!
tv.setText("" + weight);
//tv.setText("Your Input: \n" + source + "\n" + destination + "\nEnd.");
}
});
}
}
另一类的方法就是：
public class Main {
public static int callMethod (String source, String destination) {
List<Path> list = new ArrayList();
list.add(new Path("A", "C", 4));
list.add(new Path("A", "B", 6));
list.add(new Path("B", "C", 2));
list.add(new Path("B", "D", 2));
list.add(new Path("C", "D", 1));
list.add(new Path("C", "E", 2));
list.add(new Path("E", "D", 1));
list.add(new Path("E", "F", 3));
list.add(new Path("D", "F", 7));
DijkstraAlgorithm object = new DijkstraAlgorithm(list);
list = object.compute(source, destination);
int weight = 0;
for (Path path : list) {
weight = path.getWeight();
//System.out.println(path.getSource() + " -> " + path.getDestination() + " (" + path.getWeight() + ")");
}
return weight;
}
}
Logcat
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.example.nicoleta.trafficappfinal.Pair.getSource()' on a null object reference
at com.example.nicoleta.trafficappfinal.DijkstraAlgorithm.generateShortestPath(DijkstraAlgorithm.java:112)
at com.example.nicoleta.trafficappfinal.DijkstraAlgorithm.compute(DijkstraAlgorithm.java:28)
at com.example.nicoleta.trafficappfinal.Main.callMethod(Main.java:45)
at com.example.nicoleta.trafficappfinal.MainActivity$1.onClick(MainActivity.java:42)
at android.view.View.performClick(View.java:5675)
at android.view.View$PerformClick.run(View.java:22651)
at android.os.Handler.handleCallback(Handler.java:836)
at android.os.Handler.dispatchMessage(Handler.java:103)
at android.os.Looper.loop(Looper.java:208)
at android.app.ActivityThread.main(ActivityThread.java:6267)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1063)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:924)
DijkstraAlgorithm - 在这个类中，问题似乎是Path，Pair类： 修改
public class DijkstraAlgorithm {
private final List<Path> graph;
private List<Path> shortestPath;
private Map distance;
public DijkstraAlgorithm(List<Path> graph)
{
this.graph = graph;
}
public List<Path> compute(String start, String end)
{
List<Path> list = new ArrayList();
list.addAll(graph);
shortestPath = new ArrayList();
distance = new HashMap();
distance.put(start, new Pair(0, "\0"));
compute(start, end, list);
generateShortestPath(start, end);
return shortestPath;
}
private void compute(String source, String destination, List<Path> graph)
{
Path bestPath = null;
for (Path path : graph)
{
if (path.getSource() == source)
{
int pathVal = path.getWeight() + ((Pair) distance.get(source)).getValue();
if (!distance.containsKey(path.getDestination()))
{
distance.put(path.getDestination(), new Pair(pathVal, path.getSource()));
}
else
{
if (((Pair) distance.get(path.getDestination())).getValue() > pathVal)
{
distance.remove(path.getDestination());
distance.put(path.getDestination(), new Pair(pathVal, path.getSource()));
}
}
if (bestPath == null)
{
bestPath = path;
}
else
{
if (((Pair) distance.get(bestPath.getDestination())).getValue() > ((Pair) distance.get(path.getDestination())).getValue())
{
bestPath = path;
}
}
}
}
List<Path> remove = new ArrayList();
for (Path path : graph)
{
if (path.getSource() == source || path.getDestination() == source)
{
remove.add(path);
}
}
graph.removeAll(remove);
if (bestPath != null)
{
if (bestPath.getDestination() != destination)
{
boolean check = false;
for (Path path : graph)
{
if (path.getSource() == bestPath.getDestination())
{
check = true;
}
}
if (check)
{
compute(bestPath.getDestination(), destination, graph);
}
else if (!graph.isEmpty())
{
compute(graph.get(0).getSource(), destination, graph);
}
}
}
}
private void generateShortestPath(String start, String end)
{
Pair pair = (Pair) distance.get(end);
Log.d("Start", "is " + start); //this is now visible
Log.d("End", "is " + end); //this is also visible
Log.d("Msg1", "is " + "source " + pair.getSource() + //at this line is the error !!!
" value " + pair.getValue() +"\n");
shortestPath.add(new Path(pair.getSource(), end, pair.getValue()));
while (pair.getSource() != start)
{
String c = pair.getSource();
pair = (Pair) distance.get(pair.getSource());
shortestPath.add(new Path(pair.getSource(), c, pair.getValue()));
}
Collections.reverse(shortestPath);
}
}
public class Pair {
private int value;
private String source;
public Pair(int value, String source)
{
this.value = value;
this.source = source;
}
public int getValue()
{
return value;
}
public String getSource()
{
return source;
}
public void setValue(int value)
{
this.value = value;
}
public void setSource(String source)
{
this.source = source;
}
}
public class Path {
private String source;
private String destination;
private int weight;
public Path(String source, String destination, int weight)
{
this.source = source;
this.destination = destination;
this.weight = weight;
}
public String getSource()
{
return source;
}
public String getDestination()
{
return destination;
}
public int getWeight()
{
return weight;
}
public void setSource(String source)
{
this.source = source;
}
public void setDestination(String destination)
{
this.destination = destination;
}
public void setWeight(int weight)
{
this.weight = weight;
}
}
答案 0 :(得分：0)
确保EditTexts currentLoc和selectDestination不是final。并在button.setOnClickListener()之外初始化它们，这意味着在化身之后初始化它们
final Button button = findViewById(R.id.button);
或更多。
并且在声明之前始终检查length和source的{{1}}是否明显超过destinition
0