Forcing null pointer exception

Question

I am practicing JUnit testing and I am not sure what to do when NullPointerExceptions occurs.

I am currently creating my own indexOf() method and I am comparing it with the original indexOf(). I am not sure how to create a similar output from my custom indexOf() to have the same output for when the original indexOf() has a NullPointerException.

I thought about simply giving the output of the error that NullPointerException gives me but i am not sure how to do that as well

Here is my code:

public static int myIndexOf(char[] str, int ch, int index) {
    if (str == null) {
        // output NullPointerException
    }
    if (str.length <= index || index < 0) {
        return -1;
    }
    for (int i = index; i < str.length; i++) {
        if (index == str[i]) {
            return i;
        }
    }
    // if not found
    return -1;
}

Correct your code so that it doesn't throw a NullPointerException. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Mar 24, 2014 at 3:48
@Christian hmm maybe I should have reworded the question a bit better. When ex: str.indexOf(3) and str is null — Liondancer
– Liondancer, Commented Mar 24, 2014 at 3:50
@Liondancer: Java will always throw an exception in that scenario. You can't even modify this. Its "hardcoded" in every Java virtual machine. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Mar 24, 2014 at 3:53

willeM_ Van Onsem · Accepted Answer · 2014-03-24 04:01:20Z

2

indexOf() never throws a NullPointerException, except for var.indexOf with var == null. And that type of exception is handled automatically by Java.

Your code is however confusing. In most cases indexOf is an instance-method. In case of a static method it is of course possible to circumvent the problem. For instance:

public static void Foo (Bar b) {
    if(b != null) {
        //do something
    }
}

This is called a total method: in case b is null, nothing happens.

However in most cases you write instance methods. When such method is called and the instance is null, the Java Virtual Machine will throw a NullPointerException itself.

Furthermore in the static method you provide, you call str.length, since str can be null, the Java virtual machine will check this and throw a NullPointerException out of the method itself as well.

You can however test if an exception is thrown in JUnit using the following pattern:

@Test(expected = ExpectedException.class)
public void test () throws ExpectedException {
    Foo f = new Foo();
    f.methodThatThrowsException();
}

JUnit will consider this test to have failed if it does not throw an exception of type ExpectedException.

edited Mar 24, 2014 at 4:01

answered Mar 24, 2014 at 3:49

willeM_ Van Onsem

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5 Comments

Liondancer Over a year ago

This is what I meant sorry for the confusion

willeM_ Van Onsem Over a year ago

@Liondancer: When a null-instance calls a method, the Java runtime environment will throw a NullPointerException itself. You don't have to do anything for this. That makes NullPointerExceptions one of the most annoying things.

Liondancer Over a year ago

I appreciate this answer and I seem ot understand what youre saying. I have one last question. I understand that str.length will throw a NullPointerException when str is null. But I was wondering if you knew why I get a false when var.indexOf throws a NullPointerExcpetion when var is null (original) and when str.length is called (cusomt)?

willeM_ Van Onsem Over a year ago

var.indexOf will throw a NullPointerException as well. Can you provide some code (in your question) to illustrate the problem?

Liondancer Over a year ago

If you dont mind. Could you take a look at this? I thought because both methods output a NullPointerException the test case will return true. But I am assuming it will not due to the error output. pastebin.com/tFiKZWhn

Obicere · Accepted Answer · 2014-03-24 04:17:22Z

1

If you are using Java 8 you can use a fancy little method to handle this.

public static int myIndexOf(char[] str, int ch, int index) {
    Objects.requireNonNull(str);
    if (str.length <= index || index < 0) {

This method will throw a NullPointerException if the given argument is null. You can also use it like so:

Objects.requireNonNull(str, "The given string cannot be null");

To provide your own error message.

answered Mar 24, 2014 at 4:17

Obicere

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Comments

djechlin · Accepted Answer · 2014-03-24 04:20:20Z

1

throw new NullPointerException();

will do it. More "organically":

Object c = null; c.toString();

will do it.

But note that indexOf does not throw NPEs.

answered Mar 24, 2014 at 4:20

djechlin

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