Demonstrate using EasyMock to mock a method argument modification.

Example.java

import org.easymock.EasyMock;
import org.easymock.IAnswer;
import org.junit.Assert;
import org.junit.Test;

/*
 * Demonstrate using EasyMock to mock a method argument modification.
 */

// Dependency to mock...
class Service {
	public boolean update(StringBuilder builder) {
		// Challenge: Need to mock the mutation performed on the method argument.
		builder.append("the change");
		return true;
	}
}

// Class under test...
class Example {
	private Service service;
	
	public void setService(Service service) {
		this.service = service;
	}

	// Method under test
	public String doSomething() {
		// Uses a service to perform work but instead of just returning a result it modifies a method argument.
		StringBuilder builder = new StringBuilder();
		service.update(builder);
		return builder.toString();
	}
}


// JUnit verifying behavior of class 'Example'...
public class ExampleTest {

	@Test
	public void testChangeToMethodArgument() throws Exception {
		final String expectedData = "the change";
		
		Service service = EasyMock.createNiceMock(Service.class);
		EasyMock.expect(service.update(EasyMock.anyObject(StringBuilder.class))).andAnswer(new IAnswer<Boolean>() {
		    public Boolean answer() throws Throwable {
		    	// Retrieve the argument to to Service.update()
		    	StringBuilder buffer = (StringBuilder) EasyMock.getCurrentArguments()[0];
		    	
		    	// Mock change that would be applied by the method
		    	buffer.append(expectedData);
		    	
			return true;
		    }
		});

		EasyMock.replay(service);
		Example example = new Example();
		example.setService(service);
		Assert.assertEquals(expectedData, example.doSomething());
	}
}

great example...