how to extend a abstract class in java

问题:

I have a made an engine class that is as follows. based on

Engine: Interface that extends Java’s Comparable (to compare among engines) and declares an integer getter method “getForce”, which indicatesthat engine subclasses will have a force they are capable of
producing.

public interface Engine extends Comparable<Engine>{

public int getForce();

}

I am trying to make an AbstractEngine class based on the following description.

AbstractEngine: Abstract class implemented by most engines. It subclasses from Engine and has an integer field indicating its force. This field is initialized through a constructor. The class overrides getForce from Engine and compareTo from Comparable. The comparison of two engines is done by finding the difference between their forces (in such a way that sorting a collection of engines arranges. I am confused on overriding the methods from Engine and making sure AbstractEngine has the compare to the Engine has.

This is what I have at the moment, but it's failing a JUnit test checking if AbstractEngine has the getForce, equals, and CompareTo. Is there a specific way I have to extend the methods?

abstract class AbstractEngine implements Engine {

public AbstractEngine(int force){

}
public int compareTo(Engine o) {
    // TODO Auto-generated method stub
    return 0;
}
}

here is the junit test

import static org.junit.Assert.*;

import org.junit.Test;

public class EngineTest {

@Test
public void test0_EngineImplementsComparableAndDefinesGetForce() {
    Engine engine = new Engine() {
        @Override
        public int compareTo(Engine o) {
            return 0;
        }
        @Override
        public int getForce() {
            return 0;
        }
    };
    assertTrue( "Incorrect result", engine instanceof Comparable );
}
@Test
public void test1_AbstractEngineIsAnEngine() {
    Engine engine = new AbstractEngine( 2 ) { };
    assertTrue( "Incorrect result", engine instanceof Engine );
}
@Test
public void test2_AbstractEngineHasGetForce() {
    Engine engine   = new AbstractEngine( 24 ) { };
    int    actual   = engine.getForce();
    int    expected = 24;
    assertEquals( "Incorrect result", expected, actual );
}
@Test
public void test3_AbstractEngineHasEquals() {
    Engine  a, b;
    boolean actual;
    // equal to itself
    a      = new AbstractEngine( 42 ) { };
    actual = a.equals( a );
    assertTrue ( "Incorrect result", actual );
    // equal to another engine with the same force
    a      = new AbstractEngine( 19 ) { };
    b      = new AbstractEngine( 19 ) { };
    actual = a.equals( b );
    assertTrue ( "Incorrect result", actual );
    // not equal to another engine with a different force 
    a      = new AbstractEngine( 22 ) { };
    b      = new AbstractEngine( 24 ) { };
    actual = a.equals( b );
    assertFalse( "Incorrect result", actual );
    // not equal to null
    actual = a.equals( null );
    assertFalse( "Incorrect result", actual );
    // not equal to some other object
    actual = a.equals( "22" );
    assertFalse( "Incorrect result", actual );
    // not equal to some other object
    actual = a.equals(  22  );
    assertFalse( "Incorrect result", actual );
}
@Test
public void test3_AbstractEngineHasCompareTo() {
    Engine a, b;
    int    actual;
    // equal to itself
    a      = new AbstractEngine( 42 ) { };
    actual = a.compareTo( a );
    assertTrue( "Incorrect result", actual == 0 );
    // equal to another engine with the same force
    a      = new AbstractEngine( 9000 ) { };
    b      = new AbstractEngine( 9000 ) { };
    actual = a.compareTo( b );
    assertTrue( "Incorrect result", actual == 0 );
    // goes before a more powerful engine 
    a      = new AbstractEngine( 23 ) { };
    b      = new AbstractEngine( 24 ) { };
    actual = a.compareTo( b );
    assertTrue( "Incorrect result", actual < 0 );
    // goes after a less powerful engine 
    actual = b.compareTo( a );
    assertTrue( "Incorrect result", actual > 0 );
}
@Test
public void test4_OxIsAnEngine() {
    Ox ox = new Ox( 3 );
    assertTrue( "Incorrect result", ox instanceof AbstractEngine );
}
@Test
public void test5_OxHasGetForce() {
    Engine engine   = new Ox( 4 );
    int    actual   = engine.getForce();
    int    expected = 4;
    assertEquals( "Incorrect result", expected, actual );
}
@Test
public void test5_OxHasEquals() {
    Engine  a, b;
    boolean actual;
    // equal to itself
    a      = new Ox( 42 );
    actual = a.equals( a );
    assertTrue ( "Incorrect result", actual );
    // equal to another engine with the same force
    a      = new Ox( 19 );
    b      = new Ox( 19 );
    actual = a.equals( b );
    assertTrue ( "Incorrect result", actual );
    // not equal to another engine with a different force 
    a      = new Ox( 22 );
    b      = new Ox( 24 );
    actual = a.equals( b );
    assertFalse( "Incorrect result", actual );
    // not equal to another engine of equal force 
    a      = new Ox            ( 21 );
    b      = new AbstractEngine( 21 ) { };
    actual = a.equals( b );
    assertFalse( "Incorrect result", actual );
    // not equal to null
    actual = a.equals( null );
    assertFalse( "Incorrect result", actual );
    // not equal to some other object
    actual = a.equals( "blah" );
    assertFalse( "Incorrect result", actual );
    // not equal to some other object
    actual = a.equals(  111  );
    assertFalse( "Incorrect result", actual );
}
}

回答1:

• In the constructor of AbstractEngine, you need to set the force member variable.
• It is not necessary to have a getForce() method in AbstractClass, if this method looks exactly the same as in class Engine.
• You have to implement the compareTo method as described in the assignment, either in the Engine class or in the AbstractEngine class.

Reading the description, shouldn't the force member variable be in the AbstractEngine class instead of the Engine class? (If you put it in AbstractEngine, then getForce() should also be in AbstractEngine instead of Engine).