Java 如何在不获取实例化异常的情况下将这个超类更改为抽象类？

我想使我的超类成为一个抽象类，但是在尝试这样做时，我会遇到各种异常（InstanceionException、RuntimeException、InvocationTargetException）。当我没有将超类作为抽象类时，我的代码可以工作。我该如何解决这个问题

我曾尝试将我的“main”和“start”函数包含在抽象类主体之外，因为我认为在抽象类中调用这些函数可能会有问题。我将这些方法移动到扩展抽象类的子类中。然后在每个子类中重写抽象方法“start”。尽管如此，这种重新格式化并没有解决任何问题，因为我得到了相同的错误。从我在这个问题中所研究的内容来看，这个用户使用的格式接近于我试图通过扩展抽象超类“应用程序”来实现的格式

当我没有将我的超类作为抽象类，并且draw不再是一个抽象方法时，我得到了我想要的输出：

但是，否则我将收到以下错误消息：

Exception in Application constructor
java.lang.reflect.InvocationTargetException
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:567)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:464)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:363)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:567)
    at java.base/sun.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:1051)
Caused by: java.lang.RuntimeException: Unable to construct Application instance: class tutorial.application.Test
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:890)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication$2(LauncherImpl.java:195)
    at java.base/java.lang.Thread.run(Thread.java:835)
Caused by: java.lang.InstantiationException
    at java.base/jdk.internal.reflect.InstantiationExceptionConstructorAccessorImpl.newInstance(InstantiationExceptionConstructorAccessorImpl.java:48)
    at java.base/java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:500)
    at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:481)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$8(LauncherImpl.java:802)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runAndWait$12(PlatformImpl.java:455)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:428)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:389)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:427)
    at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:174)
    ... 1 more
Exception running application tutorial.application.Test

编辑： 到目前为止，我已经尝试了3个明显不同的选项，但都出现了错误。首先，我尝试在层次结构中使子类保持静态，如下所示：

static class MyLine extends Test{
.
.
.
}

public abstract class Test extends Application{

    private int width = 0;
    private int height = 0;

    public Test() {
        this.width = 0;
        this.height = 0;
    }

    public Test(int width, int height) {
        this.width = width;
        this.height = height;
    }

    public abstract void draw(GraphicsContext GC);

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

    @Override
    public void start(Stage primaryStage) {

        int canvasx = 400, canvasy = 650;
        MyLine line = new MyLine(0, 0, canvasx, canvasy);
        MyLine line2 = new MyLine(0, canvasy, canvasx, 0);
        Group root = new Group();
        Canvas canvas = new Canvas(canvasx, canvasy);
        GraphicsContext GC = canvas.getGraphicsContext2D();
        line.draw(GC);
        line2.draw(GC);
        root.getChildren().add(canvas);
        primaryStage.setScene(new Scene(root));
        primaryStage.show();
    }
}

class MyLine extends Test{

    private int x1, x2, y1, y2;

    public MyLine(int x1, int y1, int x2, int y2) {
        super(0, 0);
        this.x1 = x1;
        this.x2 = x2;
        this.y1 = y1;
        this.y2 = y2;
    }

    @Override
    public void draw(GraphicsContext GC){
        GC.strokeLine(x1, y1, x2, y2);
    }
}

package tutorial.application;

import javafx.application.Application;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.canvas.Canvas;
import javafx.scene.canvas.GraphicsContext;
import javafx.stage.Stage;
import javafx.scene.paint.Color;
import java.lang.Math;


public abstract class Test extends Application{

    private int width = 0;
    private int height = 0;

    public Test() {
        this.width = 0;
        this.height = 0;
    }

    public Test(int width, int height) {
        this.width = width;
        this.height = height;
    }

    public abstract void draw(GraphicsContext GC);


}

class MyLine extends Test{

    private int x1, x2, y1, y2;

    public MyLine(int x1, int y1, int x2, int y2) {
        super(0, 0);
        this.x1 = x1;
        this.x2 = x2;
        this.y1 = y1;
        this.y2 = y2;
    }

    @Override
    public void draw(GraphicsContext GC){
        GC.strokeLine(x1, y1, x2, y2);
    }

    @Override
    public void start(Stage arg0) throws Exception {

    }
}

class Main extends Test{
    private Main() {
        super(0,0);
    }
    public static void main(String []args){
        launch(args);
    }

    @Override
    public void start(Stage primaryStage) {

        int canvasx = 400, canvasy = 650;
        MyLine line = new MyLine(0, 0, canvasx, canvasy);
        MyLine line2 = new MyLine(0, canvasy, canvasx, 0);
        Group root = new Group();
        Canvas canvas = new Canvas(canvasx, canvasy);
        GraphicsContext GC = canvas.getGraphicsContext2D();
        line.draw(GC);
        line2.draw(GC);
        root.getChildren().add(canvas);
        primaryStage.setScene(new Scene(root));
        primaryStage.show();
    }

    @Override
    public void draw(GraphicsContext GC) {

    }
}

我还尝试将内部类“MyLine”从超类“Test”中去掉，如下所示：

static class MyLine extends Test{
.
.
.
}

public abstract class Test extends Application{

    private int width = 0;
    private int height = 0;

    public Test() {
        this.width = 0;
        this.height = 0;
    }

    public Test(int width, int height) {
        this.width = width;
        this.height = height;
    }

    public abstract void draw(GraphicsContext GC);

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

    @Override
    public void start(Stage primaryStage) {

        int canvasx = 400, canvasy = 650;
        MyLine line = new MyLine(0, 0, canvasx, canvasy);
        MyLine line2 = new MyLine(0, canvasy, canvasx, 0);
        Group root = new Group();
        Canvas canvas = new Canvas(canvasx, canvasy);
        GraphicsContext GC = canvas.getGraphicsContext2D();
        line.draw(GC);
        line2.draw(GC);
        root.getChildren().add(canvas);
        primaryStage.setScene(new Scene(root));
        primaryStage.show();
    }
}

class MyLine extends Test{

    private int x1, x2, y1, y2;

    public MyLine(int x1, int y1, int x2, int y2) {
        super(0, 0);
        this.x1 = x1;
        this.x2 = x2;
        this.y1 = y1;
        this.y2 = y2;
    }

    @Override
    public void draw(GraphicsContext GC){
        GC.strokeLine(x1, y1, x2, y2);
    }
}

package tutorial.application;

import javafx.application.Application;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.canvas.Canvas;
import javafx.scene.canvas.GraphicsContext;
import javafx.stage.Stage;
import javafx.scene.paint.Color;
import java.lang.Math;


public abstract class Test extends Application{

    private int width = 0;
    private int height = 0;

    public Test() {
        this.width = 0;
        this.height = 0;
    }

    public Test(int width, int height) {
        this.width = width;
        this.height = height;
    }

    public abstract void draw(GraphicsContext GC);


}

class MyLine extends Test{

    private int x1, x2, y1, y2;

    public MyLine(int x1, int y1, int x2, int y2) {
        super(0, 0);
        this.x1 = x1;
        this.x2 = x2;
        this.y1 = y1;
        this.y2 = y2;
    }

    @Override
    public void draw(GraphicsContext GC){
        GC.strokeLine(x1, y1, x2, y2);
    }

    @Override
    public void start(Stage arg0) throws Exception {

    }
}

class Main extends Test{
    private Main() {
        super(0,0);
    }
    public static void main(String []args){
        launch(args);
    }

    @Override
    public void start(Stage primaryStage) {

        int canvasx = 400, canvasy = 650;
        MyLine line = new MyLine(0, 0, canvasx, canvasy);
        MyLine line2 = new MyLine(0, canvasy, canvasx, 0);
        Group root = new Group();
        Canvas canvas = new Canvas(canvasx, canvasy);
        GraphicsContext GC = canvas.getGraphicsContext2D();
        line.draw(GC);
        line2.draw(GC);
        root.getChildren().add(canvas);
        primaryStage.setScene(new Scene(root));
        primaryStage.show();
    }

    @Override
    public void draw(GraphicsContext GC) {

    }
}

Laslty I设计了一个新类“Main”，以扩展“Test”，并从中启动应用程序，如下所示：

static class MyLine extends Test{
.
.
.
}

public abstract class Test extends Application{

    private int width = 0;
    private int height = 0;

    public Test() {
        this.width = 0;
        this.height = 0;
    }

    public Test(int width, int height) {
        this.width = width;
        this.height = height;
    }

    public abstract void draw(GraphicsContext GC);

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

    @Override
    public void start(Stage primaryStage) {

        int canvasx = 400, canvasy = 650;
        MyLine line = new MyLine(0, 0, canvasx, canvasy);
        MyLine line2 = new MyLine(0, canvasy, canvasx, 0);
        Group root = new Group();
        Canvas canvas = new Canvas(canvasx, canvasy);
        GraphicsContext GC = canvas.getGraphicsContext2D();
        line.draw(GC);
        line2.draw(GC);
        root.getChildren().add(canvas);
        primaryStage.setScene(new Scene(root));
        primaryStage.show();
    }
}

class MyLine extends Test{

    private int x1, x2, y1, y2;

    public MyLine(int x1, int y1, int x2, int y2) {
        super(0, 0);
        this.x1 = x1;
        this.x2 = x2;
        this.y1 = y1;
        this.y2 = y2;
    }

    @Override
    public void draw(GraphicsContext GC){
        GC.strokeLine(x1, y1, x2, y2);
    }
}

package tutorial.application;

import javafx.application.Application;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.canvas.Canvas;
import javafx.scene.canvas.GraphicsContext;
import javafx.stage.Stage;
import javafx.scene.paint.Color;
import java.lang.Math;


public abstract class Test extends Application{

    private int width = 0;
    private int height = 0;

    public Test() {
        this.width = 0;
        this.height = 0;
    }

    public Test(int width, int height) {
        this.width = width;
        this.height = height;
    }

    public abstract void draw(GraphicsContext GC);


}

class MyLine extends Test{

    private int x1, x2, y1, y2;

    public MyLine(int x1, int y1, int x2, int y2) {
        super(0, 0);
        this.x1 = x1;
        this.x2 = x2;
        this.y1 = y1;
        this.y2 = y2;
    }

    @Override
    public void draw(GraphicsContext GC){
        GC.strokeLine(x1, y1, x2, y2);
    }

    @Override
    public void start(Stage arg0) throws Exception {

    }
}

class Main extends Test{
    private Main() {
        super(0,0);
    }
    public static void main(String []args){
        launch(args);
    }

    @Override
    public void start(Stage primaryStage) {

        int canvasx = 400, canvasy = 650;
        MyLine line = new MyLine(0, 0, canvasx, canvasy);
        MyLine line2 = new MyLine(0, canvasy, canvasx, 0);
        Group root = new Group();
        Canvas canvas = new Canvas(canvasx, canvasy);
        GraphicsContext GC = canvas.getGraphicsContext2D();
        line.draw(GC);
        line2.draw(GC);
        root.getChildren().add(canvas);
        primaryStage.setScene(new Scene(root));
        primaryStage.show();
    }

    @Override
    public void draw(GraphicsContext GC) {

    }
}

前两个仍然会导致相同的错误，我认为最后一个会在线程“main”中引发新的异常，从而使问题更加严重：

---

如果超类是抽象的，则不能实例化子类的原因是子类是超类的内部类，要实例化内部类，需要包含类的实例，但根据定义，不能实例化抽象类

---

将子类设置为静态，你的问题就会得到解决。

我通过将类文件重命名为“Main”找到了一个有效的解决方案。这使我能够保留抽象类“Test”及其所有子类，并通过扩展“Application”的新公共类“Main”启动了应用程序

如果您阅读javadoc中的

实例化异常

，您将看到抽象类无法通过反射实例化。如果研究发布的堆栈跟踪，您将看到代码试图通过反射实例化

测试

类。

launch

始终启动包含调用

launch

（或类似）的方法的类，这就是

测试

类。您可以指定不同的类作为

Launch

方法的附加参数。请注意，

MyLine

也不起作用，因为它是一个非

静态的内部类，因此需要一个包含类的参数来构造。（可以反射调用的构造函数不需要传递任何参数。）是否有理由MyLine会成为它继承自的抽象的内部类？我的意思是，即使这样做有效，那有什么意义呢？@MichelFeldheim它只有在引用超类中的字段时才有意义。