Java面向对象设计

抽象类 VS 接口

Java相比于其他面向对象语言，如C++，设计上有一些基本区别，比如Java不支持多继承。这种限制，在规范了代码实现的同时，也产生了一些局限性，影响着程序设计结构。Java类可以实现多个接口，因为接口是抽象方法的集合，所以这是声明性的，但不能通过扩展多个抽象类来重用逻辑。

在一些情况下存在特定场景，需要抽象出与具体实现、实例化无关的通用逻辑，或者纯调用关系的逻辑，但是使用传统的抽象类会陷入到单继承的窘境。以往常见的做法是，实现由静态方法组成的工具类（Utils），比如java.util.Collections。

设想，为接口添加任何抽象方法，相应的所有实现了这个接口的类，也必须实现新增方法，否则会出现编译错误。对于抽象类，如果我们添加非抽象方法，其子类只会享受到能力扩展，而不用担心编译出问题。
接口的职责也不仅仅限于抽象方法的集合，其实有各种不同的实践。有一类没有任何方法的接口，通常叫作Marker Interface，顾名思义，它的目的就是为了声明某些东西，比如我们熟知的Cloneable、Serializable等。这种用法，也存在于业界其他的Java产品代码中。
从表面看，这似乎和Annotation异曲同工，也确实如此，它的好处是简单直接。对于Annotation，因为可以指定参数和值，在表达能力上要更强大一些，所以更多人选择使用Annotation。

Java 8增加了函数式编程的支持，所以又增加了一类定义，即所谓functional interface，简单说就是只有一个抽象方法的接口，通常建议使用@FunctionalInterfaceAnnotation来标记。Lambda表达式本身可以看作是一类functional interface，某种程度上这和面向对象可以算是两码事。我们熟知的Runnable、Callable之类，都
是functional interface，这里不再多介绍了，有兴趣可以参考：https://www.oreilly.com/learning/java-8-functional-interfaces 。

还有一点可能让人感到意外，严格说，Java 8以后，接口也是可以有默认方法实现的！

从Java 8开始，interface增加了对default method的支持。Java 9以后，甚至可以定义private default method。Default method提供了一种二进制兼容的扩展已有接口的办法。比如，我们熟知的java.util.Collection，它是collection体系的root interface，在Java 8中添加了一系列default method，主要是增加Lambda、Stream相关的功能。我在专栏前面提到的类似Collections之类的工具类，很多方法都适合作为default method实现在基础接口里面。
你可以参考下面代码片段：

public interface Collection<E> extends Iterable<E> {
     /**
     * Returns a sequential Stream with this collection as its source 
     * ...
     **/
     default Stream<E> sream() {
        return StreamSupport.sream(spliterator(), false);
     }
 }

面向对象设计

谈到面向对象，很多人就会想起设计模式，那些是非常经典的问题和设计方法的总结。不过在那之前还是先夯实一下基础，先来聊聊面向对象设计的基本方面。

我们一定要清楚面向对象的基本要素：封装、继承、多态。

封装的目的是隐藏事务内部的实现细节，以便提高安全性和简化编程。封装提供了合理的边界，避免外部调用者接触到内部的细节。我们在日常开发中，因为无意间暴露了细节导致难缠bug太多了，比如在多线程环境暴露内部状态，导致的并发修改问题。从另外一个角度看，封装这种隐藏，也提供了简化的界面，避免太多无意义的细节浪费调用者的精力。
继承是代码复用的基础机制，类似于我们对于马、白马、黑马的归纳总结。但要注意，继承可以看作是非常紧耦合的一种关系，父类代码修改，子类行为也会变动。在实践中，过度滥用继承，可能会起到反效果。
多态，你可能立即会想到重写（override）和重载（overload）、向上转型。简单说，重写是父子类中相同名字和参数的方法，不同的实现；重载则是相同名字的方法，但是不同的参数，本质上这些方法签名是不一样的，为了更好说明，请参考下面的样例代码：

public int doSomething() {
 return 0;
}

// 输入参数不同，意味着方法签名不同，重载的体现
public int doSomething(Lis<String> srs) {
 return 0;
}
// return类型不一样，编译不能通过
public short doSomething() {
 return 0;
}

这里你可以思考一个小问题，方法名称和参数一致，但是返回值不同，这种情况在Java代码中算是有效的重载吗？答案是不是的，编译都会出错的。

进行面向对象编程，掌握基本的设计原则是必须的，我今天介绍最通用的部分，也就是所谓的S.O.L.I.D原则。

单一职责（Single Responsibility），类或者对象最好是只有单一职责，在程序设计中如果发现某个类承担着多种义务，可以考虑进行拆分。
开关原则（Open-Close, Open for extension, close for modifcation），设计要对扩展开放，对修改关闭。换句话说，程序设计应保证平滑的扩展性，尽量避免因为新增同
类功能而修改已有实现，这样可以少产出些回归（regression）问题。
里氏替换（Liskov Substitution），这是面向对象的基本要素之一，进行继承关系抽象时，凡是可以用父类或者基类的地方，都可以用子类替换。
接口分离（Interface Segregation），我们在进行类和接口设计时，如果在一个接口里定义了太多方法，其子类很可能面临两难，就是只有部分方法对它是有意义的，这就破坏
了程序的内聚性。
对于这种情况，可以通过拆分成功能单一的多个接口，将行为进行解耦。在未来维护中，如果某个接口设计有变，不会对使用其他接口的子类构成影响。
依赖反转（Dependency Inversion），实体应该依赖于抽象而不是实现。也就是说高层次模块，不应该依赖于低层次模块，而是应该基于抽象。实践这一原则是保证产品代码之
间适当耦合度的法宝。

OOP原则实践中的取舍

值得注意的是，现代语言的发展，很多时候并不是完全遵守前面的原则的，比如，Java 10中引入了本地方法类型推断和var类型。按照，里氏替换原则，我们通常这样定义变量：
List<String> list = new ArrayLis<>();
如果使用var类型，可以简化为
var list = new ArrayLis<String>();
但是，list实际会被推断为“ArrayList ”
ArrayList<String> list = new ArrayLis<String>();
理论上，这种语法上的便利，其实是增强了程序对实现的依赖，但是微小的类型泄漏却带来了书写的变量和代码可读性的提高，所以，实践中我们还是要按照得失利弊进行选择，而不是一味得遵循原则。

OOP原则的分析

看看下面这段代码，改编自朋友圈盛传的某
伟大公司产品代码，你觉得可以利用面向对象设计原则如何改进？

public class VIPCenter {
     void serviceVIP(T extend User user>) {
     if (user insanceof SlumDogVIP) {
     // 穷X VIP，活动抢的那种
     // do somthing
     } else if(user insanceof RealVIP) {
     // do somthing
     }
     // ...
 }

这段代码的一个问题是，业务逻辑集中在一起，当出现新的用户类型时，比如，大数据发现了我们是肥羊，需要去收获一下，这就需要直接去修改服务方法代码实现，这可能会意外影响不相关的某个用户类型逻辑。
利用开关原则，我们可以尝试改造为下面的代码：

public class VIPCenter {
     private Map<User.TYPE, ServiceProvider> providers;
     void serviceVIP(T extend User user） {
        providers.get(user.getType()).service(user);
     }
}

interface ServiceProvider{
    void service(T extend User user) ;
}

class SlumDogVIPServiceProvider implements ServiceProvider{
     void service(T extend User user){
     // do somthing
     }
 }
 class RealVIPServiceProvider implements ServiceProvider{
     void service(T extend User user) {
     // do something
}

上面的示例，将不同对象分类的服务方法进行抽象，把业务逻辑的紧耦合关系拆开，实现代码的隔离保证了方便的扩展。

Exception Handling in Java

The Exception Handling in Java is one of the powerful mechanism to handle the runtime errors so that normal flow of the application can be maintained.

What is Exception in Java

Dictionary Meaning: Exception is an abnormal condition.

In Java, an exception is an event that disrupts the normal flow of the program. It is an object which is thrown at runtime.

What is Exception Handling

Exception Handling is a mechanism to handle runtime errors such as ClassNotFoundException, IOException, SQLException, RemoteException, etc.

Advantage of Exception Handling

The core advantage of exception handling is to maintain the normal flow of the application. An exception normally disrupts the normal flow of the application that is why we use exception handling. Let's take a scenario:

statement 1;  
statement 2;  
statement 3;  
statement 4;  
statement 5;//exception occurs  
statement 6;  
statement 7;  
statement 8;  
statement 9;  
statement 10;

Suppose there are 10 statements in your program and there occurs an exception at statement 5, the rest of the code will not be executed i.e. statement 6 to 10 will not be executed. If we perform exception handling, the rest of the statement will be executed. That is why we use exception handling in Java.

Hierarchy of Java Exception classes

The java.lang.Throwable class is the root class of Java Exception hierarchy which is inherited by two subclasses: Exception and Error. A hierarchy of Java Exception classes are given below:

hierarchy of exception handling

Hierarchy of Java Exception classes

Types of Java Exceptions

There are mainly two types of exceptions: checked and unchecked. Here, an error is considered as the unchecked exception. According to Oracle, there are three types of exceptions:

Checked Exception
Unchecked Exception
Error

Types of Java Exceptions
Difference between Checked and Unchecked Exceptions
1) Checked Exception
The classes which directly inherit Throwable class except RuntimeException and Error are known as checked exceptions e.g. IOException, SQLException etc. Checked exceptions are checked at compile-time.

2) Unchecked Exception
The classes which inherit RuntimeException are known as unchecked exceptions e.g. ArithmeticException, NullPointerException, ArrayIndexOutOfBoundsException etc. Unchecked exceptions are not checked at compile-time, but they are checked at runtime.

3) Error
Error is irrecoverable e.g. OutOfMemoryError, VirtualMachineError, AssertionError etc.

Java Exception Keywords

There are 5 keywords which are used in handling exceptions in Java.

Keyword Description

try The "try" keyword is used to specify a block where we should place exception code. The try block must be followed by either catch or finally. It means, we can't use try block alone.
catch The "catch" block is used to handle the exception. It must be preceded by try block which means we can't use catch block alone. It can be followed by finally block later.
finally The "finally" block is used to execute the important code of the program. It is executed whether an exception is handled or not.
throw The "throw" keyword is used to throw an exception.
throws The "throws" keyword is used to declare exceptions. It doesn't throw an exception. It specifies that there may occur an exception in the method. It is always used with method signature.

Java Exception Handling Example

Let's see an example of Java Exception Handling where we using a try-catch statement to handle the exception.

public class JavaExceptionExample{  
  public static void main(String args[]){  
   try{  
      //code that may raise exception  
      int data=100/0;  
   }catch(ArithmeticException e){System.out.println(e);}  
   //rest code of the program   
   System.out.println("rest of the code...");  
  }  
}

Test it Now
Output:

Exception in thread main java.lang.ArithmeticException:/ by zero
rest of the code...
In the above example, 100/0 raises an ArithmeticException which is handled by a try-catch block.

Common Scenarios of Java Exceptions

There are given some scenarios where unchecked exceptions may occur. They are as follows:

1) A scenario where ArithmeticException occurs
If we divide any number by zero, there occurs an ArithmeticException.

int a=50/0;//ArithmeticException

2) A scenario where NullPointerException occurs
If we have a null value in any variable, performing any operation on the variable throws a NullPointerException.

String s=null;  
System.out.println(s.length());//NullPointerException

3) A scenario where NumberFormatException occurs
The wrong formatting of any value may occur NumberFormatException. Suppose I have a string variable that has characters, converting this variable into digit will occur NumberFormatException.

String s="abc";  
int i=Integer.parseInt(s);//NumberFormatException

4) A scenario where ArrayIndexOutOfBoundsException occurs
If you are inserting any value in the wrong index, it would result in ArrayIndexOutOfBoundsException as shown below:


int a[]=new int[5];  
a[10]=50; //ArrayIndexOutOfBoundsException

Java try-catch block

Java try block

Java try block is used to enclose the code that might throw an exception. It must be used within the method.

If an exception occurs at the particular statement of try block, the rest of the block code will not execute. So, it is recommended not to keeping the code in try block that will not throw an exception.

Java try block must be followed by either catch or finally block.

Syntax of Java try-catch

try{    
//code that may throw an exception    
}catch(Exception_class_Name ref){}    
Syntax of try-finally block
try{    
//code that may throw an exception    
}finally{}

Java catch block

Java catch block is used to handle the Exception by declaring the type of exception within the parameter. The declared exception must be the parent class exception ( i.e., Exception) or the generated exception type. However, the good approach is to declare the generated type of exception.

The catch block must be used after the try block only. You can use multiple catch block with a single try block.

Let's try to understand the problem if we don't use a try-catch block.

Example 1

public class TryCatchExample1 {  

    public static void main(String[] args) {  

        int data=50/0; //may throw exception   

        System.out.println("rest of the code");  

    }  

}

Test it Now
Output:

Exception in thread "main" java.lang.ArithmeticException: / by zero

As displayed in the above example, the rest of the code is not executed (in such case, the rest of the code statement is not printed).

There can be 100 lines of code after exception. So all the code after exception will not be executed.

Solution by exception handling
Let's see the solution of the above problem by a java try-catch block.

Example 2

public class TryCatchExample2 {  

    public static void main(String[] args) {  
        try  
        {  
        int data=50/0; //may throw exception   
        }  
            //handling the exception  
        catch(ArithmeticException e)  
        {  
            System.out.println(e);  
        }  
        System.out.println("rest of the code");  
    }  

}

Test it Now
Output:


java.lang.ArithmeticException: / by zero
rest of the code

Now, as displayed in the above example, the rest of the code is executed, i.e., the rest of the code statement is printed.

Example 3
In this example, we also kept the code in a try block that will not throw an exception.

public class TryCatchExample3 {  

    public static void main(String[] args) {  
        try  
        {  
        int data=50/0; //may throw exception   
                         // if exception occurs, the remaining statement will not exceute  
        System.out.println("rest of the code");  
        }  
             // handling the exception   
        catch(ArithmeticException e)  
        {  
            System.out.println(e);  
        }  

    }  

}

Test it Now
Output:

java.lang.ArithmeticException: / by zero

Here, we can see that if an exception occurs in the try block, the rest of the block code will not execute.

Example 4
Here, we handle the exception using the parent class exception.

public class TryCatchExample4 {  

    public static void main(String[] args) {  
        try  
        {  
        int data=50/0; //may throw exception   
        }  
            // handling the exception by using Exception class      
        catch(Exception e)  
        {  
            System.out.println(e);  
        }  
        System.out.println("rest of the code");  
    }  

}

Test it Now
Output:


java.lang.ArithmeticException: / by zero
rest of the code

Example 5
Let's see an example to print a custom message on exception.

public class TryCatchExample5 {

public static void main(String[] args) {  
    try  
    {  
    int data=50/0; //may throw exception   
    }  
         // handling the exception  
    catch(Exception e)  
    {  
              // displaying the custom message  
        System.out.println("Can't divided by zero");  
    }  
}

}
Test it Now
Output:

Can't divided by zero

Example 6
Let's see an example to resolve the exception in a catch block.

public class TryCatchExample6 {  

    public static void main(String[] args) {  
        int i=50;  
        int j=0;  
        int data;  
        try  
        {  
        data=i/j; //may throw exception   
        }  
            // handling the exception  
        catch(Exception e)  
        {  
             // resolving the exception in catch block  
            System.out.println(i/(j+2));  
        }  
    }  
}

Test it Now
Output:

Example 7
In this example, along with try block, we also enclose exception code in a catch block.

public class TryCatchExample7 {  

    public static void main(String[] args) {  

        try  
        {  
        int data1=50/0; //may throw exception   

        }  
             // handling the exception  
        catch(Exception e)  
        {  
            // generating the exception in catch block  
        int data2=50/0; //may throw exception   

        }  
    System.out.println("rest of the code");  
    }  
}

Test it Now
Output:


Exception in thread "main" java.lang.ArithmeticException: / by zero

Here, we can see that the catch block didn't contain the exception code. So, enclose exception code within a try block and use catch block only to handle the exceptions.

Example 8
In this example, we handle the generated exception (Arithmetic Exception) with a different type of exception class (ArrayIndexOutOfBoundsException).

public class TryCatchExample8 {

public static void main(String[] args) {  
    try  
    {  
    int data=50/0; //may throw exception   

    }  
        // try to handle the ArithmeticException using ArrayIndexOutOfBoundsException  
    catch(ArrayIndexOutOfBoundsException e)  
    {  
        System.out.println(e);  
    }  
    System.out.println("rest of the code");  
}

}
Test it Now
Output:

Exception in thread "main" java.lang.ArithmeticException: / by zero

Example 9
Let's see an example to handle another unchecked exception.


public class TryCatchExample9 {  

    public static void main(String[] args) {  
        try  
        {  
        int arr[]= {1,3,5,7};  
        System.out.println(arr[10]); //may throw exception   
        }  
            // handling the array exception  
        catch(ArrayIndexOutOfBoundsException e)  
        {  
            System.out.println(e);  
        }  
        System.out.println("rest of the code");  
    }  

}

Test it Now
Output:

java.lang.ArrayIndexOutOfBoundsException: 10
rest of the code

Example 10
Let's see an example to handle checked exception.

import java.io.FileNotFoundException;  
import java.io.PrintWriter;  

public class TryCatchExample10 {  

    public static void main(String[] args) {  

        PrintWriter pw;  
        try {  
            pw = new PrintWriter("jtp.txt"); //may throw exception   
            pw.println("saved");  
        }  
// providing the checked exception handler  
 catch (FileNotFoundException e) {  

            System.out.println(e);  
        }         
    System.out.println("File saved successfully");  
    }  
}

Test it Now
Output:

File saved successfully

Internal working of java try-catch block

internal working of try-catch block
The JVM firstly checks whether the exception is handled or not. If exception is not handled, JVM provides a default exception handler that performs the following tasks:

Prints out exception description.
Prints the stack trace (Hierarchy of methods where the exception occurred).
Causes the program to terminate.
But if exception is handled by the application programmer, normal flow of the application is maintained i.e. rest of the code is executed.

Java catch multiple exceptions

Java Multi-catch block

A try block can be followed by one or more catch blocks. Each catch block must contain a different exception handler. So, if you have to perform different tasks at the occurrence of different exceptions, use java multi-catch block.

Points to remember

At a time only one exception occurs and at a time only one catch block is executed.
All catch blocks must be ordered from most specific to most general, i.e. catch for ArithmeticException must come before catch for Exception.

Example 1
Let's see a simple example of java multi-catch block.

public class MultipleCatchBlock1 {  

    public static void main(String[] args) {  

           try{    
                int a[]=new int[5];    
                a[5]=30/0;    
               }    
               catch(ArithmeticException e)  
                  {  
                   System.out.println("Arithmetic Exception occurs");  
                  }    
               catch(ArrayIndexOutOfBoundsException e)  
                  {  
                   System.out.println("ArrayIndexOutOfBounds Exception occurs");  
                  }    
               catch(Exception e)  
                  {  
                   System.out.println("Parent Exception occurs");  
                  }             
               System.out.println("rest of the code");    
    }  
}

Test it Now
Output:

Arithmetic Exception occurs

rest of the code
Example 2

public class MultipleCatchBlock2 {  

    public static void main(String[] args) {  

           try{    
                int a[]=new int[5];    

                System.out.println(a[10]);  
               }    
               catch(ArithmeticException e)  
                  {  
                   System.out.println("Arithmetic Exception occurs");  
                  }    
               catch(ArrayIndexOutOfBoundsException e)  
                  {  
                   System.out.println("ArrayIndexOutOfBounds Exception occurs");  
                  }    
               catch(Exception e)  
                  {  
                   System.out.println("Parent Exception occurs");  
                  }             
               System.out.println("rest of the code");    
    }  
}

Test it Now
Output:


ArrayIndexOutOfBounds Exception occurs
rest of the code

Example 3
In this example, try block contains two exceptions. But at a time only one exception occurs and its corresponding catch block is invoked.

public class MultipleCatchBlock3 {  

    public static void main(String[] args) {  

           try{    
                int a[]=new int[5];    
                a[5]=30/0;    
                System.out.println(a[10]);  
               }    
               catch(ArithmeticException e)  
                  {  
                   System.out.println("Arithmetic Exception occurs");  
                  }    
               catch(ArrayIndexOutOfBoundsException e)  
                  {  
                   System.out.println("ArrayIndexOutOfBounds Exception occurs");  
                  }    
               catch(Exception e)  
                  {  
                   System.out.println("Parent Exception occurs");  
                  }             
               System.out.println("rest of the code");    
    }  
}

Test it Now
Output:

Arithmetic Exception occurs
rest of the code

Example 4
In this example, we generate NullPointerException, but didn't provide the corresponding exception type. In such case, the catch block containing the parent exception class Exception will invoked.

public class MultipleCatchBlock4 {

public static void main(String[] args) {  

       try{    
            String s=null;  
            System.out.println(s.length());  
           }    
           catch(ArithmeticException e)  
              {  
               System.out.println("Arithmetic Exception occurs");  
              }    
           catch(ArrayIndexOutOfBoundsException e)  
              {  
               System.out.println("ArrayIndexOutOfBounds Exception occurs");  
              }    
           catch(Exception e)  
              {  
               System.out.println("Parent Exception occurs");  
              }             
           System.out.println("rest of the code");    
}

}
Test it Now
Output:


Parent Exception occurs
rest of the code

Example 5
Let's see an example, to handle the exception without maintaining the order of exceptions (i.e. from most specific to most general).

class MultipleCatchBlock5{    
  public static void main(String args[]){    
   try{    
    int a[]=new int[5];    
    a[5]=30/0;    
   }    
   catch(Exception e){System.out.println("common task completed");}    
   catch(ArithmeticException e){System.out.println("task1 is completed");}    
   catch(ArrayIndexOutOfBoundsException e){System.out.println("task 2 completed");}    
   System.out.println("rest of the code...");    
 }    
}

Test it Now
Output:

Compile-time error

Java Nested try block

The try block within a try block is known as nested try block in java.

Why use nested try block
Sometimes a situation may arise where a part of a block may cause one error and the entire block itself may cause another error. In such cases, exception handlers have to be nested.

Syntax:

....  
try  
{  
    statement 1;  
    statement 2;  
    try  
    {  
        statement 1;  
        statement 2;  
    }  
    catch(Exception e)  
    {  
    }  
}  
catch(Exception e)  
{  
}  
....

Java nested try example
Let's see a simple example of java nested try block.

class Excep6{  
 public static void main(String args[]){  
  try{  
    try{  
     System.out.println("going to divide");  
     int b =39/0;  
    }catch(ArithmeticException e){System.out.println(e);}  

    try{  
    int a[]=new int[5];  
    a[5]=4;  
    }catch(ArrayIndexOutOfBoundsException e){System.out.println(e);}  

    System.out.println("other statement);  
  }catch(Exception e){System.out.println("handeled");}  

  System.out.println("normal flow..");  
 }  
}

Finally Block in Exception Handling

Java finally block is a block that is used to execute important code such as closing connection, stream etc.

Java finally block is always executed whether exception is handled or not.

Java finally block follows try or catch block.

java finally

　Note: If you don't handle exception, before terminating the program, JVM executes finally block(if any).

Why use java finally

Finally block in java can be used to put "cleanup" code such as closing a file, closing connection etc.

Java throw exception

Java throw keyword

The Java throw keyword is used to explicitly throw an exception.

We can throw either checked or uncheked exception in java by throw keyword. The throw keyword is mainly used to throw custom exception. We will see custom exceptions later.

The syntax of java throw keyword is given below.

throw exception;

Let's see the example of throw IOException.

throw new IOException("sorry device error);

java throw keyword example

In this example, we have created the validate method that takes integer value as a parameter. If the age is less than 18, we are throwing the ArithmeticException otherwise print a message welcome to vote.

public class TestThrow1{  
   static void validate(int age){  
     if(age<18)  
      throw new ArithmeticException("not valid");  
     else  
      System.out.println("welcome to vote");  
   }  
   public static void main(String args[]){  
      validate(13);  
      System.out.println("rest of the code...");  
  }  
}

Test it Now
Output:

Exception in thread main java.lang.ArithmeticException:not valid

Java Exception propagation

An exception is first thrown from the top of the stack and if it is not caught, it drops down the call stack to the previous method,If not caught there, the exception again drops down to the previous method, and so on until they are caught or until they reach the very bottom of the call stack.This is called exception propagation.

Rule: By default Unchecked Exceptions are forwarded in calling chain (propagated).

Program of Exception Propagation

class TestExceptionPropagation1{  
  void m(){  
    int data=50/0;  
  }  
  void n(){  
    m();  
  }  
  void p(){  
   try{  
    n();  
   }catch(Exception e){System.out.println("exception handled");}  
  }  
  public static void main(String args[]){  
   TestExceptionPropagation1 obj=new TestExceptionPropagation1();  
   obj.p();  
   System.out.println("normal flow...");  
  }  
}

Test it Now

Output:exception handled
       normal flow...

exception propagation

In the above example exception occurs in m() method where it is not handled,so it is propagated to previous n() method where it is not handled, again it is propagated to p() method where exception is handled.

Exception can be handled in any method in call stack either in main() method,p() method,n() method or m() method.

Rule: By default, Checked Exceptions are not forwarded in calling chain (propagated).
Program which describes that checked exceptions are not propagated

class TestExceptionPropagation2{  
  void m(){  
    throw new java.io.IOException("device error");//checked exception  
  }  
  void n(){  
    m();  
  }  
  void p(){  
   try{  
    n();  
   }catch(Exception e){System.out.println("exception handeled");}  
  }  
  public static void main(String args[]){  
   TestExceptionPropagation2 obj=new TestExceptionPropagation2();  
   obj.p();  
   System.out.println("normal flow");  
  }  
}

Test it Now

Output:Compile Time Error

Java throws keyword

The Java throws keyword is used to declare an exception. It gives an information to the programmer that there may occur an exception so it is better for the programmer to provide the exception handling code so that normal flow can be maintained.

Exception Handling is mainly used to handle the checked exceptions. If there occurs any unchecked exception such as NullPointerException, it is programmers fault that he is not performing check up before the code being used.

Syntax of java throws

return_type method_name() throws exception_class_name{  
//method code  
}

Which exception should be declared　checked exception only, because:
unchecked Exception: under your control so correct your code.
error: beyond your control e.g. you are unable to do anything if there occurs VirtualMachineError or StackOverflowError.

Advantage of Java throws keyword

Now Checked Exception can be propagated (forwarded in call stack).

It provides information to the caller of the method about the exception.

Java throws example
Let's see the example of java throws clause which describes that checked exceptions can be propagated by throws keyword.

import java.io.IOException;  
class Testthrows1{  
  void m()throws IOException{  
    throw new IOException("device error");//checked exception  
  }  
  void n()throws IOException{  
    m();  
  }  
  void p(){  
   try{  
    n();  
   }catch(Exception e){System.out.println("exception handled");}  
  }  
  public static void main(String args[]){  
   Testthrows1 obj=new Testthrows1();  
   obj.p();  
   System.out.println("normal flow...");  
  }  
}

Test it Now
Output:


exception handled
normal flow...

Rule: If you are calling a method that declares an exception, you must either caught or declare the exception.

There are two cases:
Case1:You caught the exception i.e. handle the exception using try/catch.
Case2:You declare the exception i.e. specifying throws with the method.

Case1: You handle the exception
In case you handle the exception, the code will be executed fine whether exception occurs during the program or not.

import java.io.*;  
class M{  
 void method()throws IOException{  
  throw new IOException("device error");  
 }  
}  
public class Testthrows2{  
   public static void main(String args[]){  
    try{  
     M m=new M();  
     m.method();  
    }catch(Exception e){System.out.println("exception handled");}     

    System.out.println("normal flow...");  
  }  
}

Test it Now

Output:exception handled
       normal flow...

Case2: You declare the exception
A)In case you declare the exception, if exception does not occur, the code will be executed fine.
B)In case you declare the exception if exception occures, an exception will be thrown at runtime because throws does not handle the exception.

A)Program if exception does not occur

import java.io.*;  
class M{  
 void method()throws IOException{  
  System.out.println("device operation performed");  
 }  
}  
class Testthrows3{  
   public static void main(String args[])throws IOException{//declare exception  
     M m=new M();  
     m.method();  

    System.out.println("normal flow...");  
  }  
}

Test it Now

Output:device operation performed
       normal flow...

B)Program if exception occurs

import java.io.*;  
class M{  
 void method()throws IOException{  
  throw new IOException("device error");  
 }  
}  
class Testthrows4{  
   public static void main(String args[])throws IOException{//declare exception  
     M m=new M();  
     m.method();  

    System.out.println("normal flow...");  
  }  
}

Test it Now

Output:Runtime Exception

Difference between throw and throws

There are many differences between throw and throws keywords. A list of differences between throw and throws are given below:

throw	throws
Java throw keyword is used to explicitly throw an exception.	Java throws keyword is used to declare an exception.
Checked exception cannot be propagated using throw only.	Checked exception can be propagated with throws.
Throw is followed by an instance.	Throws is followed by class.
Throw is used within the method.	Throws is used with the method signature.
You cannot throw multiple exceptions.	You can declare multiple exceptions e.g. public void method()throws IOException,SQLException.

Que) Can we rethrow an exception?
Yes, by throwing same exception in catch block.

Java throw example

void m(){  
throw new ArithmeticException("sorry");  
}

Java throws example

void m()throws ArithmeticException{  
//method code  
}

Java throw and throws example

void m()throws ArithmeticException{  
throw new ArithmeticException("sorry");  
}

Difference between final, finally and finalize

There are many differences between final, finally and finalize. A list of differences between final, finally and finalize are given below:

No.	final	finally	finalize
1)	Final is used to apply restrictions on class, method and variable. Final class can't be inherited, final method can't be overridden and final variable value can't be changed.	Finally is used to place important code, it will be executed whether exception is handled or not.	Finalize is used to perform clean up processing just before object is garbage collected.
2)	Final is a keyword.	Finally is a block.	Finalize is a method.

Java final example

class FinalExample{  
public static void main(String[] args){  
final int x=100;  
x=200;//Compile Time Error  
}}

Java finally example

class FinallyExample{  
public static void main(String[] args){  
try{  
int x=300;  
}catch(Exception e){System.out.println(e);}  
finally{System.out.println("finally block is executed");}  
}}

Java finalize example

class FinalizeExample{  
public void finalize(){System.out.println("finalize called");}  
public static void main(String[] args){  
FinalizeExample f1=new FinalizeExample();  
FinalizeExample f2=new FinalizeExample();  
f1=null;  
f2=null;  
System.gc();  
}}

Next TopicException Handling With Method Overriding

Next →← Prev
ExceptionHandling with MethodOverriding in Java
There are many rules if we talk about methodoverriding with exception handling. The Rules are as follows:
If the superclass method does not declare an exception
If the superclass method does not declare an exception, subclass overridden method cannot declare the checked exception but it can declare unchecked exception.
If the superclass method declares an exception
If the superclass method declares an exception, subclass overridden method can declare same, subclass exception or no exception but cannot declare parent exception.
If the superclass method does not declare an exception
1) Rule: If the superclass method does not declare an exception, subclass overridden method cannot declare the checked exception.

import java.io.*;  
class Parent{  
  void msg(){System.out.println("parent");}  
}  

class TestExceptionChild extends Parent{  
  void msg()throws IOException{  
    System.out.println("TestExceptionChild");  
  }  
  public static void main(String args[]){  
   Parent p=new TestExceptionChild();  
   p.msg();  
  }  
}

Test it Now

Output:Compile Time Error

2) Rule: If the superclass method does not declare an exception, subclass overridden method cannot declare the checked exception but can declare unchecked exception.

import java.io.*;  
class Parent{  
  void msg(){System.out.println("parent");}  
}

class TestExceptionChild1 extends Parent{
void msg()throws ArithmeticException{
System.out.println("child");
}
public static void main(String args[]){
Parent p=new TestExceptionChild1();
p.msg();
}
}
Test it Now

Output:child

If the superclass method declares an exception
1) Rule: If the superclass method declares an exception, subclass overridden method can declare same, subclass exception or no exception but cannot declare parent exception.
Example in case subclass overridden method declares parent exception

import java.io.*;  
class Parent{  
  void msg()throws ArithmeticException{System.out.println("parent");}  
}  

class TestExceptionChild2 extends Parent{  
  void msg()throws Exception{System.out.println("child");}  

  public static void main(String args[]){  
   Parent p=new TestExceptionChild2();  
   try{  
   p.msg();  
   }catch(Exception e){}  
  }  
}

Test it Now

Output:Compile Time Error

Example in case subclass overridden method declares same exception

import java.io.*;  
class Parent{  
  void msg()throws Exception{System.out.println("parent");}  
}  

class TestExceptionChild3 extends Parent{  
  void msg()throws Exception{System.out.println("child");}  

  public static void main(String args[]){  
   Parent p=new TestExceptionChild3();  
   try{  
   p.msg();  
   }catch(Exception e){}  
  }  
}

Test it Now

Output:child

Example in case subclass overridden method declares subclass exception

import java.io.*;  
class Parent{  
  void msg()throws Exception{System.out.println("parent");}  
}  

class TestExceptionChild4 extends Parent{  
  void msg()throws ArithmeticException{System.out.println("child");}  

  public static void main(String args[]){  
   Parent p=new TestExceptionChild4();  
   try{  
   p.msg();  
   }catch(Exception e){}  
  }  
}

Test it Now

Output:child

Example in case subclass overridden method declares no exception

import java.io.*;  
class Parent{  
  void msg()throws Exception{System.out.println("parent");}  
}  

class TestExceptionChild5 extends Parent{  
  void msg(){System.out.println("child");}  

  public static void main(String args[]){  
   Parent p=new TestExceptionChild5();  
   try{  
   p.msg();  
   }catch(Exception e){}  
  }  
}

Test it Now

Output:child

Java Custom Exception

If you are creating your own Exception that is known as custom exception or user-defined exception. Java custom exceptions are used to customize the exception according to user need.

By the help of custom exception, you can have your own exception and message.

Let's see a simple example of java custom exception.

class InvalidAgeException extends Exception{  
 InvalidAgeException(String s){  
  super(s);  
 }  
}  
class TestCustomException1{  

   static void validate(int age)throws InvalidAgeException{  
     if(age<18)  
      throw new InvalidAgeException("not valid");  
     else  
      System.out.println("welcome to vote");  
   }  

   public static void main(String args[]){  
      try{  
      validate(13);  
      }catch(Exception m){System.out.println("Exception occured: "+m);}  

      System.out.println("rest of the code...");  
  }  
}

Test it Now

Output:Exception occured: InvalidAgeException:not valid
       rest of the code...

[end, ref: https://www.javatpoint.com/exception-handling-in-java]

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