[Java SDK] Add ValueState.read(T defaultValue) overload

[Ri1a]

Fixes #18174

This change adds a convenience overload ValueState.read(T defaultValue) to simplify handling of uninitialized state. Currently, ValueState.read() returns null if the value has never been written. As a result, users frequently need to write boilerplate code such as:

Integer count = state.read();
if (count == null) {
  count = 0;
}

or:

Integer count = MoreObjects.firstNonNull(state.read(), 0);

This PR introduces a default method:

default T read(T defaultValue)

which returns the stored value if present, or defaultValue if the state has not been written yet.

Tests

Added ValueStateTest to verify:

• read(defaultValue) returns the default when the state is empty
• read(defaultValue) returns the stored value when present
• readLater() continues to return the same state instance

Compatibility

This change is fully backward compatible:

• existing implementations of ValueState do not need modification
• the new method is implemented as a Java 8 default interface method

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the ValueState interface in the Java SDK by introducing a new read method overload. This addition simplifies the handling of uninitialized state by allowing developers to specify a default value directly, eliminating the need for manual null checks and making state management more concise and less error-prone.

Highlights

• New ValueState Overload: Introduced a new default method read(T defaultValue) to the ValueState interface, providing a convenient way to retrieve a default value when the state is uninitialized, thereby reducing boilerplate code.
• Test Coverage: Added ValueStateTest to thoroughly verify the behavior of the new read(defaultValue) method, ensuring it correctly returns default values for empty states and stored values when present, and confirming readLater() returns the same state instance.
• Backward Compatibility: Ensured full backward compatibility by implementing the new read(T defaultValue) method as a Java 8 default interface method, meaning existing implementations of ValueState do not require modification.

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Changelog

• sdks/java/core/src/main/java/org/apache/beam/sdk/state/ValueState.java
  • Added a new default method read(T defaultValue) to the ValueState interface.
• sdks/java/core/src/test/java/org/apache/beam/sdk/state/ValueStateTest.java
  • Added ValueStateTest to provide comprehensive test coverage for the ValueState interface.
  • Included tests to verify read(defaultValue) returns the specified default when the state is empty.
  • Included tests to verify read(defaultValue) returns the stored value when the state is present.
  • Included a test to confirm that readLater() returns the same state instance.

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[Ri1a]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a convenient read(T defaultValue) overload to ValueState, which simplifies handling of uninitialized state. The implementation is sound and the change is backward compatible. My review includes a suggestion to clarify the Javadoc for the new method to more accurately describe its behavior, particularly regarding null values. I also recommend adding a couple of test cases to cover edge scenarios like clear() and writing null to the state, which will make the test suite more robust.

[gemini-code-assist]

The Javadoc states that defaultValue is returned 'if the value has never been written'. However, the implementation returns defaultValue whenever read() returns null. This can occur not only when the state is uninitialized, but also when null has been explicitly written or the state has been cleared. To avoid confusion, the Javadoc should be updated to accurately reflect this behavior.

Suggested change

	/**
	* Returns the current value of the state, or {@code defaultValue} if the value has never been
	* written.
	*/
	/**
	* Returns the current value of the state, or {@code defaultValue} if the current value is {@code
	* null}. This can happen if the value has never been written or if {@code null} was written.
	*/

[gemini-code-assist]

The tests for the new functionality are a good start. To make them more comprehensive, consider adding test cases for edge scenarios. Specifically, testing the behavior after state.clear() is called, and after null is explicitly written to the state, would strengthen the test suite and clarify the contract of the new method.

For example:

@Test
public void testReadReturnsDefaultValueAfterClear() {
  TestValueState<Integer> state = new TestValueState<>();
  state.write(10);
  state.clear();
  assertEquals(Integer.valueOf(5), state.read(5));
}

@Test
public void testReadReturnsDefaultValueWhenNullIsWritten() {
  TestValueState<Integer> state = new TestValueState<>();
  state.write(null);
  assertEquals(Integer.valueOf(5), state.read(5));
}

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