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我正在爲一個家庭分配調用名爲「命名」的Java程序。該程序由五個類組成：NameSurfer，NameSurferConstants，NameSurferDataBase，NameSurferEntry和NameSurferGraph。 代碼是（我認爲）完成，但是當我將名稱放在NameSurfer控制檯上並按下Enter時，出現以下錯誤。當我點擊圖表時，它什麼都不做。Eclipse錯誤：線程「AWT-EventQueue-0」中的異常java.lang.NullPointerException
我懷疑它與NameSurferEntry類有關，但我一直在尋找幾個小時沒有成功。我真的是Java的新手，任何幫助將不勝感激。
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at NameSurfer.actionPerformed(NameSurfer.java:58)
at javax.swing.JTextField.fireActionPerformed(JTextField.java:492)
at javax.swing.JTextField.postActionEvent(JTextField.java:705)
at javax.swing.JTextField$NotifyAction.actionPerformed(JTextField.java:820)
at javax.swing.SwingUtilities.notifyAction(SwingUtilities.java:1645)
at javax.swing.JComponent.processKeyBinding(JComponent.java:2859)
at javax.swing.JComponent.processKeyBindings(JComponent.java:2894)
at javax.swing.JComponent.processKeyEvent(JComponent.java:2822)
at java.awt.Component.processEvent(Component.java:6159)
at java.awt.Container.processEvent(Container.java:2083)
at java.awt.Component.dispatchEventImpl(Component.java:4744)
at java.awt.Container.dispatchEventImpl(Container.java:2141)
at java.awt.Component.dispatchEvent(Component.java:4572)
at java.awt.KeyboardFocusManager.redispatchEvent(KeyboardFocusManager.java:1856)
at java.awt.DefaultKeyboardFocusManager.dispatchKeyEvent(DefaultKeyboardFocusManager.java:722)
at java.awt.DefaultKeyboardFocusManager.preDispatchKeyEvent(DefaultKeyboardFocusManager.java:1000)
at java.awt.DefaultKeyboardFocusManager.typeAheadAssertions(DefaultKeyboardFocusManager.java:865)
at java.awt.DefaultKeyboardFocusManager.dispatchEvent(DefaultKeyboardFocusManager.java:686)
at java.awt.Component.dispatchEventImpl(Component.java:4616)
at java.awt.Container.dispatchEventImpl(Container.java:2141)
at java.awt.Window.dispatchEventImpl(Window.java:2489)
at java.awt.Component.dispatchEvent(Component.java:4572)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:710)
at java.awt.EventQueue.access$400(EventQueue.java:82)
at java.awt.EventQueue$2.run(EventQueue.java:669)
at java.awt.EventQueue$2.run(EventQueue.java:667)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:98)
at java.awt.EventQueue$3.run(EventQueue.java:683)
at java.awt.EventQueue$3.run(EventQueue.java:681)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:680)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:296)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:211)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:196)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:188)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)
以下是NameSurfer的代碼。
/*
* File: NameSurfer.java
* ---------------------
* When it is finished, this program will implements the viewer for
* the baby-name database described in the assignment handout.
*/
import acm.program.*;
import java.awt.event.*;
import javax.swing.*;
public class NameSurfer extends Program implements NameSurferConstants {
/* private instance variables*/
private JButton graphButton;
private JButton clearButton;
private JLabel nameLabel;
private JTextField name;
private NameSurferGraph graph;
private NameSurferDataBase dataBase;
/**
* This method has the responsibility for reading in the data base
* and initializing the interactors at the top of the window.
*/
public void init() {
addActionListeners();
graph = new NameSurferGraph();
add(graph);
/* adds the control bar*/
nameLabel = new JLabel ("Name");
add(nameLabel, NORTH);
name = new JTextField(MAX_FONT_NAME);
name.addActionListener(this);
add(name, NORTH);
graphButton = new JButton ("Graph");
add(graphButton, NORTH);
clearButton = new JButton ("Clear");
add(clearButton, NORTH);
}
/**
* This class is responsible for detecting when the buttons are
* clicked, so you will have to define a method to respond to
* button actions.
*/
public void actionPerformed(ActionEvent e) {
if (e.getActionCommand().equals ("Clear")) {
graph.clear();
graph.update();
} else {
String inputName = name.getText();
NameSurferEntry entry = dataBase.findEntry(inputName);
if (entry != null) {
graph.addEntry(entry);
graph.update();
}
}
}
}
這裏是NameSurferEntry的代碼。
/*
* File: NameSurferEntry.java
* --------------------------
* This class represents a single entry in the database. Each
* NameSurferEntry contains a name and a list giving the popularity
* of that name for each decade stretching back to 1900.
*/
import acm.util.*;
import java.util.*;
import java.util.StringTokenizer;
public class NameSurferEntry implements NameSurferConstants {
/* private instance variables*/
private String name;
private int[] ranks = new int [NDECADES];
/**
* Creates a new NameSurferEntry from a data line as it appears
* in the data file. Each line begins with the name, which is
* followed by integers giving the rank of that name for each
* decade.
*/
public NameSurferEntry(String line) {
//gets the name
int nameEnd = line.indexOf(" ");
name = line.substring(0, nameEnd);
//gets the ranking and forms it into an array using StringTokenizer class
String rankingStart = line.substring(nameEnd + 1);
StringTokenizer tokenizer = new StringTokenizer(rankingStart);
for (int i = 0; tokenizer.hasMoreTokens(); i++) {
int yearRank = Integer.parseInt(tokenizer.nextToken());
ranks[i] = yearRank;
}
}
/* Method: getName() */
/**
* Returns the name associated with this entry.
*/
public String getName() {
return name;
}
/* Method: getRank(decade) */
/**
* Returns the rank associated with an entry for a particular
* decade. The decade value is an integer indicating how many
* decades have passed since the first year in the database,
* which is given by the constant START_DECADE. If a name does
* not appear in a decade, the rank value is 0.
*/
public int getRank(int decade) {
if (decade <NDECADES) {
return ranks[decade];
}
return 0;
}
/* Method: toString() */
/**
* Returns a string that makes it easy to see the value of a
* NameSurferEntry.
*/
public String toString() {
String result = "";
for (int i = 0; i < ranks.length; i++) {
result += getRank(i);
}
return ("\"" + name + "[" + result + "]\"");
}
}
這裏是NameSurferGraph的代碼。
import acm.graphics.*;
import java.awt.event.*;
import java.util.*;
import java.awt.*;
public class NameSurferGraph extends GCanvas implements NameSurferConstants, ComponentListener {
/*Private instance variables*/
private ArrayList <NameSurferEntry> dataDisplay;
/**
* Creates a new NameSurferGraph object that displays the data.
*/
public NameSurferGraph() {
addComponentListener(this);
dataDisplay = new ArrayList<NameSurferEntry>();
}
/**
* Clears the list of name surfer entries stored inside this class.
*/
public void clear() {
dataDisplay.clear();
update();
}
/**
* Adds a new NameSurferEntry to the list of entries on the display.
* Note that this method does not actually draw the graph, but
* simply stores the entry; the graph is drawn by calling update.
*/
public void addEntry(NameSurferEntry entry) {
dataDisplay.add(entry);
}
/**
* Updates the display image by deleting all the graphical objects
* from the canvas and then reassembling the display according to
* the list of entries. Your application must call update after
* calling either clear or addEntry; update is also called whenever
* the size of the canvas changes.
*/
public void update() {
removeAll();
drawGraph();
if (dataDisplay.size() >= 0) {
for (int i = 0; i < dataDisplay.size(); i++) {
NameSurferEntry entry = dataDisplay.get(i);
drawRankingGraph (entry, i);
}
}
}
/*draws the background grids and displays the years*/
private void drawGraph() {
drawMargins();
drawVerticalLines();
displayYears();
}
/*Draws the horizontal lines at the top and the bottom of the window*/
private void drawMargins() {
double x1 = 0;
double x2 = getWidth();
double y1 = GRAPH_MARGIN_SIZE;
double y2 = getHeight() - GRAPH_MARGIN_SIZE;
GLine topLine = new GLine (x1, y1, x2, y1);
GLine bottomLine = new GLine (x1, y2, x2, y2);
add(topLine);
add(bottomLine);
}
/*Draws the vertical lines*/
private void drawVerticalLines() {
double x = 0;
for (int i = 0; i < NDECADES; i++) {
GLine verticalLine = new GLine (x, 0, x, getHeight());
x += getWidth()/NDECADES;
add(verticalLine);
}
}
/*Displays the years*/
private void displayYears() {
int decade = START_DECADE;
double x = 0;
for (int i = 0; i < NDECADES; i++) {
GLabel label = new GLabel ("" + decade);
add(label, x, getHeight() - GRAPH_MARGIN_SIZE/2 + (label.getAscent()/2));
decade += NUMBER_OF_YEARS;
x += getWidth()/NDECADES;
}
}
/*Draws the ranking graph and the input name label*/
private void drawRankingGraph(NameSurferEntry entry, int n) {
int inputOrder = n;
for (int i = 0; i < NDECADES - 1; i++) {
int r1 = entry.getRank(i);
int r2 = entry.getRank(i + 1);
double x1 = i * (getWidth()/NDECADES);
double x2 = (i+1) * (getWidth()/NDECADES);
double y1 = 0;
double y2 = 0;
if (r1 == 0) {
y1 = getHeight() - GRAPH_MARGIN_SIZE;
} else {
y1 = GRAPH_MARGIN_SIZE + (getHeight() - GRAPH_MARGIN_SIZE*2) * r1/MAX_RANK;
}
if (r2 == 0) {
y2 = getHeight() - GRAPH_MARGIN_SIZE;
} else {
y2 = GRAPH_MARGIN_SIZE + (getHeight() - GRAPH_MARGIN_SIZE*2) * r2/MAX_RANK;
}
/*Sets the graph and the label on the window*/
GLine rankingGraph = new GLine (x1, y1, x2, y2);
GLabel inputName = new GLabel(entry.getName() + " " + (entry.getRank(i) == 0 ? "*" : entry.getRank(i)));
/*Sets the color*/
Color color = getColor(inputOrder%4);
rankingGraph.setColor(color);
inputName.setColor(color);
/*Displays the graph and the label*/
add(rankingGraph);
add(inputName, x1, y2);
}
}
/*Gets the color of the rankingGraph and the inputName label*/
private Color getColor(int i) {
switch (i) {
case 0: return Color.black;
case 1: return Color.red;
case 2: return Color.blue;
}
return Color.magenta;
}
/* Implementation of the ComponentListener interface */
public void componentHidden(ComponentEvent e) { }
public void componentMoved(ComponentEvent e) { }
public void componentResized(ComponentEvent e) { update(); }
public void componentShown(ComponentEvent e) { }
}
tl; dr，其中「NameSurfer」類正好是第58行？ – fge