当覆盖超类方法时，如果该方法抛出异常，您需要遵循一定的规则。
如果超类方法抛出某个异常，子类中的方法应该抛出相同的异常或者它的子类型。
在下面的示例中，超类的readFile()方法抛出了IOException异常，而子类的readFile()方法抛出了FileNotFoundException异常。
由于FileNotFoundException异常是IOException的子类型，所以该程序可以在没有任何错误的情况下编译和执行。
import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.Scanner;
abstract class Super {
public String readFile(String path) throws IOException {
throw new IOException();
}
}
public class ExceptionsExample extends Super {
@Override
public String readFile(String path) throws FileNotFoundException {
Scanner sc = new Scanner(new File("E://test//sample.txt"));
String input;
StringBuffer sb = new StringBuffer();
while (sc.hasNextLine()) {
input = sc.nextLine();
sb.append(" "+input);
}
return sb.toString();
}
public static void main(String args[]) {
String path = "E://test//sample.txt";
ExceptionsExample obj = new ExceptionsExample();
try {
System.out.println(obj.readFile(path));
}catch(FileNotFoundException e) {
System.out.println("Make sure the specified file exists");
}
}
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同样，如果子类抛出与超类相同的异常，则程序将成功编译并执行。
import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.Scanner;
abstract class Super {
public void sampleMethod()throws FileNotFoundException {
System.out.println("Method of superclass");
}
}
public class ExceptionsExample extends Super {
public void sampleMethod()throws FileNotFoundException {
System.out.println("Method of Subclass");
}
public static void main(String args[]) {
ExceptionsExample obj = new ExceptionsExample();
obj.sampleMethod();
}
}Method of Subclass
如果超类方法抛出某个异常，子类中的方法不应该抛出其超类。
在下面的示例中，超类的readFile()方法抛出FileNotFoundException异常，而子类的readFile()方法抛出IOException异常，后者是FileNotFoundException的超类。
import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.Scanner;
abstract class Super {
public String readFile(String path)throws FileNotFoundException {
throw new FileNotFoundException();
}
}
public class ExceptionsExample extends Super {
@Override
public String readFile(String path)throws IOException {
//method body ......
}
}编译时，上面的程序给出以下输出 -
ExceptionsExample.java:13: error: readFile(String) in ExceptionsExample cannot override readFile(String) in Sup
public String readFile(String path)throws IOException {
^
overridden method does not throw IOException
1 error如果父类方法抛出某些异常，你可以在重写时不抛出任何异常。
在下面的示例中，父类的sampleMethod()方法抛出FileNotFoundException异常，而子类的sampleMethod()方法根本不抛出任何异常。尽管如此，该程序可以编译和执行而不会出现任何错误。
import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.Scanner;
abstract class Super {
public void sampleMethod()throws FileNotFoundException {
System.out.println("Method of superclass");
}
}
public class ExceptionsExample extends Super {
public void sampleMethod() {
System.out.println("Method of Subclass");
}
public static void main(String args[]) {
ExceptionsExample obj = new ExceptionsExample();
obj.sampleMethod();
}
}Method of Subclass
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