Why does newInstance() return null?

Question

According to the JavaDocs, the newInstance() Method of java.lang.Class is not intented to return null. But my code seems to prove the opposite. Why?

public Assessment createAssessment() {
    Class<? extends Assessment> assessmentClass = (Class<? extends Assessment>) assessmentClassDataTable.getRowData();
    try {
        System.out.println("ASSESSMENTCLASS " + assessmentClass);
        // -> 'ASSESSMENTCLASS class my.model.ManualSelectAssessment'
        Assessment a = assessmentClass.newInstance();
        System.out.println("ASSESSMENT " + a);
        // -> 'ASSESSMENT null'
        return a;
    } catch (Exception e) {
        Application.handleError(e);
    }
    return null;
}

It returns null.

Can the `toString` method of `ManualSelectAssessment` return `"null"`? — Mike Samuel, Sep 03 '13 at 14:28
Presumably you are sure it's not throwing an exception, being caught and then returning null? — Dave Richardson, Sep 03 '13 at 14:28
You're printing the results of `a.toString()`, which is not "returning" `null`. See whether `a == null`. — chrylis -cautiouslyoptimistic-, Sep 03 '13 at 14:29
@Mike Good though, but no. I can be sure that the return value is null. — Zeemee, Sep 03 '13 at 14:30
@DaveRlz Yes, I'm sure because `System.out.println("ASSESSMENT " + a);` is executed. — Zeemee, Sep 03 '13 at 14:32
there is something in your tostring method that doesn't have value, when you instantiate with the default constructor. — Felquir, Sep 03 '13 at 14:33
@All - you were all right. `toString` returns null. Shame on me. I will delete this useless question. — Zeemee, Sep 03 '13 at 14:41

Peter Lawrey · Accepted Answer · 2013-09-03T14:35:17.107

7

newInstance() never returns null. However newInstance().toString() can return "null"

Note: One gotcha with newInstance() is that it can throw a CheckedException !

public Main() throws IOException {
    throw new IOException();
}

public static void main(String[] args) throws InstantiationException, IllegalAccessException {
    Main.class.newInstance(); // throws IOException silently.
}

Even though IOException is a check exception, the compiler has not idea the newInstance() will throw this checked exception. If you try to catch it the compiler will complain it cannot be thrown !!

edited Sep 03 '13 at 14:35

answered Sep 03 '13 at 14:30

Peter Lawrey

525,659
79
751
1,130

1

Wouldn't the `IOException` be wrapped in the `InstantiationException`? – Paul Bellora Sep 03 '13 at 15:23
InstantiationException is when `the class object represents an abstract class, an interface, an array class, a primitive type, or void OR the class has no nullary constructor` There is an `ExecutionException` for this when calling `Constructor.newInstance()` – Peter Lawrey Sep 03 '13 at 15:24
Interesting, I always though it functioned like `InvocationTargetException`. That's definitely a gotcha. – Paul Bellora Sep 03 '13 at 15:28
@PaulBellora One of four ways to throw a CheckedException without the compiler knowing (without JNI) :P – Peter Lawrey Sep 03 '13 at 15:34
1

I'll have to update [an answer of mine](http://stackoverflow.com/a/11943139/697449) about that, thanks for teaching me something. – Paul Bellora Sep 03 '13 at 15:39
@PaulBellora A puzzle for you, can you guess another three ;) – Peter Lawrey Sep 03 '13 at 15:40

Why does newInstance() return null?

1 Answers1

Linked