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What is an abstract class?

Abstract classes are used in java to create a class with some default method implementation for subclasses. An abstract class can have abstract method without body and it can have methods with implementation also.

abstract keyword is used to create a abstract class. Abstract classes can’t be instantiated and mostly used to provide base for sub-classes to extend and implement the abstract methods and override or use the implemented methods in abstract class.

What is an interface?

Interfaces are core part of java programming language and used a lot not only in JDK but also java design patterns, most of the frameworks and tools. Interfaces provide a way to achieve abstraction in java and used to define the contract for the subclasses to implement.

Interfaces are good for starting point to define Type and create top level hierarchy in our code. Since a java class can implements multiple interfaces, it’s better to use interfaces as super class in most of the cases. 

What is static block?

Java static block is the group of statements that gets executed when the class is loaded into memory by Java ClassLoader. It is used to initialize static variables of the class. Mostly it’s used to create static resources when class is loaded.

What is multi-catch block in java?

Java 7 one of the improvement was multi-catch block where we can catch multiple exceptions in a single catch block. This makes are code shorter and cleaner when every catch block has similar code.

If a catch block handles multiple exception, you can separate them using a pipe (|) and in this case exception parameter (ex) is final, so you can’t change it.

Read more at Java multi catch block.

Catching Multiple Exceptions in single catch and Rethrowing Exceptions with Improved Type Checking – Java 7 Feature

In Java 7, catch block has been improved to handle multiple exceptions in a single catch block. If you are catching multiple exceptions and they have similar code, then using this feature will reduce code duplication. Let’s understand this with an example.

Before Java 7:

catch (IOException ex) {
     logger.error(ex);
     throw new MyException(ex.getMessage());
catch (SQLException ex) {
     logger.error(ex);
     throw new MyException(ex.getMessage());
}catch (Exception ex) {
     logger.error(ex);
     throw new MyException(ex.getMessage());
}
In Java 7, we can catch all these exceptions in a single catch block as:


catch(IOException | SQLException | Exception ex){
     logger.error(ex);
     throw new MyException(ex.getMessage());
}
If a catch block handles multiple exception, you can separate them using a pipe (|) and in this case exception parameter (ex) is final, so you can’t change it. The byte code generated by this feature is smaller and reduce code redundancy.

Another improvement is done in Compiler analysis of rethrown exceptions. This feature allows you to specify more specific exception types in the throws clause of a method declaration.

Let’s see this with a small example:

package com.journaldev.util;

public class Java7MultipleExceptions {

    public static void main(String[] args) {
        try{
            rethrow("abc");
        }catch(FirstException | SecondException | ThirdException e){
            //below assignment will throw compile time exception since e is final
            //e = new Exception();
            System.out.println(e.getMessage());
        }
    }

    static void rethrow(String s) throws FirstException, SecondException,
            ThirdException {
        try {
            if (s.equals("First"))
                throw new FirstException("First");
            else if (s.equals("Second"))
                throw new SecondException("Second");
            else
                throw new ThirdException("Third");
        } catch (Exception e) {
            //below assignment disables the improved rethrow exception type checking feature of Java 7
            // e=new ThirdException();
            throw e;
        }
    }

    static class FirstException extends Exception {

        public FirstException(String msg) {
            super(msg);
        }
    }

    static class SecondException extends Exception {

        public SecondException(String msg) {
            super(msg);
        }
    }

    static class ThirdException extends Exception {

        public ThirdException(String msg) {
            super(msg);
        }
    }

}

As you can see that in rethrow method, catch block is catching Exception but it’s not part of throws clause. Java 7 compiler analyze the complete try block to check what types of exceptions are thrown and then rethrown from the catch block.

Note that this analysis is disabled if you change the catch block argument.
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