當覆寫超類別方法時，如果該方法拋出異常，您需要遵循一定的規則。
如果超類別方法拋出某個異常，子類別中的方法應該拋出相同的異常或它的子類型。
在下面的範例中，超類別的readFile()方法拋出了IOException異常，而子類別的readFile()方法拋出了FileNotFoundException異常。
由於FileNotFoundException異常是IOException的子類型，所以程式可以在沒有任何錯誤的情況下編譯和執行。
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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同樣，如果子類別拋出與超類別相同的異常，則程式將成功編譯並執行。
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
如果超類別方法拋出某個異常，子類別中的方法不應該拋出其超類。
在下面的範例中，超類別的readFile()方法拋出FileNotFoundException異常，而子類別的readFile()方法拋出IOException異常，後者是FileNotFoundException的超類。
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 ......
}
}編譯時，上面的程式給出以下輸出-
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如果父類別方法拋出出某些異常，你可以在重寫時不拋出任何異常。
在下面的範例中，父類別的sampleMethod()方法會拋出FileNotFoundException異常，而子類別的sampleMethod()方法根本不會拋出任何例外。儘管如此，該程式可以編譯和執行而不會出現任何錯誤。
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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