增加自定义异常的例子

04：异常处理/01.md

@@ -440,7 +440,95 @@ public class Test {
 
 ## 4. 自定义异常
 
-略，留给大家自己去探索。
+> Java provides quite a few exception classes. Use them whenever possible instead of defining your own exception classes. However, if you run into a problem that cannot be adequately described by the predefined exception classes, you can create your own exception class, derived from Exception or from a subclass of Exception, such as IOException.
+>
+> In Listing 12.7, CircleWithException.java, the setRadius method throws an exception if the radius is negative. Suppose you wish to pass the radius to the handler. In that case, you can define a custom exception class, as shown in Listing 12.10.
+
+InvalidRadiusException.java
+
+```java
+public class InvalidRadiusException extends Exception {
+	private double radius;
+
+	/** Construct an exception */
+	public InvalidRadiusException(double radius) {
+		super("Invalid radius " + radius);
+		this.radius = radius;
+	}
+
+	/** Return the radius */
+	public double getRadius() {
+		return radius;
+	}
+}
+```
+
+![image-20260410132946665](/run/media/danny/Data/CUIT/Java-Book/04：异常处理/img/image-20260410132946665.png)
+
+TestCircleWithCustomException.java
+
+```java
+public class TestCircleWithCustomException {
+	public static void main(String[] args) {
+		try {
+			new CircleWithCustomException(5);
+			new CircleWithCustomException(-5);
+			new CircleWithCustomException(0);
+		} catch (InvalidRadiusException ex) {
+			System.out.println(ex);
+		}
+
+		System.out.println("Number of objects created: " + CircleWithCustomException.getNumberOfObjects());
+	}
+}
+
+class CircleWithCustomException {
+	/** The radius of the circle */
+	private double radius;
+
+	/** The number of the objects created */
+	private static int numberOfObjects = 0;
+
+	/** Construct a circle with radius 1 */
+	public CircleWithCustomException() throws InvalidRadiusException {
+		this(1.0);
+	}
+
+	/** Construct a circle with a specified radius */
+	public CircleWithCustomException(double newRadius) throws InvalidRadiusException {
+		setRadius(newRadius);
+		numberOfObjects++;
+	}
+
+	/** Return radius */
+	public double getRadius() {
+		return radius;
+	}
+
+	/** Set a new radius */
+	public void setRadius(double newRadius) throws InvalidRadiusException {
+		if (newRadius >= 0)
+			radius = newRadius;
+		else
+			throw new InvalidRadiusException(newRadius);
+	}
+
+	/** Return numberOfObjects */
+	public static int getNumberOfObjects() {
+		return numberOfObjects;
+	}
+
+	/** Return the area of this circle */
+	public double findArea() {
+		return radius * radius * 3.14159;
+	}
+}
+
+```
+
+
+
+
 
 ## 5. 本章重点