Develop Reference

Boolean value differs with calling println for method call and calling print ln for variable containing boolean return value of method call [duplicate]

I ran into an interesting (and very frustrating) issue with the equals() method today which caused what I thought to be a well tested class to crash and cause a bug that took me a very long time to track down.
Just for completeness, I wasn't using an IDE or debugger - just good old fashioned text editor and System.out's. Time was very limited and it was a school project.
Anyhow -
I was developing a basic shopping cart which could contain an ArrayList of Book objects. In order to implement the addBook(), removeBook(), and hasBook() methods of the Cart, I wanted to check if the Book already existed in the Cart. So off I go -
public boolean equals(Book b) {
... // More code here - null checks
if (b.getID() == this.getID()) return true;
else return false;
}
All works fine in testing. I create 6 objects and fill them with data. Do many adds, removes, has() operations on the Cart and everything works fine. I read that you can either have equals(TYPE var) or equals(Object o) { (CAST) var } but assumed that since it was working, it didn't matter too much.
Then I ran into a problem - I needed to create a Book object with only the ID in it from within the Book class. No other data would be entered into it. Basically the following:
public boolean hasBook(int i) {
Book b = new Book(i);
return hasBook(b);
}
public boolean hasBook(Book b) {
// .. more code here
return this.books.contains(b);
}
All of a sudden, the equals(Book b) method no longer works. This took a VERY long time to track down without a good debugger and assuming the Cart class was properly tested and correct. After swaapping the equals() method to the following:
public boolean equals(Object o) {
Book b = (Book) o;
... // The rest goes here
}
Everything began to work again. Is there a reason the method decided not to take the Book parameter even though it clearly was a Book object? The only difference seemed to be it was instantiated from within the same class, and only filled with one data member. I'm very very confused. Please, shed some light?

In Java, the equals() method that is inherited from Object is:
public boolean equals(Object other);
In other words, the parameter must be of type Object. This is called overriding; your method public boolean equals(Book other) does what is called overloading to the equals() method.
The ArrayList uses overridden equals() methods to compare contents (e.g. for its contains() and equals() methods), not overloaded ones. In most of your code, calling the one that didn't properly override Object's equals was fine, but not compatible with ArrayList.
So, not overriding the method correctly can cause problems.
I override equals the following everytime:
#Override
public boolean equals(Object other){
if (other == null) return false;
if (other == this) return true;
if (!(other instanceof MyClass)) return false;
MyClass otherMyClass = (MyClass)other;
...test other properties here...
}
The use of the #Override annotation can help a ton with silly mistakes.
Use it whenever you think you are overriding a super class' or interface's method. That way, if you do it the wrong way, you will get a compile error.

If you use eclipse just go to the top menu
Source --> Generate equals() and
hashCode()

Slightly off-topic to your question, but it's probably worth mentioning anyway:
Commons Lang has got some excellent methods you can use in overriding equals and hashcode. Check out EqualsBuilder.reflectionEquals(...) and HashCodeBuilder.reflectionHashCode(...). Saved me plenty of headache in the past - although of course if you just want to do "equals" on ID it may not fit your circumstances.
I also agree that you should use the #Override annotation whenever you're overriding equals (or any other method).

Another fast solution that saves boilerplate code is Lombok EqualsAndHashCode annotation. It is easy, elegant and customizable. And does not depends on the IDE. For example;
import lombok.EqualsAndHashCode;
#EqualsAndHashCode(of={"errorNumber","messageCode"}) // Will only use this fields to generate equals.
public class ErrorMessage{
private long errorNumber;
private int numberOfParameters;
private Level loggingLevel;
private String messageCode;
See the options avaliable to customize which fields to use in the equals. Lombok is avalaible in maven. Just add it with provided scope:
<dependency>
<groupId>org.projectlombok</groupId>
<artifactId>lombok</artifactId>
<version>1.14.8</version>
<scope>provided</scope>
</dependency>

in Android Studio is
alt + insert ---> equals and hashCode
Example:
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Proveedor proveedor = (Proveedor) o;
return getId() == proveedor.getId();
}
#Override
public int hashCode() {
return getId();
}

Consider:
Object obj = new Book();
obj.equals("hi");
// Oh noes! What happens now? Can't call it with a String that isn't a Book...

the instanceOf statement is often used in implementation of equals.
This is a popular pitfall !
The problem is that using instanceOf violates the rule of symmetry:
(object1.equals(object2) == true) if and only if (object2.equals(object1))
if the first equals is true, and object2 is an instance of a subclass of
the class where obj1 belongs to, then the second equals will return false!
if the regarded class where ob1 belongs to is declared as final, then this
problem can not arise, but in general, you should test as follows:
this.getClass() != otherObject.getClass(); if not, return false, otherwise test
the fields to compare for equality!

recordId is property of the object
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Nai_record other = (Nai_record) obj;
if (recordId == null) {
if (other.recordId != null)
return false;
} else if (!recordId.equals(other.recordId))
return false;
return true;
}

Deep comparison for HashSet in Java [duplicate]

I ran into an interesting (and very frustrating) issue with the equals() method today which caused what I thought to be a well tested class to crash and cause a bug that took me a very long time to track down.
Just for completeness, I wasn't using an IDE or debugger - just good old fashioned text editor and System.out's. Time was very limited and it was a school project.
Anyhow -
I was developing a basic shopping cart which could contain an ArrayList of Book objects. In order to implement the addBook(), removeBook(), and hasBook() methods of the Cart, I wanted to check if the Book already existed in the Cart. So off I go -
public boolean equals(Book b) {
... // More code here - null checks
if (b.getID() == this.getID()) return true;
else return false;
}
All works fine in testing. I create 6 objects and fill them with data. Do many adds, removes, has() operations on the Cart and everything works fine. I read that you can either have equals(TYPE var) or equals(Object o) { (CAST) var } but assumed that since it was working, it didn't matter too much.
Then I ran into a problem - I needed to create a Book object with only the ID in it from within the Book class. No other data would be entered into it. Basically the following:
public boolean hasBook(int i) {
Book b = new Book(i);
return hasBook(b);
}
public boolean hasBook(Book b) {
// .. more code here
return this.books.contains(b);
}
All of a sudden, the equals(Book b) method no longer works. This took a VERY long time to track down without a good debugger and assuming the Cart class was properly tested and correct. After swaapping the equals() method to the following:
public boolean equals(Object o) {
Book b = (Book) o;
... // The rest goes here
}
Everything began to work again. Is there a reason the method decided not to take the Book parameter even though it clearly was a Book object? The only difference seemed to be it was instantiated from within the same class, and only filled with one data member. I'm very very confused. Please, shed some light?

In Java, the equals() method that is inherited from Object is:
public boolean equals(Object other);
In other words, the parameter must be of type Object. This is called overriding; your method public boolean equals(Book other) does what is called overloading to the equals() method.
The ArrayList uses overridden equals() methods to compare contents (e.g. for its contains() and equals() methods), not overloaded ones. In most of your code, calling the one that didn't properly override Object's equals was fine, but not compatible with ArrayList.
So, not overriding the method correctly can cause problems.
I override equals the following everytime:
#Override
public boolean equals(Object other){
if (other == null) return false;
if (other == this) return true;
if (!(other instanceof MyClass)) return false;
MyClass otherMyClass = (MyClass)other;
...test other properties here...
}
The use of the #Override annotation can help a ton with silly mistakes.
Use it whenever you think you are overriding a super class' or interface's method. That way, if you do it the wrong way, you will get a compile error.

If you use eclipse just go to the top menu
Source --> Generate equals() and
hashCode()

Slightly off-topic to your question, but it's probably worth mentioning anyway:
Commons Lang has got some excellent methods you can use in overriding equals and hashcode. Check out EqualsBuilder.reflectionEquals(...) and HashCodeBuilder.reflectionHashCode(...). Saved me plenty of headache in the past - although of course if you just want to do "equals" on ID it may not fit your circumstances.
I also agree that you should use the #Override annotation whenever you're overriding equals (or any other method).

Another fast solution that saves boilerplate code is Lombok EqualsAndHashCode annotation. It is easy, elegant and customizable. And does not depends on the IDE. For example;
import lombok.EqualsAndHashCode;
#EqualsAndHashCode(of={"errorNumber","messageCode"}) // Will only use this fields to generate equals.
public class ErrorMessage{
private long errorNumber;
private int numberOfParameters;
private Level loggingLevel;
private String messageCode;
See the options avaliable to customize which fields to use in the equals. Lombok is avalaible in maven. Just add it with provided scope:
<dependency>
<groupId>org.projectlombok</groupId>
<artifactId>lombok</artifactId>
<version>1.14.8</version>
<scope>provided</scope>
</dependency>

in Android Studio is
alt + insert ---> equals and hashCode
Example:
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Proveedor proveedor = (Proveedor) o;
return getId() == proveedor.getId();
}
#Override
public int hashCode() {
return getId();
}

Consider:
Object obj = new Book();
obj.equals("hi");
// Oh noes! What happens now? Can't call it with a String that isn't a Book...

the instanceOf statement is often used in implementation of equals.
This is a popular pitfall !
The problem is that using instanceOf violates the rule of symmetry:
(object1.equals(object2) == true) if and only if (object2.equals(object1))
if the first equals is true, and object2 is an instance of a subclass of
the class where obj1 belongs to, then the second equals will return false!
if the regarded class where ob1 belongs to is declared as final, then this
problem can not arise, but in general, you should test as follows:
this.getClass() != otherObject.getClass(); if not, return false, otherwise test
the fields to compare for equality!

recordId is property of the object
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Nai_record other = (Nai_record) obj;
if (recordId == null) {
if (other.recordId != null)
return false;
} else if (!recordId.equals(other.recordId))
return false;
return true;
}

getting error while overrideing equals method [duplicate]

I ran into an interesting (and very frustrating) issue with the equals() method today which caused what I thought to be a well tested class to crash and cause a bug that took me a very long time to track down.
Just for completeness, I wasn't using an IDE or debugger - just good old fashioned text editor and System.out's. Time was very limited and it was a school project.
Anyhow -
I was developing a basic shopping cart which could contain an ArrayList of Book objects. In order to implement the addBook(), removeBook(), and hasBook() methods of the Cart, I wanted to check if the Book already existed in the Cart. So off I go -
public boolean equals(Book b) {
... // More code here - null checks
if (b.getID() == this.getID()) return true;
else return false;
}
All works fine in testing. I create 6 objects and fill them with data. Do many adds, removes, has() operations on the Cart and everything works fine. I read that you can either have equals(TYPE var) or equals(Object o) { (CAST) var } but assumed that since it was working, it didn't matter too much.
Then I ran into a problem - I needed to create a Book object with only the ID in it from within the Book class. No other data would be entered into it. Basically the following:
public boolean hasBook(int i) {
Book b = new Book(i);
return hasBook(b);
}
public boolean hasBook(Book b) {
// .. more code here
return this.books.contains(b);
}
All of a sudden, the equals(Book b) method no longer works. This took a VERY long time to track down without a good debugger and assuming the Cart class was properly tested and correct. After swaapping the equals() method to the following:
public boolean equals(Object o) {
Book b = (Book) o;
... // The rest goes here
}
Everything began to work again. Is there a reason the method decided not to take the Book parameter even though it clearly was a Book object? The only difference seemed to be it was instantiated from within the same class, and only filled with one data member. I'm very very confused. Please, shed some light?

In Java, the equals() method that is inherited from Object is:
public boolean equals(Object other);
In other words, the parameter must be of type Object. This is called overriding; your method public boolean equals(Book other) does what is called overloading to the equals() method.
The ArrayList uses overridden equals() methods to compare contents (e.g. for its contains() and equals() methods), not overloaded ones. In most of your code, calling the one that didn't properly override Object's equals was fine, but not compatible with ArrayList.
So, not overriding the method correctly can cause problems.
I override equals the following everytime:
#Override
public boolean equals(Object other){
if (other == null) return false;
if (other == this) return true;
if (!(other instanceof MyClass)) return false;
MyClass otherMyClass = (MyClass)other;
...test other properties here...
}
The use of the #Override annotation can help a ton with silly mistakes.
Use it whenever you think you are overriding a super class' or interface's method. That way, if you do it the wrong way, you will get a compile error.

If you use eclipse just go to the top menu
Source --> Generate equals() and
hashCode()

Slightly off-topic to your question, but it's probably worth mentioning anyway:
Commons Lang has got some excellent methods you can use in overriding equals and hashcode. Check out EqualsBuilder.reflectionEquals(...) and HashCodeBuilder.reflectionHashCode(...). Saved me plenty of headache in the past - although of course if you just want to do "equals" on ID it may not fit your circumstances.
I also agree that you should use the #Override annotation whenever you're overriding equals (or any other method).

Another fast solution that saves boilerplate code is Lombok EqualsAndHashCode annotation. It is easy, elegant and customizable. And does not depends on the IDE. For example;
import lombok.EqualsAndHashCode;
#EqualsAndHashCode(of={"errorNumber","messageCode"}) // Will only use this fields to generate equals.
public class ErrorMessage{
private long errorNumber;
private int numberOfParameters;
private Level loggingLevel;
private String messageCode;
See the options avaliable to customize which fields to use in the equals. Lombok is avalaible in maven. Just add it with provided scope:
<dependency>
<groupId>org.projectlombok</groupId>
<artifactId>lombok</artifactId>
<version>1.14.8</version>
<scope>provided</scope>
</dependency>

in Android Studio is
alt + insert ---> equals and hashCode
Example:
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Proveedor proveedor = (Proveedor) o;
return getId() == proveedor.getId();
}
#Override
public int hashCode() {
return getId();
}

Consider:
Object obj = new Book();
obj.equals("hi");
// Oh noes! What happens now? Can't call it with a String that isn't a Book...

the instanceOf statement is often used in implementation of equals.
This is a popular pitfall !
The problem is that using instanceOf violates the rule of symmetry:
(object1.equals(object2) == true) if and only if (object2.equals(object1))
if the first equals is true, and object2 is an instance of a subclass of
the class where obj1 belongs to, then the second equals will return false!
if the regarded class where ob1 belongs to is declared as final, then this
problem can not arise, but in general, you should test as follows:
this.getClass() != otherObject.getClass(); if not, return false, otherwise test
the fields to compare for equality!

recordId is property of the object
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Nai_record other = (Nai_record) obj;
if (recordId == null) {
if (other.recordId != null)
return false;
} else if (!recordId.equals(other.recordId))
return false;
return true;
}

Is it okay to have a hashcode as a simple increment?

Is this code below okay for a hashCodeto be a simple increment starting with a negative number?
private volatile static AtomicInteger creations = new AtomicInteger(Integer.MIN_VALUE);
private final int creation;
{
creation = creations.getAndIncrement();
}
#Override
public int hashCode() {
return creation;
}
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
if (creation == ((Stuff) obj).creation)
return true;
else
return false;
}
How come hashCodes are usually made with prime numbers instead of using just a simple increment? (The reason why I want it to be this way is because each object I create with this class will be unique and I want to be able to remove it from a HashMap later even though the state of the object will change, because it is my understanding if a Hash changes then you cannot remove it from a HashMap after it was added) To prevent this from being a simple yes or no answer, maybe explain to me the pros and cons of implementing a Hash this way.

In general it's not okay. Two objects for which object1.equals(object2) returns true must have the same hashCode.
However, in your specific case, where the only criteria for object equality is the creation property, which is the same property used in the hashCode, your hashCode is fine.
However, your hashCode and equals don't offer much benefit over the default implementations of Object, since a.equals(b) will be true if and only if a==b.

Yes it's ok (if two instances are equals, they have the same hashcode) although your equals/hashcode implementation is not doing much vs. the default implementation of Object so there is no point doing that - unless you want objects to be equal every 2^32 instance creation.
Based on your additions, I would agree with Patricia Shanahan's suggestion to use identityHashCode:
#Override
public int hashCode() {
return System.identityHashCode(this);
}
#Override
public boolean equals(Object obj) {
return this == obj;
}
That is essentially what Object does by default. In terms of collisions, I doubt that you will have more collision with this implementation than yours.

In a Java method that takes an Object argument, how can you access fields that only exist for instances of specific classes?

-- Yes, this is a question asking for help regarding something that was assigned as homework. No, this is not me asking you to do my homework for me. The deadline was a half an hour ago; I literally can't change my submission. You'll just have to take my word for it. Moving on...
I know that testing for the type of objects is supposed to be unnecessary. When I was looking for details about 'instanceof', I found a half dozen threads where people responded only to tell the original poster that they were making a mistake if they had to test to find out what kind of object they were dealing with before processing it. Yes, I know, I would love to follow the conventions. Unfortunately, my professor asked for us to override the equals method of a class we defined, and specifically required an Object type parameter. If you see my code, you'll probably understand better:
public boolean equals(Course other){
if(!(other instanceof Course)){
return false;
} else if(other.name==this.name && other.days==this.days &&
other.start==this.start && other.end==this.end){
return true;
}
return false;
}
You can probably understand where I'm going with this. The 'other' parameter should be an Object type, but the program has a fit if I leave it as an Object and use the name / days / start / end fields. If I change it to Course, of course it works (no pun intended), but that would be a completely different method. The desired behavior is for all objects other than Course instances to make the method return false, and additionally, for mismatched data between Course instances to make it return false.
I'm sorry to all of you who know Java well enough to be frustrated by seeing questions like these.

If you want to override the "equals" method, you should use Object as a parameter, and thus you have to check for the object's type. Usually your own implementation would look like this:
#Override
public boolean equals(Object obj) {
if (obj == this)
return true; // object's references are identical
else if (!(obj instanceof Course))
return false;
Course that = (Course) obj;
return (this.name.equals(that.name)) && (this.days == that.days)
&& (this.start.equals(that.start)) && (this.end.equals(that.end));
}
Of course you should override "hashCode" as well, using the same significant fields.
Instead, you overloaded the method with your own parameter of type Course. So if you call myobject.equals(anotherObject) and anotherObject is not of type Course, your "equals" method will never be called, instead the Object#equals method will be called, which does only the following: return this == obj.
The reason why overloading the "equals" method is not enough is the necessity to overload "hashCode" as well, which takes no parameters and thus cannot be overloaded.
If you write your own implementation of boolean equals(Object), you must also implement int hashCode()
Both methods should use the same significant fields for "hashCode" and "equals".
If a.equals(b) == true than the following must be also true: a.hashCode() == b.hashCode()
Also if a.hashCode() != b.hashCode() then a.equals(b) == false
The last point is the main reason why you should not just overload "equals" with your own type:
Course c1 = new Course("myname");
Course c2 = new Course("myname");
c1.equals(c2); // true
c1.hashCode() == c2.hashCode(); // false

You can cast Object as Course:
Course course = (Course)object;
Then do all the comparisons on the course object. Obviously, still do the instanceof check before casting to avoid a ClassCastException.

You're trying to cast it:
Cource c = (Course)other;

Your code :
public boolean equals(Course other){
if(!(other instanceof Course)){ <-- other can only be Course here
return false;
} else if(other.name==this.name && other.days==this.days &&
other.start==this.start && other.end==this.end){
return true;
}
return false;
}
Correct code :
public boolean equals(Object other){
if(!(other instanceof Course)){
return false;
} else{
Course c = (Course) other;
if(c.name==this.name && c.days==this.days &&
c.start==this.start && c.end==this.end){
return true;
}
}
}
return false;
}