[FIXED] Set null to object but no NPE

Issue

Java 8

public class OidDTO {
    private String value;

    public OidDTO(String value) {
        this.value = value;
    }
}

Here Junit test:

  @Test
    public void change_object_by_ref() {
        OidDTO oidDTO = new OidDTO("init_value");
        changeOidDTO(oidDTO);

        String expected = "new_value";
        String actual = oidDTO.getValue();
        
        assertEquals(expected, actual);
    }

    private void changeOidDTO(OidDTO oidDTO) {
        oidDTO.setValue("new_value");
    }

As result the test is pass because expected and actual are equals. It's equals because in the method changeOidDTO I change value.

Nice.

Now another test:

 @Test
    public void change_object_by_ref() {
        OidDTO oidDTO = new OidDTO("init_value");
        changeOidDTO(oidDTO);

        String expected = "new_value";
        String actual = oidDTO.getValue(); // Why no NPE

        assertEquals(expected, actual);
    }

    private void changeOidDTO(OidDTO oidDTO) {
        oidDTO = null;
    }

And test is fail.

expected: <new_value> but was: <init_value>
Expected :new_value
Actual   :init_value

As you can see I set null in this line:

oidDTO = null;

The question is: Why not throw NPE in the next line:

String actual = oidDTO.getValue()

?

Solution

You asked why no NPE on this line

    String actual = oidDTO.getValue(); // Why no NPE

Because oidDTO is not null.

    OidDTO oidDTO = new OidDTO("init_value");

This line

changeOidDTO(oidDTO);

Changes the LOCAL variable oidDTO, and not the original one being passed to the function.

Answered By - hfontanez