stackoverflow-60622844

60622844.groovy

import java.time.LocalDate

public class AmountModel {
  private Integer id;
  private Double amount;
//   Not defined in OP
//   private CategoryModel categoryModel;
  private LocalDate localDate;
}

/* ******************** */

public class MyBudget {
  // Map "amounts" by ID for easy O(1) lookup.
  Map<Integer, AmountModel> amountMap

  // Load some dummy data.
  MyBudget() {
    List<AmountModel> models = (1..10).collect { i -> new AmountModel(id: i, amount: i) }
    this.amountMap = models.collectEntries(model -> [(model.id): model])
  }

  public void deleteAmount(Integer id) {
    if (!amountMap.containsKey(id)) { 
      // (Handle invalid input)
      throw new Exception()
    }

    amountMap.remove(id)
    return
  }
}

/* ******************** */

MyBudget budget = new MyBudget()
assert budget.amountMap.containsKey(1)
budget.deleteAmount(1)
assert !budget.amountMap.containsKey(1)