[Canonical page](<https://docs.dhtmlx.com/planning-engine/java/api/project-preview/>)
[Full documentation index](<https://docs.dhtmlx.com/planning-engine/llms.txt>)


# Project#preview

Applies hypothetical changes to a copy of the solved baseline of a [`Project`](<https://docs.dhtmlx.com/planning-engine/java/api/project/project.md>) and schedules the copy. The project isn't modified. Use it to show what an edit would do before committing it with [`apply`](<https://docs.dhtmlx.com/planning-engine/java/api/project-apply.md>). Needs a solved project.

## Signature

```java
public ScheduleResult preview(SchedulingChanges changes, PreviewOptions options)
```

Synchronous and CPU-only. Not synchronized: serialize calls on one `Project` instance.

## Parameters

| Name | Type | Description |
|---|---|---|
| `changes` | [`SchedulingChanges`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/scheduling-changes.md>) | The edits to try. Not `null` |
| `options` | [`PreviewOptions`](<https://docs.dhtmlx.com/planning-engine/java/api/project/preview-options.md>) | Subtree, analytics, reasons, baseline check and an optional load profile. Not `null`; `new PreviewOptions()` for defaults |

## Behavior

- `updates` are relative to the baseline. An empty change set gives empty `updates`.
- The result's `baselineId` is the baseline the preview read, so a following `preview` accepts it.
- The change set isn't partially accepted: one rejected operation fails the whole call.
- `options.baselineId` and `changes.baselineId` are checked separately. `null` skips a check.

## Returns

A [`ScheduleResult`](<https://docs.dhtmlx.com/planning-engine/java/api/results/schedule-result.md>) for the previewed state.

## Errors

All engine exceptions are unchecked.

| Error | When |
|---|---|
| [`ProjectException`](<https://docs.dhtmlx.com/planning-engine/java/api/errors/project-exception.md>) `NO_BASELINE` | The project is unsolved, including after `apply` |
| `ProjectException` `STALE_BASELINE` | `options.baselineId` or `changes.baselineId` doesn't name the current baseline |
| [`InputException`](<https://docs.dhtmlx.com/planning-engine/java/api/errors/input-exception.md>) | An operation in `changes` is rejected (the exception reports the first one, with its `path` and message), or the `profile` query is invalid |
| `NullPointerException` | `changes` or `options` is `null` |

## Example

Preview a drag of task `b`, guarded by the baseline token, and check that the project keeps its dates.

```java
import java.util.List;
import org.dhtmlx.plen.*;

public class PreviewDrag {
  static Moment day(int d) {
    return Moment.fromYmdHm(2026, 1, d, 0, 0);
  }

  public static void main(String[] args) throws Exception {
    ProjectData data = new ProjectData()
        .withTasks(List.of(new TaskData(Id.str("b")).withStartDate(day(1)).withDuration(1.0)));
    data.config.useCalendars = false;

    Project project = new Project();
    ScheduleResult solved = project.solve(data, new SolveOptions());

    SchedulingChanges drag = new SchedulingChanges();
    drag.tasks.update.add(new TaskUpdate(Id.str("b")).withStartDate(Patch.of(day(5))));

    PreviewOptions options = new PreviewOptions().withBaselineId(solved.baselineId);
    ScheduleResult preview = project.preview(drag, options);
    for (Mutation update : preview.updates) {
      // task b: 2026-01-05 00:00 .. 2026-01-06 00:00
      System.out.println("task " + update.id + ": " + update.startDate + " .. " + update.endDate);
    }

    // The project is unchanged: an empty preview reports no updates.
    ScheduleResult unchanged = project.preview(new SchedulingChanges(), new PreviewOptions());
    System.out.println("updates: " + unchanged.updates.size()); // updates: 0
  }
}
```

## Related

- [`PreviewOptions`](<https://docs.dhtmlx.com/planning-engine/java/api/project/preview-options.md>) - scope, analytics, baseline guard and load profile
- [`SchedulingChanges`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/scheduling-changes.md>) - the edits to try
- [`ScheduleResult`](<https://docs.dhtmlx.com/planning-engine/java/api/results/schedule-result.md>) - the previewed dates
- [`Project#apply`](<https://docs.dhtmlx.com/planning-engine/java/api/project-apply.md>) - commits the same change set
- [Previewing changes](<https://docs.dhtmlx.com/planning-engine/java/live-projects/previewing-changes.md>) - previews, baseline tokens and copies