Java-为什么Scanner.nextLine（）在scanner.nextDouble（）之后返回空的多余行？

Question

import java.util.Scanner;
import java.lang.Math;

public class ClassNine {    
public static Scanner INPUT = new Scanner(System.in);

public static void nine() {
    boolean loop = true;
    while(loop) {
        loop = areaMenu();
    }
    Seperator.addLine();
}

public static boolean areaMenu() {
    boolean loop2 = true;
    boolean validChoice = false;

    while(loop2) {
        System.out.print("\n1 - Triangle\n"
                + "2 - Square\n"
                + "3 - Rectangle\n"
                + "4 - Parallelogram\n"
                + "5 - Trapezium\n"
                + "6 - Circle\n"
                + "7 - Ellipse\n"
                + "8 - Sector\n"
                + "\t>>enter 'exit' to leave.\n"
                + "\nChoice: ");

        String choice = INPUT.nextLine();

        if(choice.matches("[0-8]")) {
            validChoice = true;
        }else if(choice.matches("exit")){
            return false;
        }else{
            System.out.println("Invalid Input. Exiting 3.");        //TODO
            validChoice = false;
        }

        if(validChoice) {
            try {
                switch(choice){
                    case "1": triangle();
                    break;
                    case "2": square();
                    break;
                    case "3": rectangle();
                    break;
                    case "4": parallelogram();
                    break;
                    case "5": trapezium();
                    break;
                    case "6": circle();
                    break;
                    case "7": ellipse();
                    break;
                    case "8": sector();
                    break;
                    case "exit": loop2 = false;
                    break;
                }       
            }catch(Exception e) {
                System.out.println("Invalid Input. Exiting.");
                break;
            }
        }

    }
    return true;
}


public static void triangle() {
    System.out.print("\nTriangle Base: ");
    double base =  INPUT.nextDouble();
    System.out.print("Triangle Height: ");
    double height =  INPUT.nextDouble();

    System.out.println("\n\nArea of Traingle: " + 0.5*base*height + "\n");
}

public static void square() {
    System.out.print("\nSquare Side Lenght: ");
    double side =  INPUT.nextDouble();

    System.out.println("\n\nArea of Square: " + Math.pow(side, 2) + "\n");
}

public static void rectangle() {
    System.out.print("\nRectangle Lenght: ");
    double lenght =  INPUT.nextDouble();
    System.out.print("Rectangle Height: ");
    double height =  INPUT.nextDouble();

    System.out.println("\n\nArea of Rectangle: " + lenght*height + "\n");
}

public static void parallelogram() {
    System.out.print("\nParallelogram Lenght: ");
    double lenght =  INPUT.nextDouble();
    System.out.print("Parallelogram Height: ");
    double height =  INPUT.nextDouble();

    System.out.println("\n\nArea of Parallelogram: " + lenght*height + "\n");
}

public static void trapezium() {
    System.out.print("\nTrapezium Narrow Width: ");
    double narrow =  INPUT.nextDouble();
    System.out.print("Trapezium Wide Width: ");
    double wide =  INPUT.nextDouble();
    System.out.print("Trapezium Height: ");
    double height =  INPUT.nextDouble();

    System.out.println("\n\nArea of Trapezium: " + 0.5*(narrow + wide)*height + "\n");
}

public static void circle() {
    System.out.print("\nCircle Radius: ");
    double radius = INPUT.nextDouble();

    System.out.println("\n\nArea of Circle: " + radius*Math.pow(Math.PI, 2) + "\n");
}

public static void ellipse() {
    System.out.print("\nEllipse Narrow Radius: ");
    double narrow =  INPUT.nextDouble();
    System.out.print("Ellipse Wide Radius: ");
    double wide =  INPUT.nextDouble();

    System.out.println("\n\nArea of Ellipse: " + Math.PI*narrow*wide + "\n");
}

public static void sector() {
    System.out.print("\nSector Radius: ");
    double radius =  INPUT.nextDouble();
    System.out.print("Value of angle subtending the sector (in radians): ");
    double angle =  INPUT.nextDouble();

    System.out.println("\n\nArea of sector: " + 0.5*Math.pow(radius, 2)*angle + "\n");
}

}

在选择一个选项并执行了相关的方法后，程序将循环并再次通过选项菜单显示，这是应该的，但是由于没有任何进一步的输入，INPUT.nextLine()返回一个空字符串并直接转到'其他”语句中包含“退出3”，然后再循环一遍并实际上等待用户提示。

为什么Scanner.nextLine()在scanner.nextDouble()之后返回一个多余的空行？

另一个半问题是输入“ exit”会执行应做的操作并退出程序，但是查看代码不应结束程序，而只是结束程序 再次循环菜单，因为它中断了返回true的循环以再次开始循环。对？ （反问，我显然缺少了一些东西）