JUnit Test cases Using Mockito: null pointer exception

Question

I am writing a JUnit test cases using Mockito. I have populated a list using reflection with mock objects of IIOImage class of java. Though my list has objects, my test case is failing and I am getting a null pointer exception. Can anyone help?

_sourceList=(List) Whitebox.getInternalState(OneClass, "_sourceListContent");

_sourceList.add(Mockito.mock(IIOImage.class)); 
_sourceList.add(Mockito.mock(IIOImage.class));
Field listfield=OneClass.class.getDeclaredField("_sourceListContent");
listfield.setAccessible(true);
listfield.set(oneclass, _sourceImages);
System.out.println(_sourceList.size()+"is size");

for(int i=0;i<=_sourceImages.size(); i++) //this loop is giving null
{   
      IIOImage img = (IIOImage)_sourceImages.get(i);
      SomeClass data=Mockito.mock(SomeClass.class);
      data=(SomeClass)img.getMetadata();
      data.dumpData();
}

It returns the size here, but causes problem when I run the test case saying null pointer.

here's the stack trace

java.lang.NullPointerException
at SomeClass.SomeClassTest.dumpRequestTest(IPRequestTest.java:285)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:44)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:41)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
at org.junit.runners.BlockJUnit4ClassRunner.runNotIgnored(BlockJUnit4ClassRunner.java:79)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:71)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:49)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184)
at org.junit.runners.ParentRunner.run(ParentRunner.java:236)
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)

Answer 1

You are creating instances as mocks:

_sourceList.add(Mockito.mock(IIOImage.class)); 

Later on you retireive them one by one and call this method on each:

IIOImage img = (IIOImage)_sourceImages.get(i);
data=(SomeClass)img.getMetadata();

You have not declared what Mockito should return when this method is called (using the when(..).thenReturn(..) syntax), thus Mockito returns a default null value.

When you then call the:

data.dumpData();

You get a NullPointerException as data is null.

Fix

• Declare what should be return by when(img.getMetadata()).thenReturn(..).
• Use Mockito.spy instead of mock.