JAVA concatenating double array => String(with special format)

Question

How do I format a String to convert a double array to the form: [0.0, 2.4, 5.5, 6.3] ?

I am assigning double array elements to one string to have a string representation in the format above.

My code to change double array to a single string representation:

public String toString() {
    String toString="[";
     for(int i=0; i<data.length; i++) {
         toString = toString + data[i] + ", ";
      }
    return toString + "]";
}

I am getting the array in the format [0.0, 2.4, 5.5, 6.3, ], which is not expected and wrong.

How can I get it formatted to [0.0, 2.4, 5.5, 6.3]?

I have tried to use Arrays.toString() method but, I am not allowed to use that in this project.

How should I change my code to have the desired format?

Answer 1

You could stream the data array and let Collectors.joining do the heavy lifting:

@Override
public String toString() {
    return Arrays.stream(data)
                 .mapToObj(Double::toString).collect(Collectors.joining(", ", "[", "]"));
}

Edit:
If you can't stream the array, you could iterate it yourself and give special treatment to the first element, which is the only element that isn't preceded by a comma:

@Override
public String toString() {
    StringBuilder sb = new StringBuilder("[");
    if (data.length > 0) {
        sb.append(data[0]);
    }
    for (int i = 1; i < data.length; ++i) {
        sb.append(", ").append(data[i]);
    }
    sb.append("]");
    return sb.toString();
}

Answer 2

You can stop your loop before the last element of the array and then manually add the last element to your string in the result. Like this:

if (data.length == 0) {
  return "[]";
}

int lastElementIndex = data.length - 1;
String result = "[";

for (int i = 0; i < lastElementIndex; i++) {
  result += data[i] + ", "; 
// or result += String.format("%.1f, ", data[i]); if you are allowed
}

return result + data[lastElementIndex] + "]"; 
// or String.format("%s%.1f]", result, data[lastElementIndex]);

Answer 3

---

Just check the length of an array if it is greater than equal to one then concatenate it with toString and then start traversing from index 1 and return it.

 public String toString() {
    String toString="[";
    //check length of an array
     if(data.length >= 1){
        toString += data[0];
     }
    //start from 1st index
    for(int i=1; i<data.length; i++) {
        toString += ", " + data[i];
    }
    return toString + "]";
}

Answer 4

Why not just check if you are on last element and not add the , then:

       public String toString2(double[] data) {
        String toString = "[";
        for (int i = 0; i < data.length; i++) {
            if (i == data.length - 1) {
                toString = toString + data[i];
            } else {
                toString = toString + data[i] + ", ";
            }
        }
        return toString + "]";
    }

Answer 5

Just add an if-block to check for last element:

public String toString() {
    String toString = "[";
    for (int i = 0; i < data.length; i++) {
        toString = toString + data[i];
        if (i != data.length - 1) {
            toString = toString + ", ";
        }
    }
    return toString + "]";
}

Or alternatively add your separator before the elements except the first

public String toString() {
    String separator = "";
    String toString = "[";
    for (int i = 0; i < data.length; i++) {
        toString = toString + separator;
        toString = toString + data[i];
        separator = ", ";
    }
    return toString + "]";
}