JUnit测试公共布尔等于（对象obj）

Question

I am working on JUnit testing a project I'm working on, and ran into a wall testing the following method:

public boolean equals(Object obj) {
  if (!(obj instanceof Vehicle)) {
     return false;
  }
  else {
     Vehicle other = (Vehicle) obj;
     return (owner + yearMakeModel + value).
        equals(other.owner + other.yearMakeModel
        + other.value);

I am using the following tests to test instnaceof, however, it does not seem to be testing the correct code. After submitting, This section of my code is highlighted and I am seeing a message that this method is not executed in my testing.

/** 
 * instanceof test false.
 */ 
 @Test public void instanceOfFalseTest() {
      Car car1 = new Car("Jones, Sam", "2017 Honda Accord", 222000, true);
      Object obj = new Object();
      Assert.assertFalse(obj instanceof Vehicle);
   } 
/** 
 * instanceof test true.
 */ 
@Test public void instanceOfTrueTest() {
      Car car1 = new Car("Jones, Sam", "2017 Honda Accord", 222000, true);
      Object obj = new Object();
      Assert.assertTrue(car1 instanceof(Vehicle);

Answer 1

There is no need to test Java's instanceof operator, because it is a built-in functionality of the JVM, not developed by you.

You should rather test your equals method. For example like this:

@Test public void equalsFalseTest1() {
    Car car1 = new Car("Jones, Sam", "2017 Honda Accord", 222000, true);
    Object obj = new Object();
    Assert.assertNotEquals(car1, obj);
}

@Test public void equalsFalseTest2() {
    Car car1 = new Car("Jones, Sam", "2017 Honda Accord", 222000, true);
    Car car2 = new Car("Doe, John", "2017 Honda Accord", 222000, true);
    Assert.assertNotEquals(car1, car2);
}

@Test public void equalsTrueTest() {
    Car car1 = new Car("Jones, Sam", "2017 Honda Accord", 222000, true);
    Car car2 = new Car("Jones, Sam", "2017 Honda Accord", 222000, true);
    Assert.assertEquals(car1, car2);
}

Then your equals method is actually called.

Answer 2

The other answer by Nikolas nicely explains why your current tests are pointless. A more reasonable test could look like this:

Car car1 = new Car("Jones, Sam", "2017 Honda Accord", 222000, true);
Car car2 = new Car("Jones, Sam", "2017 Honda Accord", 222000, true);
Car car3 = new Car("Jones, Samual", "2017 Honda Accord", 222000, true);

assertThat(car1, is(car2));
assertThat(car1, not(car3));

where is() refers to a Java Hamcrest matcher (they allow you to write down your test cases in very elegant ways).

You want to think up all the possible ways how two objects could be equal, nor not equal. And then, you (at least) one test case for each of those cases. Ideally, they each go into a separate, independent @Test method.

Answer 3

The Java's instanceof is part of the JVM instruction set, it's a specific same-named instruction. If interested, read the Chapter 6. The Java Virtual Machine Instruction Set: 6.5. Instructions .

Luckily, there is neither a way to @Override the implementation nor change the behavior - this implies this feature is untestable and there is no need to test if an object is an instance of another since the definition using extends and implements key-words.

Note that the method Object::equals is irrelevant to do with this operator/instruction.

Answer 4

The other answer:

 @Test public void instanceOfFalseTest() {
  Object obj = new Object();
  assertThat(obj, instanceOf(Vehicle.class));
} 

 @Test public void instanceOfTrueTest() {
  Car car1 = new Car("Jones, Sam", "2017 Honda Accord", 222000, true);
  Object obj = new Object();
  assertThat(car1, instanceOf(Vehicle.class));
}

, where instanceOf() check if an Object is an instance of a given class.

Click reference hamcrest instanceOf()