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Java

JJ Brandler
JJ Brandler
988 Points
Posted by JJ Brandler
JJ Brandler
988 Points

what is being compared in "if(equals(other))"

when we call the compareTo method, we use the mDescription and mCreationDate. but when we call the equals method, what is it being compared to? (from the Java Data Structures, Interfaces)

4 Answers

Kourosh Raeen
Kourosh Raeen
23,733 Points
Kourosh Raeen
Kourosh Raeen
23,733 Points

The equals method is called on the current object. So this code:

if (equals(other)) {
    return 0;
}

is equivalent to this code:

if (this.equals(other)) {
    return 0;
}

In the first version the this keyword is omitted.

Dennis Addo
PLUS
Dennis Addo
Courses Plus Student 2,943 Points
Dennis Addo
Dennis Addo
Courses Plus Student 2,943 Points

can you post in an example so that we can help you solve the problem or do you want a general understanding of the CompareTo Interface?

JJ Brandler
JJ Brandler
988 Points
JJ Brandler
JJ Brandler
988 Points

I am trying to get better understanding of what is going on.

here is the code:

@Override
public int compareTo(Object obj){
    Treet other = (Treet)obj;

    if(this.equals(other))
        return 0;
    int dateComp  = mCreationDate.compareTo(other.mCreationDate);
    if (dateComp==0)
        return mDescription.compareTo(other.mDescription);
    return dateComp;
}

I found that the code still works if i use 

    if(this.equals(other))

in place of if(equals(other))

I found the use of if(equals(other)) hard to understand, and I am trying to wrap my head around how it is being used in this case, and when I can/should use it like that.

Kourosh Raeen
Kourosh Raeen
23,733 Points
Kourosh Raeen
Kourosh Raeen
23,733 Points

Hi JJ - See my answer above.

JJ Brandler
JJ Brandler
988 Points
JJ Brandler
JJ Brandler
988 Points

Kourosh Raeen, when would you "this" and when would you leave it out?

Kourosh Raeen
Kourosh Raeen
23,733 Points
Kourosh Raeen
Kourosh Raeen
23,733 Points

If there is any ambiguity then you need to use this. As an example, consider the constructor in the Treet class:

public Treet(String author, String description, Date creationDate) {
    mAuthor = author;
    mDescription = description;
    mCreationDate = creationDate;
}

Now, suppose that when defining the member variables we hadn't started the variable names with m:

    private String author;
    private String description;
    private Date creationDate;

Then suppose you write the constructor code like this:

public Treet(String author, String description, Date creationDate) {
    author = author;
    description = description;
    creationDate = creationDate;
}

This code is now ambiguous in the sense that when you say author = author it is not clear that which one is the member variable and which one is the parameter. So to fix it we use the this keyword:

public Treet(String author, String description, Date creationDate) {
    this.author = author;
    this.description = description;
    this.creationDate = creationDate;
}