Immutability test failing

import java.util.ArrayList;
import java.util.List;

public final class Student {
    private final int id;
    private final String name;
    private final List<String> hobbies;

    Student(int id, String name, List<String> hobbies) {
        this.id = id;
        this.name = name;
        this.hobbies = hobbies;
    }

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public List<String> getHobbies() {
        return new ArrayList<> (hobbies);
    }

}

When I tried to test for immutability using “assertImmutable(Student.class);”, I am getting following error.

Attempt to wrap mutable collection using a non-whitelisted unmodifiable wrapper method. [Field: hobbies…]

[Grundlefleck]

Hi,

While Stephan is correct that wrapping in unmodifiable in the getter makes the class immutable, unfortunately Mutability Detector does not realise that. It’s analysis is a bit too naïve.

I suggest copying and wrapping in the constructor:

this.hobbies = Collections.unmodifiableList(new ArrayList<>(hobbies));

That should make the test pass and have other benefits:

• only one instance of the unmodifiable wrapper needs to be constructed even if the getter is called many times
• it’s harder to accidentally mutate the collection within this class

Let us know how that works for you.

Cheers!

P.S. really satisfying to see other people helping out on issues, thanks Stephan. On 15 May 2016 8:13 pm, “harikrishna553” [e-mail] wrote:

Stephen, I got it what you are saying. I am still getting same error when testing ‘assertImmutable(Student.class)’ the Student class.

org.mutabilitydetector.unittesting.MutabilityAssertionError: Expected: mutability.Student to be IMMUTABLE but: mutability.Student is actually NOT_IMMUTABLE Reasons: Field can have a mutable type (java.util.ArrayList) assigned to it. [Field: hobbies, Class: mutability.Student] Allowed reasons: None. at org.mutabilitydetector.unittesting.internal.AssertionReporter.assertThat(AssertionReporter.java:48) at org.mutabilitydetector.unittesting.MutabilityAsserter.assertImmutable(MutabilityAsserter.java:108) at org.mutabilitydetector.unittesting.MutabilityAssert.assertImmutable(MutabilityAssert.java:672) at mutability.TestMutability.checkMyClassIsImmutable(TestMutability.java:11) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:497) at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50) at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12) at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47) at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17) at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325) at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78) at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57) at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290) at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71) at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288) at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58) at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268) at org.junit.runners.ParentRunner.run(ParentRunner.java:363) at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50) at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:675) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)

Following i my Test class.

import static org.mutabilitydetector.unittesting.MutabilityAssert.assertImmutable;

import org.junit.Test;

public class TestMutability {

@Test
public void checkMyClassIsImmutable() {
    assertImmutable(Student.class);
}

}

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[Stephan202]

@Grundlefleck, good suggestion on moving everything in the constructor; I almost feel dumb for not suggesting it myself. I guess it’s because in my own code I always use Guava’s immutable collections, so I haven’t recently used the “JDK-only” approach. Anyway, happy to help!