For immutable classes, I like to set the hashCode value when the object is constructed. (I sometimes do this for the toString value too.) I want to do this in a Record as well. For example, I have a game that uses rows and columns, and rows never exceed 18, and columns never exceed 3. So if I'm writing it as a class, I could write this:

public final class RowColumn {
  private final int row;
  private final int column;
  private final int hashCode
  private final String toString;

  public RowColumn(int row, int column) {
    this.row = row;
    this.column = column;
    this.hash = row * 100 + column;
    this.toString = String.format("%02d %2d", row, column);
  }

  @Override
  public int hashCode() { return hash; }

  @Override
  public String toString() { return toString; }

  // ... getters, equals methods go here.
}

I like this hashCode because I can read the row and column from it, which may be very helpful in debugging. And since the hashcode is computed from two final values, there's no point in recomputing it each time we call hashCode(). And since I intend to use this in a hash set, there's no point in lazy instantiation like this:

private int hash = -1;
@Override
public int hashCode() {
  if (hash == -1) {
    hash = row * 100 + column;
  }
  return hash;
}

So I pre-compute the hash value. But as far as I can tell, if I write this class as a record, I can't pre-compute these values. (I'm less concerned about the toString() method, since I will only use it in debugging.)

Before you tell me that this class is so simple that any performance hit is minor, I should point out that most records are more complicated. If the Record has a lot of fields, of various immutable types, pre-computing the hash value and String become more useful.

Is there a way to do this in a Record?

For immutable classes, I like to set the hashCode value when the object is constructed. (I sometimes do this for the toString value too.) I want to do this in a Record as well. For example, I have a game that uses rows and columns, and rows never exceed 18, and columns never exceed 3. So if I'm writing it as a class, I could write this:

public final class RowColumn {
  private final int row;
  private final int column;
  private final int hashCode
  private final String toString;

  public RowColumn(int row, int column) {
    this.row = row;
    this.column = column;
    this.hash = row * 100 + column;
    this.toString = String.format("%02d %2d", row, column);
  }

  @Override
  public int hashCode() { return hash; }

  @Override
  public String toString() { return toString; }

  // ... getters, equals methods go here.
}

I like this hashCode because I can read the row and column from it, which may be very helpful in debugging. And since the hashcode is computed from two final values, there's no point in recomputing it each time we call hashCode(). And since I intend to use this in a hash set, there's no point in lazy instantiation like this:

private int hash = -1;
@Override
public int hashCode() {
  if (hash == -1) {
    hash = row * 100 + column;
  }
  return hash;
}

So I pre-compute the hash value. But as far as I can tell, if I write this class as a record, I can't pre-compute these values. (I'm less concerned about the toString() method, since I will only use it in debugging.)

Before you tell me that this class is so simple that any performance hit is minor, I should point out that most records are more complicated. If the Record has a lot of fields, of various immutable types, pre-computing the hash value and String become more useful.

Is there a way to do this in a Record?

• java
• java-record

• if I write this class as a record, I can't pre-compute these values Why not? – Sotirios Delimanolis Commented 1 hour ago

1 Answer 1

record MyRecord(int a, int b, int hash) {

    MyRecord(int a, int b) {
        this(a, b, Objects.hash(a, b));
    }

    @Deprecated
    MyRecord {
    }

    @Override
    public int hashCode() {
        return hash;
    }
}

That said, I consider this an odd usage of records and would never do it myself. If I was convinced that I needed to precompute hash codes I would go with an old fashioned hand written value class.