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發表時間：2018/2/13 2:27 |
- Webmaster
- 註冊日: 2008/2/19
- 來自:
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- [轉貼]Java How to Throw Exceptions
- How to Throw Exceptions
Before you can catch an exception, some code somewhere must throw one. Any code can throw an exception: your code, code from a package written by someone else such as the packages that come with the Java platform, or the Java runtime environment. Regardless of what throws the exception, it's always thrown with the throw statement.
As you have probably noticed, the Java platform provides numerous exception classes. All the classes are descendants of the Throwable class, and all allow programs to differentiate among the various types of exceptions that can occur during the execution of a program.
You can also create your own exception classes to represent problems that can occur within the classes you write. In fact, if you are a package developer, you might have to create your own set of exception classes to allow users to differentiate an error that can occur in your package from errors that occur in the Java platform or other packages.
You can also create chained exceptions. For more information, see the Chained Exceptions section.
The throw Statement
All methods use the throw statement to throw an exception. The throw statement requires a single argument: a throwable object. Throwable objects are instances of any subclass of the Throwable class. Here's an example of a throw statement.
throw someThrowableObject;
Let's look at the throw statement in context. The following pop method is taken from a class that implements a common stack object. The method removes the top element from the stack and returns the object.
public Object pop() {
Object obj;
if (size == 0) {
throw new EmptyStackException();
}
obj = objectAt(size - 1);
setObjectAt(size - 1, null);
size--;
return obj;
}
The pop method checks to see whether any elements are on the stack. If the stack is empty (its size is equal to 0), pop instantiates a new EmptyStackException object (a member of java.util) and throws it. The Creating Exception Classes section in this chapter explains how to create your own exception classes. For now, all you need to remember is that you can throw only objects that inherit from the java.lang.Throwable class.
Note that the declaration of the pop method does not contain a throws clause. EmptyStackException is not a checked exception, so pop is not required to state that it might occur.
Throwable Class and Its Subclasses
The objects that inherit from the Throwable class include direct descendants (objects that inherit directly from the Throwable class) and indirect descendants (objects that inherit from children or grandchildren of the Throwable class). The figure below illustrates the class hierarchy of the Throwable class and its most significant subclasses. As you can see, Throwable has two direct descendants: Error and Exception.
The Throwable class and its most significant subclasses.
The Throwable class.
Error Class
When a dynamic linking failure or other hard failure in the Java virtual machine occurs, the virtual machine throws an Error. Simple programs typically do not catch or throw Errors.
Exception Class
Most programs throw and catch objects that derive from the Exception class. An Exception indicates that a problem occurred, but it is not a serious system problem. Most programs you write will throw and catch Exceptions as opposed to Errors.
The Java platform defines the many descendants of the Exception class. These descendants indicate various types of exceptions that can occur. For example, IllegalAccessException signals that a particular method could not be found, and NegativeArraySizeException indicates that a program attempted to create an array with a negative size.
One Exception subclass, RuntimeException, is reserved for exceptions that indicate incorrect use of an API. An example of a runtime exception is NullPointerException, which occurs when a method tries to access a member of an object through a null reference. The section Unchecked Exceptions — The Controversy discusses why most applications shouldn't throw runtime exceptions or subclass RuntimeException.
原文出處：How to Throw Exceptions (The Java™ Tutorials > Essential Classes > Exceptions)
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冷日
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發表時間：2018/2/13 2:32 |
- Webmaster
- 註冊日: 2008/2/19
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- [轉貼]How to throw a general exception in Java?
- How to throw a general exception in Java?
Consider this simple program. The program has two files:
Vehicle.java:
class Vehicle {
private int speed = 0;
private int maxSpeed = 100;
public int getSpeed()
{
return speed;
}
public int getMaxSpeed()
{
return maxSpeed;
}
public void speedUp(int increment)
{
if(speed + increment > maxSpeed){
// throw exception
}else{
speed += increment;
}
}
public void speedDown(int decrement)
{
if(speed - decrement < 0){
// throw exception
}else{
speed -= decrement;
}
}
}
And HelloWorld.java:
public class HelloWorld {
/**
* @param args
*/
public static void main(String[] args) {
Vehicle v1 = new Vehicle();
Vehicle v2 = new Vehicle();
// do something
// print something useful, TODO
System.out.println(v1.getSpeed());
}
}
As you can see in the first class, I have added a comment ("// throw exception") where I would like to throw an exception. Do I have to define my own class for exceptions or is there some general exception class in Java I can use?
java exception
You could create your own Exception class:
public class InvalidSpeedException extends Exception {
public InvalidSpeedException(String message){
super(message);
}
}
In your code:
throw new InvalidSpeedException("TOO HIGH");
You could use IllegalArgumentException:
public void speedDown(int decrement)
{
if(speed - decrement < 0){
throw new IllegalArgumentException("Final speed can not be less than zero");
}else{
speed -= decrement;
}
}
Well there are lots of exceptions to throw but here is how you throw an exception:
throw new IllegalArgumentException("INVALID");
Also yes you can create your own custom exceptions.
Edit: Also note about exceptions. When you throw an exception (like above) and you catch the exception: the String that you supply in the exception can be accessed throw the getMessage() method.
try{
methodThatThrowsException();
}catch(IllegalArgumentException e)
{
e.getMessage();
}
It really depends on what you want to do with that exception after you catch it. If you need to differentiate your exception then you have to create your custom Exception. Otherwise you could just throw new Exception("message goes here");
The simplest way to do it would be something like:
throw new java.lang.Exception();
However the following lines would be unreachable in your code. So, we have two ways:
原文出處：How to throw a general exception in Java? - Stack Overflow
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