Solution guide for TIJ4 for exercise 28 from chapter 17 says:
When a class extends a concrete Comparable class (as T3<A,B,C>
extends T2<A,B> in the original program), and adds a significant
field, there is no way to simultaneously implement compareTo()
correctly. (...) The same holds true for equals()"
I did this exercise using inheritance as it was used in "original
program" and it works as well (?). In exercise some needs to implement
equals() method and Comparable interface.
public class TwoTuple<A extends Comparable, B extends Comparable>
implements Comparable {
public final A first;
public final B second;
public TwoTuple(A a, B b) {
first = a;
second = b;
}
public String toString() {
return "(" + toString2() + ")";
}
protected String toString2() {
return first + ", " + second;
}
public boolean equals(Object o) {
return o != null &&
o.getClass() == getClass() &&
(first == null
? getClass().cast(o).first == null
: first.equals(getClass().cast(o).first)) &&
(second == null
? getClass().cast(o).second == null
: second.equals(getClass().cast(o).second));
}
public int hashCode() {
int result = 17;
if(first != null) {
result = result * 37 + first.hashCode();
}
return second == null ? result : result * 37 + second.hashCode();
}
public int compareTo(Object o) {
if(o.getClass() != getClass()) {
throw new ClassCastException();
}
int comparation = first.compareTo(getClass().cast(o).first);
return comparation != 0
? comparation
: second.compareTo(getClass().cast(o).second);
}
}
public class ThreeTuple<A extends Comparable, B extends Comparable, C extends Comparable>
extends TwoTuple<A, B> {
public final C third;
public ThreeTuple(A a, B b, C c) {
super(a, b);
third = c;
}
protected String toString2() {
return super.toString2() + ", " + third;
}
public boolean equals(Object o) {
return super.equals(o) &&
(third == null
? getClass().cast(o).third == null
: third.equals(getClass().cast(o).third));
}
#Override
public int hashCode() {
int result = super.hashCode();
return third == null ? result : result * 37 + third.hashCode();
}
#Override
public int compareTo(Object o) {
int comparation = super.compareTo(o);
return comparation != 0
? comparation
: third.compareTo(getClass().cast(o).third);
}
}
Is my solution correct or I missed something?
To make it working I needed to skip generics implementing Comparable interface, so not this way:
TwoTuple(...) implements Comparable<TwoTuple> {
(...)
int compareTo(TwoTuple o) {
but just:
TwoTuple(...) implements Comparable {
(...)
int compareTo(Object o) {
Maybe this is wrong statement which not obeying docs (?).
--
regards
Pawel
Related
This question already has answers here:
Implementing equals and hashCode for objects with circular references in Java
(4 answers)
Closed 3 years ago.
I'm trying to calculate hashcode of one class, but I got stackoverflow. How can I do this correctly? I genered it by IntelliJ idea, but still. Got stackoverflow, I know the reason (probably) but I really want to calculate proper hashcode..
public class Main {
public static void main(String[] args) {
TestA testA = new TestA();
TestB testB = new TestB();
testA.id = 1;
testA.name = "test";
testA.testB = testB;
testB.testA = testA;
testB.id = 1;
testB.name = "test";
System.out.println(testA.hashCode());
}
}
class TestB {
int id;
String name;
TestA testA;
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (!(o instanceof TestB)) return false;
TestB testB = (TestB) o;
if (id != testB.id) return false;
if (name != null ? !name.equals(testB.name) : testB.name != null) return false;
return testA != null ? testA.equals(testB.testA) : testB.testA == null;
}
#Override
public int hashCode() {
int result = id;
result = 31 * result + (name != null ? name.hashCode() : 0);
result = 31 * result + (testA != null ? testA.hashCode() : 0);
return result;
}
}
class TestA {
int id;
String name;
TestB testB;
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (!(o instanceof TestA)) return false;
TestA testA = (TestA) o;
if (id != testA.id) return false;
if (name != null ? !name.equals(testA.name) : testA.name != null) return false;
return testB != null ? testB.equals(testA.testB) : testA.testB == null;
}
#Override
public int hashCode() {
int result = id;
result = 31 * result + (name != null ? name.hashCode() : 0);
result = 31 * result + (testB != null ? testB.hashCode() : 0);
return result;
}
}
I included main function too. You can easily open this..
What you are looking for is a way to walk the object tree without entering into an infinite loop. This can be achieved by storing the visited objects in a thread-local Set and stopping when entering a hashcode while this is in that set.
And you can't just willy-nilly use a HashSet to store the 'visited' objects, because it internally calls your hashcode so the problem is just shifted elsewhere and you still get a stack overflow. Luckily there's a container that uses identity instead of equality, however it's the Map variant, not the Set. Ideally you want IdentityHashSet, but it doesn't exist, however the still useful IdentityHashMap exists. Just use the keys as the actual contents and use dummy values.
public class Main {
public static void main(String[] args) {
TestA testA = new TestA();
TestB testB = new TestB();
testA.id = 1;
testA.name = "test";
testA.testB = testB;
testB.testA = testA;
testB.id = 1;
testB.name = "test";
System.out.println(testA.hashCode());
}
}
class TestB {
int id;
String name;
TestA testA;
#Override
public boolean equals(Object o) {
if (this == o)
return true;
if (!(o instanceof TestB))
return false;
TestB testB = (TestB)o;
if (id != testB.id)
return false;
if (name != null ? !name.equals(testB.name) : testB.name != null)
return false;
return testA != null ? testA.equals(testB.testA) : testB.testA == null;
}
private static final ThreadLocal<Set<Object>> VISITED = ThreadLocal.withInitial(() -> new HashSet(10));
#Override
public int hashCode() {
Set<Object> visited = VISITED.get();
if (visited.contains(this))
return 0;
visited.add(this);
try {
int result = id;
result = 31 * result + (name != null ? name.hashCode() : 0);
result = 31 * result + (testA != null ? testA.hashCode() : 0);
return result;
} finally {
visited.remove(this);
}
}
}
class TestA {
int id;
String name;
TestB testB;
#Override
public boolean equals(Object o) {
if (this == o)
return true;
if (!(o instanceof TestA))
return false;
TestA testA = (TestA)o;
if (id != testA.id)
return false;
if (name != null ? !name.equals(testA.name) : testA.name != null)
return false;
return testB != null ? testB.equals(testA.testB) : testA.testB == null;
}
private static final ThreadLocal<Map<Object, Object>> VISITED =
ThreadLocal.withInitial(() -> new IdentityHashMap<>(10));
#Override
public int hashCode() {
Map<Object, Object> visited = VISITED.get();
if (visited.containsKey(this))
return 0;
visited.put(this, this);
try {
int result = id;
result = 31 * result + (name != null ? name.hashCode() : 0);
result = 31 * result + (testB != null ? testB.hashCode() : 0);
return result;
} finally {
visited.remove(this);
}
}
}
Note: The two VISITED variables can be a single variable, but since your classes don't have a common superclass (other than Object) I had to make two of them.
Caveat: When the tree contains multiple times the same instance of a class, the hashcode of that instance will be calculated multiple times. This is because everytime that instance is done visiting, it's removed from the list. This is because you don't want hard references to these instances to remain in the thread-local Map, preventing garbage collection.
I need to write abstract class, which looks like this.
public abstract class Value {
public abstract String toString();
public abstract Value add(Value v);
public abstract Value sub(Value v);
public abstract boolean eq(Value v);
public abstract boolean lte(Value v);
public abstract boolean gte(Value v);
public abstract boolean neq(Value v);
public abstract boolean equals(Object other);
public abstract int hashCode();
public abstract Value create(String s);
}
Now I need to make few classe, which inherit from that one. I started from Int class and implemented it like this:
public class Int extends Value {
int val;
public String toString() {
String toStr = Integer.toString(val);
return toStr;
}
public Int add(Value v) {
Int result = new Int();
if(v instanceof Int) {
Int temp = (Int) v;
result.val = val + temp.val;
}
return result;
}
public Int sub(Value v) {
Int result = new Int();
if(v instanceof Int) {
Int temp = (Int) v;
result.val = val - temp.val;
}
return result;
}
public boolean eq(Value o) {
if(this == o) return true;
if(this == null) return false;
if(getClass() != o.getClass()) return false;
Int other = (Int) o;
return toString() == other.toString();
}
public boolean lte(Value v) {
if(v instanceof Int) {
Int temp = (Int) v;
return this.val < temp.val;
}
return false;
}
public boolean gte(Value v) {
if(v instanceof Int) {
Int temp = (Int) v;
return this.val > temp.val;
}
return false;
}
public boolean neq(Value v) {
if(v instanceof Int) {
Int temp = (Int) v;
return !eq(temp);
}
return true;
}
public boolean equals(Object o) {
if(this == o) return true;
if(this == null) return false;
if(getClass() != o.getClass()) return false;
Int other = (Int) o;
return toString() == other.toString();
}
public int hashCode() {
Integer hash = val;
return hash.hashCode();
}
public Int create(String s) {
val = Integer.parseInt(s);
return this;
}
}
Everything is compiling and working, but I have no clue if my hashcode() function and equals() are good. Furthermore i want to use create() to make objects like this:
getInstance().create("1234");
Is my method also sufficient?
Everything is compiling and working, but I have no clue if my hashcode() function and equals() are good.
Your equals() should compare int val and not result of toString() of compared objects (this.val == other.val).
Your hashCode() looks good, though I would add #Override to it (same with equals()).
Furthermore i want to use create() to make objects like this: getInstance().create("1234");
Looking at its implementation, it looks fine (i.e. would work according to your needs):
public Int create(String s) {
val = Integer.parseInt(s);
return this;
}
though I don't think you really want to use it with getInstance(). Simply Int.create() would be enough:
public static Int create(String s) {
val = Integer.parseInt(s);
return new Int(val);
}
Note that you would need a private constructor.
Also, as someone noted in the comments, consider using generics instead of inheritance.
The hashCode() method is fine (although I'd add an #Override annotation, just to make the code easier to maintain and avoid mistakes), but the equals(Object) definitely isn't.
Following the logic you have in place, == isn't the right way to compare strings. You should use equals instead (see, e.g., How do I compare strings in Java?). In addition, as Joakim Danielson noted in the comments, this can never be null - you should check if o is null instead:
public boolean equals(Object o) {
if (this == o) {
return true;
}
if (o == null) {
return false;
}
if(getClass() != o.getClass()) {
return false;
}
Int other = (Int) o;
return toString().equals(other.toString()); // Here!
}
But in all fairness, there's no reason to use toString - you could just compare the internal val:
public boolean equals(Object o) {
if (this == o) {
return true;
}
if (o == null) {
return false;
}
if(getClass() != o.getClass()) {
return false;
}
Int other = (Int) o;
return val == other.val; // Here!
}
First when you override Methods please do it with #Override Annotation. Then i would implement your equals method in another way. Just do return this.val == other.val instead of doing this.toString() == other.toString(). Your toString() method implementation is ok. Your hashCode is good as well. But please remove that create method. Use a constructor instead.
Can I implement equals() method using eq() like this ?
public boolean equals(Object o) {
Value compare = (Value) o;
return eq(compare);
}
I have the problem, that my equals method doesnt work as i want it to. I want to implement a deterministic turing machine, so I want to add the method findCommand(), which searchs through a arraylist of commands. So I decided to create a searchDummy to find all Transitions that are available for the Configuration I have.
Class States:
public class States {
private int stateId;
private boolean rejState;
private boolean accState;
private boolean stopState;
private List<Commands> commands = new ArrayList<Commands>();
equals in class States:
#Override
public boolean equals(Object other) {
if (this == other) {
return true;
} else if (other instanceof States) {
States otherState = (States) other;
return (stateId == otherState.stateId);
} else {
return false;
}
}
hashCode:
#Override public int hashCode() {
StringBuilder b = new StringBuilder(stateId);
return b.toString().hashCode();
}
this is the findCommand method in States:
public Commands findCommand(States state, char inputTapeChar,
char[] tapeChars) {
Commands searchDummy = new Commands(state, inputTapeChar, tapeChars,
null, null, null, null);
int pos = commands.indexOf(searchDummy);
return pos >= 0 ? commands.get(pos) : null;
}
commands is my arraylist, so I want to find the searchDummy with indexOf().
I have the class Commands, which holds the attribute Configuration configuration, the class Configuration, which holds the attributes of a Configuration and the attribute Transition transition and the class transition that holds the attributes for itself.
Class Commands:
public class Commands implements Comparable<Commands> {
private Configuration configuration;
Class Configuration:
public class Configuration {
private Transition transition;
private States state;
private char inputTapeChar;
private char[] tapeChars;
Class Transition:
public class Transition {
private States targetState;
private Direction inputTapeHeadMove;
private char[] newTapeChars;
private Direction[] tapeHeadMoves;
i have this equals method in Commands:
#Override public boolean equals(Object other) {
if (this == other) {
return true;
} else if (other instanceof Commands) {
Commands otherCmd = (Commands) other;
return (configuration.equals(otherCmd.configuration));
} else {
return false;
}
}
and this hashcode
#Override
public int hashCode() {
StringBuilder b = new StringBuilder(configuration.getState() + ","
+ configuration.getInputTapeChar());
for (char c : configuration.getTapeChars()) {
b.append("," + c);
}
return b.toString().hashCode();
}
then almost the same in Configuration:
#Override
public boolean equals(Object other) {
if (this == other) {
return true;
} else if (other instanceof Configuration) {
Configuration otherConfi = (Configuration) other;
return (state.equals(otherConfi.state))
&& (inputTapeChar == otherConfi.inputTapeChar)
&& (Arrays.equals(tapeChars, otherConfi.tapeChars));
} else {
return false;
}
}
hashcode:
#Override
public int hashCode() {
StringBuilder b = new StringBuilder(state + "," + inputTapeChar);
for (char c : tapeChars) {
b.append("," + c);
}
return b.toString().hashCode();
}
equales in class State:
#Override
public boolean equals(Object other) {
if (this == other) {
return true;
} else if (other instanceof States) {
States otherState = (States) other;
return (stateId == otherState.stateId);
} else {
return false;
}
}
so my question:
when I debug this it goes through until it's finished with the checks but when it should return the value it stucks at Configuration.equals(...) and shows the error no source found!
what is the problem? Are the hashcodes wrong? Or are the equals wrong?
I never used equals before so I dont know when i need to use it or how i need to fix this. thanks for your help.
Your hashCode implementation looks suspect - all that String stuff is not standard.
For example for your Transition class should be something like this:
#Override
public int hashCode() {
int result = 17;
result = 31 * result + targetState.hashCode();
result = 31 * result + inputTapeHeadMove.hashCode();
result = 31 * result + newTapeChars.hashCode();
result = 31 * tapeHeadMoves.hashCode();
return result;
}
Most IDEs will offer autogen of hashCode and equals methods.
I have 2 custom Java classes;
private MyCustomClass1 obj1;
private MyCustomClass2 obj2;
Each of them has multiple attributes as below;
MyCustomClass1 {
attr1,
attr2,
commonattrId,
attr3
}
MyCustomClass2 {
attr4,
attr5,
commonattrId,
attr6
}
So as you can see, there is a common attribute in each of them (commonattrId) which just to add is a Long
There is also a composite class defined as below;
MyCompositeClass {
MyCustomClass1 obj1;
MyCustomClass2 obj2;
}
Now one of my query execution returns below list;
List myList1
and there is another query execution which returns me below list;
List myList2
My question is can I combine the above 2 lists given I have a commonattrId ?
slightly long but the idea is to override equals in MyClass1 and MyClass2:
public static void main(String[] args) throws IOException {
List<MyClass1> myClass1s = new ArrayList<MyClass1>();
myClass1s.add(new MyClass1(1, 1));
myClass1s.add(new MyClass1(2, 2));
List<MyClass2> myClass2s = new ArrayList<MyClass2>();
myClass2s.add(new MyClass2(3, 1));
myClass2s.add(new MyClass2(4, 2));
List<MyComposite> allMyClasses = new ArrayList<MyComposite>();
for(MyClass1 m : myClass1s) { // note: you should take the shorte of the two lists
int index = myClass2s.indexOf(m);
if(index != -1) {
allMyClasses.add(new MyComposite(m, myClass2s.get(index)));
}
}
System.out.println(allMyClasses);
}
static class MyClass1 {
int attr1;
long commonAttrId;
public MyClass1(int attr, long commonAttr) {
this.attr1 = attr;
this.commonAttrId = commonAttr;
}
#Override
public int hashCode() {
int hash = 5;
hash = 83 * hash + (int) (this.commonAttrId ^ (this.commonAttrId >>> 32));
return hash;
}
#Override
public boolean equals(Object obj) {
if (obj == null) {
return false;
}
if(obj instanceof MyClass2) {
return this.commonAttrId == ((MyClass2)obj).commonAttrId;
}
if(obj instanceof MyClass1) {
return this.commonAttrId == ((MyClass1)obj).commonAttrId;
}
return false;
}
#Override
public String toString() {
return "attr1=" + attr1 + ", commonAttrId=" + commonAttrId;
}
}
static class MyClass2 {
int attr2;
long commonAttrId;
public MyClass2(int attr, long commonAttr) {
this.attr2 = attr;
this.commonAttrId = commonAttr;
}
#Override
public int hashCode() {
int hash = 5;
hash = 83 * hash + (int) (this.commonAttrId ^ (this.commonAttrId >>> 32));
return hash;
}
#Override
public boolean equals(Object obj) {
if (obj == null) {
return false;
}
if(obj instanceof MyClass1) {
return this.commonAttrId == ((MyClass1)obj).commonAttrId;
}
if(obj instanceof MyClass2) {
return this.commonAttrId == ((MyClass2)obj).commonAttrId;
}
return false;
}
#Override
public String toString() {
return "attr2=" + attr2 + ", commonAttrId=" + commonAttrId;
}
}
static class MyComposite {
MyClass1 myClass1;
MyClass2 myClass2;
public MyComposite(MyClass1 a, MyClass2 b) {
myClass1 = a;
myClass2 = b;
}
#Override
public String toString() {
return "myClass1=" + myClass1 + ", myClass2=" + myClass2;
}
}
I don't know all the parameters of your problem but there are probably better ways to do this. For example: have both MyClass1 and MyClass2 inherit from a common class (i.e. MyBaseClass) and create a collection of that instead of the composite class MyCompositeClass.
Or instead of Lists you could have sets and create a set intersection.
You could create a map from id to the object for one of the lists and then iterate through the other to create the new List using the data from the map.
List<MyCompositeClass> combine(List<MyCustomClass1> myList1, List<MyCustomClass2> myList2) {
// create map
Map<Long, MyCustomClass1> idToObj = new HashMap<>();
for (MyCustomClass1 o : myList1) {
idToObj.put(o.commonattrId, o);
}
// construct result list
List<MyCompositeClass> result = new ArrayList<>();
for (MyCustomClass2 o : myList2) {
MyCustomClass1 o1 = map.get(o.commonattrId);
if (o1 != null) {
MyCompositeClass combined = new MyCompositeClass();
combined.obj1 = o1;
combined.obj2 = o;
result.add(combined);
}
}
return result;
}
This will only add all possible combinations of objects from both lists, if commonattrId values are pairwise distinct in each list, but since the field name has "Id" as suffix, I made an educated guess...
I'd like my EqualTester generic class to call the overridden equals(...) method of its generic parameter, but it seems to call Object.equals instead. Here is my test code:
import junit.framework.TestCase;
public class EqualityInsideGenerics extends TestCase {
public static class EqualTester<V> {
public boolean check(V v1, V v2) {
return v1.equals(v2);
}
}
public static class K {
private int i;
private Object o;
public K(Object o, int i) {
this.o = o;
this.i = i;
}
public boolean equals(K k) {
return ((k.o != null && k.o.equals(o)) || o == null) && (k.i == i);
}
};
public void testEqual() {
K k1 = new K(null, 0);
K k2 = new K(null, 0);
assertTrue(k1.equals(k2)); // This one ok
EqualTester<K> tester = new EqualTester<K>();
assertTrue(tester.check(k1, k2)); // This one KO!
}
}
Could you please explain why this does not work, and how I could change my EqualTester class?
Is it because K does not actually override the Object.equals() method (because the parameter does not have the correct type)?
Thanks.
You need to code as public boolean equals(Object k), and then cast to k.
Right now you are just overloading the equals method.
It's also useful to add #Override annotation to the method.
When overriding the method signature must match exactly.
Because equals(K k) does not actually override the equals(Object o) method.
You must override equals(Object o) exactly in order for it to work.
Thanks Padmarag and Phill!
A solution that works:
#Override
public boolean equals(Object obj) {
if (!(obj instanceof K)) {
return false;
}
K k = (K)obj;
return ((k.o != null && k.o.equals(o)) || o == null) && (k.i == i);
}
Comments welcome: I started programming in Java only a few days ago...