事件处理程序有编译错误：“从lambda表达式引用的局部变量必须是final”
提问于 2020-04-03 02:56:08
我已经将所有代码从ActionEvent转移到了一个方法中，但是编译器仍然给我同样的错误：“从lambda表达式引用的局部变量必须是final”。
它来自这里的playerMove()调用：
for (int row = 0; row < shooterBoard.length; row++) {
for (int col = 0; col < shooterBoard[row].length; col++) {
shooterBoard[row][col].setOnAction((ActionEvent e) -> {
playerMove(memoryBoard, shooterBoard, playerText, winCounter, row, col);
});
}
}我如何解决这个问题，你能解释为什么我会得到它吗？我是编程新手，想要更好地理解它。
下面是我的完整程序：
import javafx.application.Application;
import javafx.application.Platform;
import javafx.beans.value.ChangeListener;
import javafx.beans.value.ObservableValue;
import javafx.event.ActionEvent;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.control.TextArea;
import javafx.scene.control.TextField;
import javafx.scene.Scene;
import javafx.scene.image.Image;
import javafx.scene.image.ImageView;
import javafx.scene.layout.GridPane;
import javafx.scene.paint.Color;
import javafx.scene.text.Font;
import javafx.scene.text.FontWeight;
import javafx.stage.Stage;
import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;
public class BattleshipP2 extends Application{
Image hit = new Image(getClass().getResourceAsStream("H"));
Image miss = new Image(getClass().getResourceAsStream("M"));
public static void grid(String[] args) {
launch(args);
}
public void playerMove(char[][] memoryBoard, Button[][] shooterBoard, TextArea playerText, int winCounter, int row, int col) {
if (memoryBoard[row][col] != 'X') {
if (memoryBoard[row][col] == '*') {
memoryBoard[row][col] = 'X';
shooterBoard[row][col].setGraphic(new ImageView(hit));
playerText.setText("You attacked " + row + " " + col + " and hit! Great job captain!");
winCounter++;
} else {
memoryBoard[row][col] = 'X';
shooterBoard[row][col].setGraphic(new ImageView(miss));
playerText.setText("You attacked " + row + " " + col + " and missed! Nice try captain!");
}
} else {
playerText.setText("Invalid Move!");
}
}
public static void loadFile(Button[][] board, char[][] memoryBoard, String fileName, TextArea text, boolean status) {
try {
Scanner file = new Scanner(new File(fileName));
while (file.hasNextLine()) {
for (int row = 0; row < board.length; row++) {
for (int col = 0; col < board[row].length; col++) {
if (status) {
board[row][col].setText(file.next());
} else {
memoryBoard[row][col] = file.next().charAt(0);
board[row][col].setText("*");
}
}
}
}
}
catch (FileNotFoundException exception) {
text.setText("Error opening file");
}
}
public void start(Stage primaryStage) {
GridPane gridPane = new GridPane();
TextArea playerText = new TextArea("PLAYER.txt");
TextArea cpuText = new TextArea("CPU.txt");
Label promptP = new Label("Player Messages");
Label promptC = new Label("CPU Messages");
Label xAxis = new Label("");
Label yAxis = new Label("");
Label xAxis2 = new Label("");
Label yAxis2 = new Label("");
Button[][] shooterBoard = new Button[10][10];
Button[][] playerBoard = new Button[10][10];
char[][] memoryBoard = new char[10][10];
int winCounter = 0;
Scene scene = new Scene(gridPane, 440, 300);
gridPane.add(xAxis, 1, 0, 10, 1);
gridPane.add(yAxis, 0, 1, 1, 10);
xAxis.setGraphic(new ImageView(new Image(getClass().getResourceAsStream("cols.png"))));
yAxis.setGraphic(new ImageView(new Image(getClass().getResourceAsStream("rows.png"))));
for (int row = 0; row < shooterBoard.length; row++) {
for (int col = 0; col < shooterBoard[row].length; col++) {
playerBoard[row][col] = new Button();
playerBoard[row][col].setPrefWidth(50);
playerBoard[row][col].setPrefHeight(50);
gridPane.add(playerBoard[row][col], 1 + col, 1 + row, 1, 1);
}
}
gridPane.add(xAxis2, 12, 0, 10, 1);
gridPane.add(yAxis2, 11, 1, 1, 10);
xAxis2.setGraphic(new ImageView(new Image(getClass().getResourceAsStream("cols.png"))));
yAxis2.setGraphic(new ImageView(new Image(getClass().getResourceAsStream("rows.png"))));
for (int row = 0; row < shooterBoard.length; row++) {
for (int col = 0; col < shooterBoard[row].length; col++) {
shooterBoard[row][col] = new Button();
shooterBoard[row][col].setPrefWidth(50);
shooterBoard[row][col].setPrefHeight(50);
gridPane.add(shooterBoard[row][col], 12 + col, 1 + row, 1, 1);
}
}
promptP.setFont(Font.font("Boulder", FontWeight.BOLD, 14));
promptC.setFont(Font.font("Boulder", FontWeight.BOLD, 14));
playerText.setFont(Font.font("Boulder", FontWeight.BOLD, 22));
cpuText.setFont(Font.font("Boulder", FontWeight.BOLD, 22));
promptP.setTextFill(Color.web("#0000ff", 0.8));
promptC.setTextFill(Color.web("#FF0000" , 0.8));
playerText.setStyle("-fx-text-inner-color: blue;");
cpuText.setStyle("-fx-text-inner-color: red;");
gridPane.add(promptP, 0, 11, 3, 1);
gridPane.add(playerText, 0, 12, 22, 1);
gridPane.add(promptC, 0, 13, 3, 1);
gridPane.add(cpuText, 0, 14, 22, 1);
primaryStage.setTitle("Battleship GUI");
primaryStage.setScene(scene);
primaryStage.show();
loadFile(playerBoard, memoryBoard, playerText.getText(), playerText, true);
loadFile(shooterBoard, memoryBoard, cpuText.getText(), cpuText, false);
while (winCounter < 17) {
for (int row = 0; row < shooterBoard.length; row++) {
for (int col = 0; col < shooterBoard[row].length; col++) {
shooterBoard[row][col].setOnAction((ActionEvent e) -> {
playerMove(memoryBoard, shooterBoard, playerText, winCounter, row, col);
});
}
}
}
}
}回答 1
Stack Overflow用户
回答已采纳
发布于 2020-04-03 06:51:32
lambda表达式中的变量必须是final，否则会产生意外的行为。
lambda表达式是“匿名类实现，只有一个方法”的简写--过度简化它
在新类的作用域中，任何事情都可能发生在一个变量上，外部不会看到它，也不会有任何影响。(按引用传递vs按值传递)
您有两个选项。使用(有效的)最后一个参数
// declaring them like this, makes them effectivly final because they aren't be assigned again, but instead declared and instanciated every time wich makes them "effectivley final"
// the underscore is just for the naming and has no meaning otherwise. Its just, a exact copy of each Value
// but will be "effectivly final"
int row_ = row;
int col_ = col;
String playerText_ = playerText;
... // and so on
shooterBoard[row][col].setOnAction((ActionEvent e) -> {
playerMove(memoryBoard_, shooterBoard_, playerText_, winCounter_, row_, col_);
});使用对象作为包装器，这实际上是最终的。
在javaFX中，你有一个叫做属性的东西，它对于这样的事情非常方便。
不使用原始类型，而是使用“属性包装”，它将只实例化一次，并且将是有效的最终类型，然后每个作用域都能够访问更改。
对于javaFx中的其他事情，这些属性也非常方便。就像更改时的回调。
Playertext示例
StringProperty playerText = new SimpleStringProperty().
playerText.set("bla bla")
System.out.println(playerText.get());对于每个参数都应该这样做。
也可以为所有的Patrameters编写自己的对象包装器。
因为您需要: memoryBoard、shooterBoard、playerText、winCounter、row、col
你可以创建一个(有效地)最终的对象，然后传递它。
public class GameInstanceModel{
String playerText;
int winCounter;
int row;
int col;
//... your other Variables.
}
// on lunch
public void start(Stage primaryStage) {
GridPane gridPane = new GridPane();
GameInstanceModel currentGame = new GameInstanceModel();
// ...
shooterBoard[row][col].setOnAction((ActionEvent e) -> {
playerMove(currentGame);
});
}然后创建
页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/61004407
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