How to incorporate a boolean in a method JAVA

Question

I having a hard time getting the right output because i dont know how boolean exactly works inside a method. I have an arraylist and I'm check for any duplicates in the arraylist Here's my code

public void rentOneInstrument(List<Instrument> instrumentList){
        String a="";
        String b="";
        boolean found = false;
        for (int j = 0; j < instrumentList.size(); j++) {
                a ="";
                a = instrumentList.get(j).getName();

                for (int i = j+1; i < instrumentList.size(); i++) {
                    b = instrumentList.get(i).getName();
                    System.out.println("a" + a + " b" + b);
                    if(a.equals(b)){
                        found = true;
                    }else {
                        found = false;
                    }
                }
        }
        if (found) {
            System.out.println("duplicate");
        }else{
            System.out.println("no duplicate");
        }
    }

Here is my output

a Cymbals b Drums,..
a Cymbals b Cello,..
a Cymbals b Cymbals,..
a Drums b Cello,..
a Drums b Cymbals,..
a Cello b Cymbals

no duplicate // prints no duplicate when there is clearly a duplicate in the output. How can i correct this?

Edit.. Btw I just a want a single out put that prints whether it found a duplicate or not inside the loop

Answer 1

if(a.equals(b)){
  found = true
}else {
    found = false;
}

This is your problem. This way only the last iteration of your loop will be stored in found . Since you are initializing it as false you don't need to set it to that again here.

   for (int i = j+1; i < instrumentList.size(); i++) {
      b = instrumentList.get(i).getName();
      System.out.println("temp1 " + a + " temp2 " + b);
      if(a.equals(b)){
         found = true;
      }
   }

Alternatively, you can use a break statement when you have found a match to get out of the loop like so:

if(a.equals(b)){
  found = true;
  break;
}else {
    found = false;
}

This way, found will be true and no other iterations will be performed, continuing after the end of the loop instead.

Answer 2

Consider the following input :

a,b,c,a,d 

Now whit your code it would be false as d is not repeated. It would over right the value for a that is repeated. Also once a element is fond that it is replanted you don't need to go through the entire loop of all elements hence there is a break.

public void rentOneInstrument(List<Instrument> instrumentList){
        String a="";
        String b="";
        boolean found = false;
        for (int j = 0; j < instrumentList.size(); j++) {
                a ="";
                a = instrumentList.get(j).getName();
                if(found) { break; } // Changed line here 
                for (int i = j+1; i < instrumentList.size(); i++) {
                    b = instrumentList.get(i).getName();
                    System.out.println("temp1 " + a + " temp2 " + b);
                    if(a.equals(b)){
                        found = true;
                        break; // Changed line here 
                    }
                }
        }
        if (found) {
            System.out.println("duplicate");
        }else{
            System.out.println("no duplicate");
        }
    }

Another way of doing it without a break would be

public void rentOneInstrument(List<Instrument> instrumentList){
        String a="";
        String b="";
        boolean found = false;
        for (int j = 0; j < instrumentList.size() && !found; j++) {
                a ="";
                a = instrumentList.get(j).getName();

                for (int i = j+1; i < instrumentList.size() && !found; i++) { // Changed for condition to look at the found variable too.
                    b = instrumentList.get(i).getName();
                    System.out.println("temp1 " + a + " temp2 " + b);
                    if(a.equals(b)){
                        found = true;
                    }
                }
        }
        if (found) {
            System.out.println("duplicate");
        }else{
            System.out.println("no duplicate");
        }
    }

A much better way of doing this would be using sets that doesn't contain duplicates.

public void rentOneInstrument(List<Instrument> instrumentList){
     Set< Instrument > instrumentSet = new HashSet< Instrument >(instrumentList);
     if(instrumentList.size()== instrumentSet.size()) {
          System.out.println("no duplicate");
     } else {
          System.out.println("duplicate");
     }
}

Answer 3

Let found be false by default. and just set it true if any duplicate is found.

 if(a.equals(b)){
      found = true;
 }else {
      // found = false;
      // don't do this otherwise it will override previous `found` value
 }

Answer 4

public void rentOneInstrument(List<Instrument> instrumentList){

    boolean found=false;
    List<String> listString=new ArrayList<String>();

    for (Instrument inst: instrumentList){
        if(listString.contains(inst.getName())){

            found=true;
        }
        else{
            listString.add(inst.getName());
        }


    }

  if (found) {
            System.out.println("duplicate");
        }else{
            System.out.println("no duplicate");
        }


}