Cache stuff immutable in an Java object

ImmutableCacheExample.java

public final class Foo {
  private final int a;
  private final int b;
  
  //Immutabel cache even without final because it's the result of final/immutable data
  private int calculateCache; //init = 0
  
  public Foo(int a, int b) {
    this.a = a;
    this.b = b;
  }
  
  public int calculate() {
    if(calculateCache == 0) { //check if cache is in init state
      calculateCache = a * b;
    }
    returne calculateCache;
  }
}