How to create a function to retains only the elements in the list that are contained in the specified list without using any third 3rd party

Question

I have a class StudentArrayList inherit interface SimpleArrayList

the SimpleArrayList have the function: retainAll

 /**
 * Retains only the elements in this list that are contained in the
 * specified student.SimpleArrayList. In other words, removes from this list all of
 * its elements that are not contained in the specified student.SimpleArrayList.
 *
 * @param c
 *            student.SimpleArrayList containing elements to be retained in this
 *            list
 * @return <tt>true</tt> if this list changed as a result of the call
 */
 boolean retainAll(SimpleArrayList<E> c);

I created a function corresponding in StudentArrayList like this:

@Override
public boolean retainAll(SimpleArrayList c) {        
    if (c.size()==0){
        this.clear();
        return true;
    }
    int temp=this.size;
    int count=0;
    boolean check=false;
    for(int i=0;i<this.size;i++){
        for(int j=0;j<c.size();j++){
            if(this.studentList[i].equals(c.get(j))) check=true;
        }
        if(!check) {
            this.remove(i);
            //this.size--;
            check=false;
        }
    } 
    if(temp==this.size) return false;
    return true;

I don't know why my coding get wrong answer. Could anyone please explain for me why?

Sorry, but we cannot do anything with the statement _"my coding get wrong answer"_. Clearly explain the problem you are having, with a [mcve]. — Asteroids With Wings, Jul 08 '20 at 23:44
*Incorrect behavior:* If both lists are empty, your method returns `true`, even though the list wasn't changed. — Andreas, Jul 09 '20 at 00:28
*Performance:* You should `break` the inner loop once you set `check=true`. There is no need to keep searching the `c` list. — Andreas, Jul 09 '20 at 00:28

score 0 · Answer 1 · answered Jul 09 '20 at 00:18

Try this:

@Override
public boolean retainAll(SimpleArrayList c) {        
    if (c.size()==0){
        this.clear();
        return true;
    }
    int temp=this.size;
    int count=0;
    
    for(int i=0;i<this.size;i++){
        boolean check=false;
        for(int j=0;j<c.size();j++){
            if(this.studentList[i].equals(c.get(j))) check=true;
        }
        if(!check) {
            this.remove(i);
            //this.size--;
            check=false;
        }
    } 
    if(temp==this.size) return false;
    return true;

If not post more code, but it appears your logic is wrong.

How to create a function to retains only the elements in the list that are contained in the specified list without using any third 3rd party

1 Answers1