倒数计时器 Java JavaFX

消息“时间线可能未初始化”表示您正在尝试在构造它之前使用。您正在调用一个事件处理程序，该处理程序正在传递给 Timeline 构造函数;由于构造函数尚未完成，因此编译器不能保证在事件处理程序运行时实际上已为其分配了值。换句话说，可能被初始化。timelinetimeline.stop()timelinetimeline

解决此问题的一种方法是单独添加关键帧。这样，Timeline 构造函数就完全完成了：

Timeline timeline = new Timeline();
// Construction complete;  now timeline can be used in an event handler.

timeline.getKeyFrames().add(new KeyFrame(Duration.seconds(1), event -> {
    // Decrease one second
    if (seconds > 0) {
        seconds--;
    } else {
        // If seconds reach zero, decrement minutes and set seconds to 59
        if (minutes > 0) {
            minutes--;
            seconds = 59;
        } else {
            // If minutes reach zero, decrement hours and set minutes and seconds to 59
            if (hours > 0) {
                hours--;
                minutes = 59;
                seconds = 59;
            } else {
                // If hours, minutes, and seconds reach zero, stop the timer
                hours = 0;
                minutes = 0;
                seconds = 0;
                timeline.stop();
            }
        }
    }
    // Update the label with the new values
    timerLabel.setText(String.format("Time Left: %02d:%02d:%02d", hours, minutes, seconds));
}));

另一种方法是声明为私有字段，而不是局部变量。这是有效的，因为一旦封闭类完成了自己的构造函数，就可以保证每个字段都被初始化。（如果您的代码从未为某个字段赋值，则该字段将自动初始化为 null。timeline

眼前的问题（“时间线可能未初始化”）由 @VGR 的回答回答。但是，使用这样的方法根本不是创建倒数计时器的方法。问题在于时间线不能保证每秒调用一次事件处理程序;它只会在呈现 UI 时执行，如果至少经过了一秒钟。Timeline

当倒计时开始时，您应该计算它应该结束的时间。然后通过计算距离结束时间的剩余时间来“定期”更新标签。您可以使用时间轴或（每次渲染 UI 时都会调用其方法）来执行此部分。这将增加一些额外的计算时间，但代价是尽可能精确 JavaFX 系统;在这里，我认为计算足够小，这不是问题，并且更喜欢这种方法。AnimationTimerhandle()AnimationTimer

以下是使用此方法的方法：startComputation()

import javafx.animation.Animation;
import javafx.animation.AnimationTimer;
import javafx.animation.KeyFrame;
import javafx.animation.Timeline;
import javafx.application.Application;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.control.TextField;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

import java.time.Duration;
import java.time.LocalTime;

public class CountdownTimer extends Application {
    private int hours;
    private int minutes;
    private int seconds;

    private Label timerLabel = new Label();

    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage primaryStage) {
        VBox root = new VBox(10);
        root.setAlignment(Pos.CENTER);

        TextField hoursField = new TextField();
        hoursField.setPromptText("Hours");
        TextField minutesField = new TextField();
        minutesField.setPromptText("Minutes");
        TextField secondsField = new TextField();
        secondsField.setPromptText("Seconds");

        Button startButton = new Button("Start Timer");
        startButton.setOnAction(event -> {
            // Parse user input for hours, minutes, and seconds
            hours = Integer.parseInt(hoursField.getText());
            minutes = Integer.parseInt(minutesField.getText());
            seconds = Integer.parseInt(secondsField.getText());

            // Start the countdown timer
            startCountdown();
        });

        root.getChildren().addAll(hoursField, minutesField, secondsField,   startButton, timerLabel);

        Scene scene = new Scene(root, 300, 200);
        primaryStage.setTitle("Countdown Timer");
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    private void startCountdown() {
        LocalTime end = LocalTime.now()
                .plusHours(hours)
                .plusMinutes(minutes)
                .plusSeconds(seconds);
        AnimationTimer timer = new AnimationTimer() {
            @Override
            public void handle(long l) {
                Duration remaining = Duration.between(LocalTime.now(), end);
                if (remaining.isPositive()) {
                    timerLabel.setText(format(remaining));
                } else {
                    timerLabel.setText(format(Duration.ZERO));
                    stop();
                }
            }

            private String format(Duration remaining) {
                return String.format("%02d:%02d:%02d",
                        remaining.toHoursPart(),
                        remaining.toMinutesPart(),
                        remaining.toSecondsPart()
                );
            }
        };

        timer.start();
    }
}

如果您更喜欢 ，请将该方法替换为TimelinestartCountdown()

    private void startCountdown() {
        LocalTime end = LocalTime.now()
                .plusHours(hours)
                .plusMinutes(minutes)
                .plusSeconds(seconds);
        Timeline timeline = new Timeline();
        timeline.getKeyFrames().add(new KeyFrame(Duration.seconds(1), e -> {
            java.time.Duration remaining = java.time.Duration.between(LocalTime.now(), end);
            if (remaining.isPositive()) {
                timerLabel.setText(format(remaining));
            } else {
                timerLabel.setText(format(java.time.Duration.ZERO));
                timeline.stop();
            }
        }));
        timeline.setCycleCount(Animation.INDEFINITE);
        timeline.play();
    }

    private String format(java.time.Duration remaining) {
        return String.format("%02d:%02d:%02d",
                remaining.toHoursPart(),
                remaining.toMinutesPart(),
                remaining.toSecondsPart()
        );
    }