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Add react exercise (exercism#2266)
* Add react exercise * Tidy up react exercise - Remove unneeded generics parameter - Remove unneeded projectsEvaluated config from gradle.build - Add ComputeCell stub for student - Change stubs to use interface instead of static inner classes - Update config.json to use prerequisites and practices instead of topics * Refactor example solution * Refactor to example solution to propagate changes instead of tracking a tentative value and requiring a separate commit step. This approach is more consistent with the example solution in other tracks for this exercise. * Change Cells to use just inner classes instead of interfaces.
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‎config.json‎

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"strings"
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]
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},
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{
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"slug": "react",
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"name": "React",
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"uuid": "33531718-1d8a-4abd-bd2a-090303ad0c39",
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"practices": [
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"classes",
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"generic-types"
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],
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"prerequisites": [
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"classes",
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"generic-types"
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],
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"difficulty": 8,
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"topics": []
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},
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{
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"slug": "rectangles",
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"name": "Rectangles",

‎exercises/practice/react/.docs/instructions.md‎

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# Instructions
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Implement a basic reactive system.
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Reactive programming is a programming paradigm that focuses on how values are computed in terms of each other to allow a change to one value to automatically propagate to other values, like in a spreadsheet.
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Implement a basic reactive system with cells with settable values ("input" cells) and cells with values computed in terms of other cells ("compute" cells).
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Implement updates so that when an input value is changed, values propagate to reach a new stable system state.
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In addition, compute cells should allow for registering change notification callbacks.
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Call a cell’s callbacks when the cell’s value in a new stable state has changed from the previous stable state.

‎exercises/practice/react/.meta/config.json‎

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{
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"authors": [
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"kahgoh"
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],
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"files": {
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"solution": [
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"src/main/java/React.java"
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],
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"test": [
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"src/test/java/ReactTest.java"
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],
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"example": [
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".meta/src/reference/java/React.java"
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],
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"invalidator": [
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"build.gradle"
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]
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},
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"blurb": "Implement a basic reactive system."
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}

‎exercises/practice/react/.meta/src/reference/java/React.java‎

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import java.util.*;
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import java.util.function.Consumer;
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import java.util.function.Function;
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import java.util.function.Supplier;
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import java.util.stream.Collectors;
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public class React {
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public static class Cell<T> {
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T value;
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public T getValue() {
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return value;
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}
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final Collection<ComputeCell<T>> dependentCells = new ArrayList<>();
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}
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public static class InputCell<T> extends Cell<T> {
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InputCell(T initialValue) {
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this.value = initialValue;
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}
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public void setValue(T newValue) {
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this.value = newValue;
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dependentCells.forEach(cell -> cell.propagateUpdate());
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dependentCells.forEach(cell -> cell.fireCallbacks());
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}
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}
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public static class ComputeCell<T> extends Cell<T> {
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private T lastFiredValue;
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private final Supplier<T> formula;
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final Collection<Consumer<T>> callbacks = new ArrayList<>();
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ComputeCell(Supplier<T> formula) {
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this.formula = formula;
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this.value = formula.get();
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this.lastFiredValue = value;
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}
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private void propagateUpdate() {
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value = formula.get();
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dependentCells.forEach(cell -> cell.propagateUpdate());
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}
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private void fireCallbacks() {
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if (!Objects.equals(value, lastFiredValue)) {
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callbacks.forEach(callback -> callback.accept(value));
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dependentCells.forEach(cell -> cell.fireCallbacks());
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lastFiredValue = value;
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}
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}
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public void addCallback(Consumer<T> callback) {
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callbacks.add(callback);
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}
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public void removeCallback(Consumer<T> callback) {
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callbacks.remove(callback);
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}
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}
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public static <T> InputCell<T> inputCell(T initialValue) {
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return new InputCell<>(initialValue);
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}
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public static <T> ComputeCell<T> computeCell(Function<List<T>, T> function, List<Cell<T>> cells) {
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Supplier<T> formula = () -> {
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List<T> cellValues = cells.stream().map(Cell::getValue).collect(Collectors.toList());
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return function.apply(cellValues);
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};
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var computeCell = new ComputeCell<>(formula);
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cells.forEach(cell -> cell.dependentCells.add(computeCell));
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return computeCell;
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}
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}

‎exercises/practice/react/.meta/tests.toml‎

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# This is an auto-generated file.
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#
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# Regenerating this file via `configlet sync` will:
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# - Recreate every `description` key/value pair
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# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
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# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
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# - Preserve any other key/value pair
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#
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# As user-added comments (using the # character) will be removed when this file
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# is regenerated, comments can be added via a `comment` key.
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[c51ee736-d001-4f30-88d1-0c8e8b43cd07]
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description = "input cells have a value"
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[dedf0fe0-da0c-4d5d-a582-ffaf5f4d0851]
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description = "an input cell's value can be set"
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[5854b975-f545-4f93-8968-cc324cde746e]
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description = "compute cells calculate initial value"
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[25795a3d-b86c-4e91-abe7-1c340e71560c]
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description = "compute cells take inputs in the right order"
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[c62689bf-7be5-41bb-b9f8-65178ef3e8ba]
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description = "compute cells update value when dependencies are changed"
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[5ff36b09-0a88-48d4-b7f8-69dcf3feea40]
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description = "compute cells can depend on other compute cells"
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[abe33eaf-68ad-42a5-b728-05519ca88d2d]
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description = "compute cells fire callbacks"
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[9e5cb3a4-78e5-4290-80f8-a78612c52db2]
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description = "callback cells only fire on change"
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[ada17cb6-7332-448a-b934-e3d7495c13d3]
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description = "callbacks do not report already reported values"
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[ac271900-ea5c-461c-9add-eeebcb8c03e5]
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description = "callbacks can fire from multiple cells"
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[95a82dcc-8280-4de3-a4cd-4f19a84e3d6f]
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description = "callbacks can be added and removed"
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[f2a7b445-f783-4e0e-8393-469ab4915f2a]
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description = "removing a callback multiple times doesn't interfere with other callbacks"
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[daf6feca-09e0-4ce5-801d-770ddfe1c268]
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description = "callbacks should only be called once even if multiple dependencies change"
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[9a5b159f-b7aa-4729-807e-f1c38a46d377]
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description = "callbacks should not be called if dependencies change but output value doesn't change"

‎exercises/practice/react/build.gradle‎

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apply plugin: "java"
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apply plugin: "eclipse"
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apply plugin: "idea"
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// set default encoding to UTF-8
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compileJava.options.encoding = "UTF-8"
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compileTestJava.options.encoding = "UTF-8"
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repositories {
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mavenCentral()
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}
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dependencies {
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testImplementation "junit:junit:4.13"
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testImplementation "org.assertj:assertj-core:3.15.0"
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}
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test {
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testLogging {
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exceptionFormat = 'full'
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showStandardStreams = true
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events = ["passed", "failed", "skipped"]
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}
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}

‎exercises/practice/react/src/main/java/React.java‎

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import java.util.List;
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import java.util.function.Consumer;
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import java.util.function.Function;
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public class React {
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public static class Cell<T> {
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public T getValue() {
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throw new UnsupportedOperationException("Please implement the Cell.getValue() method");
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}
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}
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public static class InputCell<T> extends Cell<T> {
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public void setValue(T newValue) {
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throw new UnsupportedOperationException("Please implement the InputCell.setValue() method");
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}
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}
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public static class ComputeCell<T> extends Cell<T> {
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public void addCallback(Consumer<T> callback) {
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throw new UnsupportedOperationException("Please implement the ComputeCell.addCallback() method");
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}
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public void removeCallback(Consumer<T> callback) {
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throw new UnsupportedOperationException("Please implement the ComputeCell.removeCallback() method");
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}
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}
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public static <T> InputCell<T> inputCell(T initialValue) {
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throw new UnsupportedOperationException("Please implement the React.inputCell() method");
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}
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public static <T> ComputeCell<T> computeCell(Function<List<T>, T> function, List<Cell<T>> cells) {
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throw new UnsupportedOperationException("Please implement the React.computeCell() method");
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}
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}

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