What does <init> signify in a Java exception?

Question

What does <init> signify in a Java exception?

For example:

BlahBlahException...

at java.io.FileInputStream.<init>(FileInputStream.java:20)

Solution 1

That the exception is thrown in the construction of the object, there are two options:

• in the constructor
• while initializing variables

Check out this demo I wrote: http://ideone.com/Mm5w5

---

class Main
{
        public static void main (String[] args) throws java.lang.Exception
        {
                try
                { new Test(); } catch (Exception e) { e.printStackTrace(); }

                try
                { new Test2(); } catch (Exception e) { e.printStackTrace(); }

                try
                { new Test3(); } catch (Exception e) { e.printStackTrace(); }


        }

        static class Test
        {
                Object obj = getObject();
                Object getObject()
                { throw new RuntimeException("getObject"); }
        }

        static class Test2
        {
                Test2()
                {
                        throw new RuntimeException("constructor");
                }
        }

        static class Test3
        {
                Object obj1 = null;
                String str = obj1.toString();
        }
}

Produces:

java.lang.RuntimeException: getObject
    at Main$Test.getObject(Main.java:24)
    at Main$Test.<init>(Main.java:22)
    at Main.main(Main.java:9)
java.lang.RuntimeException: constructor
    at Main$Test2.<init>(Main.java:31)
    at Main.main(Main.java:12)
java.lang.NullPointerException
    at Main$Test3.<init>(Main.java:38)
    at Main.main(Main.java:15)

Solution 2

<init>

is called

Instance Initialization method

which is created by your java compiler from the constructor you have defined. Though it is not valid method definition, your JVM expects this and anything that you put in the constructor will be executed in method. So when you an exception with from , you can be sure that it is from the constructor of the executed java class. Read more about this on Bill venner's design technique articles on Object Initialization.

Comments

[Jon Skeet]

Quick note - your tag of "compiler-errors" suggests you may not be drawing an appropriate distinction between compile-time errors and execution-time exceptions. This isn't a compiler-error, it's an exception. (I'll edit the tag now, but it's something to be aware of.)