CODEWITHSUNDEEP
javafor-looparraylistboolean
Milwaukoholic
Milwaukoholic

Reputation: 1161

Comparing one ArrayList to another ArrayList using boolean method in java

I am trying to get a boolean method to return true or false on wether two arrayLists are equal to each other. The arraysLists are array and array1. The user inputs them. Right now here is the code that I thought would work:

public boolean equals(){
//if both are equal return true, else false
boolean test = false;
for(int i = 0; i < array1.size() && !test; i++){
    if(array1.get(i) == (array.get(i))){
        test = true;
    }       
}
return test;
  }

except even when all the arrayLists numbers match the other arrayLists numbers, it returns false.

Upvotes: 0

Views: 4101

Answers (5)

Konrad H&#246;ffner
Konrad H&#246;ffner

Reputation: 12207

You don't need to overwrite the equals method, as there is one already provided for lists that does exactly what you need.

If you insist of writing it yourself there is a simple error in your code. Because you initialize test to be false, "&& !test" lets your loop exist right at the start.

The correct version would be:

public boolean equals(){
if(array.size()!=array1.size) return false; // test for different length
for(int i = 0; i < array1.size(); i++){
    if(!array1.get(i).equals(array.get(i))){
        return false;
    }       
 }
 return true;
}

Upvotes: 2

Mike Lentini
Mike Lentini

Reputation: 1358

You'll need to change your code to this:

public boolean equals(){
    if (array1.size() != array.size()) return false;

    for(int i = 0; i < array1.size(); i++){
        if(!array1.get(i).equals(array.get(i))){
            return false;
        }       
    }

    return true;
}

First off, you have to start with test being true and return false if you find something that isn't equal, because this clearly shows that the ArrayLists are not equal. You actually don't need the test variable at all, so I took it out. Just return false if you find something that isn't equal. If you don't find something that isn't equal, it will never return false and will just return true at the end. Second, you have to use the equals() method, because ArrayLists use the Integer class, not the int primitive so == will check if they are the same object, not if they are the same number. Lastly, to deal with comparing arrays of different sizes, you should compare their size and return false if they are not the same size, since there is no way they can be equal.

Upvotes: 0

SunKing2
SunKing2

Reputation: 319

You should just 'reverse' your method. Assume the arrays are equal first. It should then check on each iteration if the element differs. If the element differs, then set a "not equal" flag. In pseudo-codee

boolean different = false;
for (each element of array 1) {
    if (element != element of array 2) different = true
    break;
}

Upvotes: 0

ssedano
ssedano

Reputation: 8432

Think that you are only iterating one array. Think what can go wrong there. Also take a look at your control statement.

If you carefully follow the flux of your code you quickly will realize why is false.

Upvotes: 0

ControlAltDel
ControlAltDel

Reputation: 35096

double equals (==) is dangerous. You are actually returning objects in your code up there, so you should definitely use equals() instead

Upvotes: 0

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